RADIOACTIVE ISOTOPES OF NITROGEN

A. G. Lundin and M. B. Neiman

Radioactive nitrogen \(N_7^{13}\) was one of the three elements (the other two were \(Si^{27}\) and \(P^{30}\)) in which Irène Curie and Frédéric Joliot\(^1\) first discovered the phenomenon of artificial radioactivity in 1933.

The possibility of artificially producing radioactive elements placed in the hands of scientists—physicists, chemists, biologists—and engineers a most powerful tool for investigating a wide variety of processes. Numerous studies carried out since then, and continuing at the present time, have led to the discovery of artificial radioactivity in almost all elements of the periodic system. In most elements several radioactive isotopes have been found.

The existence of a second radioactive isotope of nitrogen—\(N_7^{16}\)—was established in 1934. In 1948, when particles with energies in the hundreds of MeV were obtained, yet another radioactive isotope of nitrogen, \(N_7^{17}\), was discovered; it decays with the emission of \(\beta^-\)-particles and neutrons.

Finally, in 1949, the short-lived isotope of nitrogen \(N_7^{12}\) was discovered; in its decay, positrons with energies greater than 16 MeV are emitted.

Below we set forth the methods of production, properties, and certain applications of the listed isotopes of nitrogen.

1. ISOTOPE \(N_7^{12}\). \(T_{1/2}=0.0125 \pm 0.001\) sec.

When a carbon target was bombarded with high-energy protons (32 MeV), obtained by means of a linear accelerator, a radioactive product with very interesting properties\(^2\) was discovered: it proved to be the shortest-lived of all \(\beta\)-radioactive elements known up to the present time. In addition, the positrons emitted by this substance,

possess a colossal energy—16.6 MeV, which is more than three times greater than the maximum energy of the β-spectrum of any of the known elements possessing positron activity. This radioactive element turned out to be a new, lightest isotope of nitrogen—\( \mathrm{N}_7^{12} \), formed as a result of the reaction

\[ \mathrm{C}_6^{12}+\mathrm{H}_1^1=\mathrm{N}_7^{12}+\mathrm{n}_0^1+Q \tag{1} \]

and undergoing positron decay:

\[ \mathrm{N}_7^{12}=\mathrm{C}_6^{12}+\beta^+ + E_\beta+\nu . \tag{2} \]

The linear accelerator used to obtain high-energy protons operated at a frequency of 7.5 pulses of proton current per second. The interval between pulses in this case reached approximately 130 msec. During this interval a decrease in the activity of the sample by about a factor of 1000 was observed. It proved quite simple to prepare samples of the required activity. According to the data of the author of the work\(^2\), at a peak current in the accelerator of 1 μA the source of \( \mathrm{N}_7^{12} \) had an activity of the order of 1 millicurie, which made it necessary to conduct the work at a considerably smaller current in order to avoid overloading the counters.

Measurements of the half-life were carried out in the following manner: the positrons from the decay of \( \mathrm{N}_7^{12} \) were recorded by means of two Geiger counters connected in a coincidence circuit, and the current pulses arising in them were fed to the screen of a cathode oscilloscope. However, the recording circuits did not function during the entire interval between pulses of the linear accelerator, but each time were opened only for a short interval of time equal to 0.008 sec. The position of the moment during which the pulses were recorded could be changed relative to the end of the pulse of the linear accelerator, i.e., the moment at which bombardment ended. By measuring the number of positrons falling within the interval of 0.008 sec as a function of the displacement of the latter relative to the moment at which bombardment ended, it was possible to obtain the decay curve of \( \mathrm{N}_7^{12} \).

In the path of the protons that irradiated the carbon target there was placed polystyrene foil 0.025 mm thick, in which, under the action of proton bombardment, the radioactive isotope of carbon \( \mathrm{C}_6^{11} \) was produced. By measuring the activity of the foil, it was possible to determine the magnitude of the irradiation received by the carbon target. Figure 1 shows one of the decay curves of \( \mathrm{N}_7^{12} \) obtained in this manner. Its half-life, determined with the aid of this curve, proved to be

\[ T_{1/2}=0.0125 \pm 0.001\ \text{sec}. \]

The author also determined the threshold of reaction (1). The energy of the protons was varied by slowing them down in a layer of aluminum placed in front of the target being bombarded. The curve obtained in this way—

the excitation activity in a polystyrene foil of thickness 0.12 mm is presented in Fig. 2. From these data the threshold value of the reaction \(C^{12}(p,n)N^{12}\) was found to be 20 MeV. It should be noted that the threshold of the reaction \(C^{12}(p,pn)C^{11}\) (which also occurs when carbon is bombarded by protons), calculated from the masses of the atoms participating in it, is 20.2 MeV.

Fig. 1. Decay of \(N^{12}_7\). \(I\)—activity in arbitrary units.

This indicates that the binding energy of the proton in \(N^{12}\) is very small and is approximately 200 keV.

By observing the absorption of positrons from the decay of \(N^{12}_7\) in aluminum, the \(\beta\)-spectrum shown in Fig. 3 was obtained. Its maximum energy proved to be

\[ E_\beta = 16.6\ \text{MeV} \]

(curve 2). Such an unusually large value of the \(\beta\)-spectrum energy makes \(N^{12}_7\) a very convenient isotope for testing the modern “neutrino theory.” Indeed, the recoil nucleus \(N^{12}_7\) should have an energy of approximately 13,000 eV. Such a nucleus will have a range of the order of 2 cm in a Wilson chamber filled with water vapor and operating at room temperature (i.e., at low vapor pressure). Thus, the energy and momentum of the recoil nucleus can be measured comparatively accurately. Determining the energy and momentum of the positron of the decay presents no particular difficulties.

Knowing the energies and momenta of the positron and the recoil nucleus, it will be possible, on the basis of the conservation laws, to determine the energy and momentum of the neutrino. According to the existing theory, the sum of the energies of the positron and neutrino found in this way must always be equal to the maximum energy of the \(\beta\)-spectrum.

On the basis of the found \(\beta\)-spectrum energy, equal to 16.6 MeV, and taking the mass of \(C^{12}_6\) to be 12.0038, we find that the mass of \(N^{12}_7\) must be

\[ M_{N^{12}} = 12.0228 \pm 0.00015 . \]

Exactly the same value for the mass of \(N^{12}_7\) is obtained when calculating it from the data of reaction (1) at the proton threshold energy

\[ E_p = 20\ \text{MeV}. \]

From this, incidentally, it follows that the arising

when carbon is irradiated with protons the 0.0125-second activity belongs precisely to \(N_7^{12}\). This also follows from the fact that, as can easily be shown by direct calculations, none of the isotopes with \(Z<8\) and \(A<1\) can give such an activity. In fact, starting from the energies of the positrons, one can calculate the mass of any of these isotopes that transform by \(\beta\)-decay into known isotopes. Then, having calculated the mass of the excited nucleus \(N_7^{13}\) for a proton energy equal to \(20\) MeV, one can show that none of the reactions leading to isotopes with \(Z<8\) and \(A<14\), except reaction (1), is energetically possible.

Let us carry out such a calculation, for example, for \(Li_3^4\), i.e., let us suppose that the 0.0125-second isotope is \(Li_3^4\), which transforms upon emission of a positron into \(He_2^4\). Then its mass should be equal to:

\[ Li_3^4 = He_2^4 + 2\beta + E_\beta = \]

\[ = 4.0227\, MU. \]

The mass of the excited nucleus \(N_7^{13}\) is equal to

\[ !N_7^{13} = C_6^{12} + H_1^1 + E_p = \]

\[ = 13.0335\, MU. \]

Fig. 2. Excitation curve of the activity in a polystyrene target. The thickness of the polystyrene target is \(17\,mg/cm^2\). \(I\) is the intensity in arbitrary units.

Fig. 3. Absorption curve of positrons of the decay of \(N_7^{12}\) in aluminum. The maximum energy of the \(\beta\)-spectrum is \(16.6\) MeV. \(I\) is the intensity in arbitrary units.

In order to transform into \(Li_3^4\), two \(\alpha\)-particles and a neutron must be ejected from the nucleus \(N_7^{13}\), i.e., a mass equal to

\[ 2He_2^4 + n_0^1 = 9.01673\, MU. \]

In this case only \(4.0168\, MU\) will be left for \(Li_3^4\). Hence it is clear that for the formation of \(Li_3^4\) approximately \(5\) MeV of energy is lacking.

II. ISOTOPE \(N_7^{13}\). \(T_{1/2}=9.9\) min.

1. Methods of production

As already mentioned, radio-nitrogen \(N_7^{13}\) was first obtained by Curie and Joliot\(^3\) by irradiating a boron target with \(\alpha\)-particles.

Curie and Joliot\(^{3,4}\) carried out a chemical identification of the activity obtained. They irradiated a boron nitride BN target for several minutes with \(\alpha\)-particles and then heated it with caustic soda. The gaseous ammonia \(NH_3\) formed in this process was the only substance in which the activity was concentrated. The half-life proved to be equal to the half-lives obtained with other boron targets, whereas bombardment of nitrogen with \(\alpha\)-particles did not lead to the appearance of analogous activity. This indicated the formation, upon irradiation of boron with \(\alpha\)-particles, of the radioactive isotope of nitrogen \(N_7^{13}\) according to the reaction

\[ B_5^{10}+He_2^4=N_7^{13}+n_0^1+Q. \tag{3} \]

The formation of radio-nitrogen upon irradiation of boron with \(\alpha\)-particles was studied by many other authors\(^{5-13}\).

Ridenour and Henderson\(^ {10}\) give, for \(\alpha\)-particles with an energy of 9 MeV, a yield of \(N_7^{13}\) of \(4.6\cdot 10^{-6}\) atoms per one \(\alpha\)-particle (calculated for the pure isotope \(B_5^{10}\)).

For the yield of \(N_7^{13}\) by reaction (3), resonance values were observed by various authors at the following \(\alpha\)-particle energies: 3.6 and 4.4 MeV\(^ {11}\), 3.02 and 4.42 MeV\(^ {12}\), and 2.72; 2.98; 3.43; 3.70; 4.05; 4.33; 4.59; 4.94 and 5.22 MeV\(^ {13}\).

For the energy of reaction (3), \(^{14}\) gives the value \(Q=1.18\) MeV.

Data for another reaction by which \(N_7^{13}\) is also obtained,

\[ C_6^{13}+H_1^1=N_7^{13}+n_0^1+Q, \tag{4} \]

are given in \(^{13}\). The authors bombarded a graphite target with protons from an electrostatic generator and determined the threshold energy at which neutron emission begins. The neutron indicator was an ionization chamber filled with \(BF_3\) and surrounded by paraffin, connected to a linear amplifier and a mechanical pulse counter. Figure 4 gives the dependence of the neutron yield on proton energy obtained in this way. From it the reaction threshold was determined, located at

\[ E_p=3.20\pm0.03\ \text{MeV}. \]

This makes it possible to calculate the energy of reaction (4)

\[ Q=-2.97\pm0.05\ \text{MeV}, \]

as well as the mass difference of the atoms

\[ N^{13}-C^{13}=2.21\pm0.03\ \text{MeV}. \]

For the maximum energy of the positrons in the decay \( \mathrm{N}^{13} = \mathrm{C}^{13} + \beta^{+} + E_\beta + \nu \), we obtain:

\[ E^{\beta}=1.19 \pm 0.03\ \text{MeV}. \]

Let us note that direct measurements of the limits of the \(\beta\)-spectrum give values from \(E_\beta = 1.19\ \text{MeV}\) to \(E_\beta = 1.25\ \text{MeV}\).

Cockcroft, Gilbert, and Walton \(^{16,17}\) discovered the existence of an activity with a period of \(10.5 \pm 0.5\) min, produced as a result of bombarding graphite with protons by the reaction

\[ {}^{12}_{6}\mathrm{C}+{}^{1}_{1}\mathrm{H}\to{}^{13}_{7}\mathrm{N}+Q(\gamma). \tag{5} \]

The formation of radioactive nitrogen \({}^{13}_{7}\mathrm{N}\) under bombardment of carbon by protons was subsequently observed by a number of authors \(^{17-24}\). The yield of the reaction for protons with an energy of 520 keV was estimated \(^{18}\)

Fig. 4. Dependence of the neutron yield on the energy of the bombarding protons in the reaction
\({}^{13}_{6}\mathrm{C}(p,n){}^{13}_{7}\mathrm{N}\). \(I\) — intensity in arbitrary units.

at \(1 \cdot 10^{-10}\) atoms per proton. Investigation of the dependence of the radioactivity induced in graphite on the proton energy in the interval from 200 to 900 keV \(^{19}\), when bombarded with protons from a Van de Graaff generator, led to the conclusion that in this region there are two resonance peaks at \(E_p = 400\) keV and \(E_p = 480\) keV, and that the yield of \({}^{13}_{7}\mathrm{N}\) is, in order of magnitude, close to \(1 \cdot 10^{-10}\) atoms per proton. In other measurements \(^{20}\), one resonance peak was found at \(E_p = 450\) keV with a width of 30 keV, and for the yield of \({}^{13}_{7}\mathrm{N}\) a value was obtained approximately 7.5 times larger than in \(^{19}\) and \(^{20}\).

For the \(\gamma\)-rays arising as a result of reaction (5), a resonance peak was observed at \(E_p = 480\) keV \(^{21}\). The energy of these \(\gamma\)-rays, determined from their absorption in the material between two Geiger counters operating in coincidence, proved to be

\[ E_\gamma = 2.6\ \text{MeV}. \]

In work \(^{22}\), for the resonance energy of the protons the value \(E_p = 460\) keV is given, and for the resonance width \(\Delta E_p = 50\) keV. The most

A. G. LUNDIN AND M. B. NEIMAN

Reliable results of the investigation of reaction (5) are evidently given in \(^{23}\) and \(^{24}\). The authors studied the dependence of the yield of \(\gamma\)-rays arising as a consequence of reaction (5), and also of positrons from the decay of \(N^{13}_{7}\), on the energy of the protons bombarding graphite, and obtained very good agreement of the results in both cases. In Fig. 5 is shown the resonance curve obtained by them for the yield of \(N^{13}_{7}\) (from decay positrons). For the resonance energy the authors give the value \(E_p = 453\) keV and for the width of the resonance (at a height equal to one-half the maximum) \(\Delta E_p = 35\) keV.

Fig. 5. Yield curve of \(N^{13}_{7}\) for the reaction \(C^{12}_{6}(p,\gamma)N^{13}_{7}\).

In a later work \(^{21}\) the same authors refined the value of the resonance energy of the protons and gave for it the figure \(E_p = 456\) keV. The energy of the \(\gamma\)-rays produced in this reaction is \(2.3\) MeV; the yield is \(7.3 \cdot 10^{-10}\) \(\gamma\)-quanta per proton; the reaction cross section is \(\sigma = 1.2 \cdot 10^{-4}\) barn. It is interesting to calculate the energy of the \(\gamma\)-rays arising as a result of reaction (5) from the masses of the atoms participating in the reaction. Substituting in equation (5) the mass values of \(C^{12}_{6}\) and \(H^{1}_{1}\) from the tables of S. Petrovich \(^{25}\), and taking the mass of \(N^{13}_{7}\) equal to 13.00994 (see below), we find: \(Q' = 1.85\) MeV.

If, in addition, one takes into account the kinetic energy of the protons, equal to \(E_t = 2.30\) MeV, in complete agreement with the results of the works \(^{23,24}\).

Quite recently \(^{1a}\) one more resonance was discovered for the \(\gamma\)-rays arising in reaction (5), located at \(E_p = 1.697 \pm 0.012\) MeV. The width of the resonance is \(74 \pm 94\) keV. The yield is \((-9.4 \pm 1.4)\cdot 10^{-10}\) quanta per proton. In this work, from the resonance proton energies equal to 0.456 MeV and 1.697 MeV, two excited levels in \(N^{13}\) were determined, located at \(2.83 \pm 0.018\) MeV and \(3.53 \pm 0.027\) MeV.

The formation of \(N^{13}_{7}\) when carbon is bombarded by deuterons according to the reaction

\[ C^{12}_{6} + H^{2}_{1} = N^{13}_{7} + n^{1}_{0} + Q, \tag{6} \]

which was simultaneously observed as early as 1934 by two different groups of authors \(^{26,27,28}\), subsequently became the most widely disseminated

RADIOACTIVE ISOTOPES OF NITROGEN

A widespread method for obtaining \(N^{13}_{7}\) for applied purposes. The excitation curve of the activity in carbon under bombardment by deuterons for deuteron energies from 200 to 900 keV, given in \(^{19}\), indicates a smooth increase in the yield of \(N^{13}_{7}\) with the energy of the deuterons in this region. The yield of \(N^{13}_{7}\) was measured at several points for deuteron energies from 2 to 5 MeV \(^{29,30}\), and a considerable maximum was found at 3 MeV, associated with reaching the top of the Coulomb barrier. In this case the reaction cross section was equal to \(4.0\cdot 10^{-2}\) barn.

The excitation curve of the activity in carbon under its bombardment by deuterons with energy up to 6 MeV is given in work \(^{31}\). This reaction was also studied in a number of older works \(^{32—34}\).

In a more recent work \(^{35}\), in the interval of deuteron energies from 0.7 to 1.9 MeV, five resonance maxima were found for the yield of neutrons formed in the reaction \(C^{12}(d,n)N^{13}\), located at 0.92, 1.16, 1.30, 1.74, and 1.82 MeV. At the same time, with the aid of a Geiger counter with thin walls, the yield of positrons from the decay of \(N^{13}_{7}\) was observed. Resonance peaks were found at the same values of the deuteron energies as for the neutron yield.

Similar results were obtained in investigations \(^{36}\). In addition to the five resonances listed above, the authors also found a considerable peak at \(E_d = 2.3\) MeV.

In 1947 the spectrum of neutrons obtained in the reaction \(C^{12}_{6} + d\) was measured from recoil protons in a Wilson chamber \(^{37}\). The authors found that the neutrons formed in the bombardment of carbon by deuterons with energy 2 MeV are monochromatic. From the ranges of the recoil protons in the Wilson chamber the energy of these neutrons was determined, and hence the reaction energy (6)

\[ Q = -0.27 \pm 0.02 \text{ MeV}. \]

The results of measuring the yield of neutrons and \(\gamma\)-rays from carbon under the action of deuterons with energies from 0.85 to 3.25 MeV are given in \(^{38}\). The deuteron energy was controlled with an accuracy up to 0.2%. A strong asymmetry of the neutron yield with angle was discovered. The maximum of their intensity is obtained at \(0^\circ\), i.e., in the direction of the deuteron beam. In the case \(E_d = 3\) MeV the intensity of neutrons at \(0^\circ\) exceeds their intensity at \(90^\circ\) by approximately 20 times.

Fig. 6. Dependence of the yield of \(N^{13}_{7}\) on the deuteron energy (curve 1). On curve 2 the same data are plotted in the coordinates \([\text{yield } N^{13}]^{2/3}\) and \(E_d\).

In one of the recent works\(^{39}\), the threshold energy of reaction (6) was determined. The obtained dependence of the positron yield from \(N_7^{13}\) in the threshold region on the deuteron energy is shown in Fig. 6. The dependence \((\text{yield } N^{13})^{2/3}\)—deuteron energy proved to be linear in this region. The threshold value, equal to \(E_p = 328 \pm 3\) keV, was obtained by extrapolating this dependence. The reaction energy \(Q\) calculated from this value is:

\[ Q = -0.281 \pm 0.003\ \text{MeV}. \]

Fig. 7. Angular distribution of neutrons in the reaction \(C_6^{12}(d,n)N_7^{13}\) for the deuteron energies indicated in the drawing. The coordinate system is associated with the center of inertia. \(\sigma\)—reaction cross section in barns; \(\omega\)—solid angle.

Resonance peaks in the yield of neutrons produced by reaction (6), and of \(\gamma\)-rays arising as a result of the competing reaction

\[ C_6^{12} + H_2^1 = C_6^{13} + H_1^1 + Q(\gamma), \tag{6′} \]

in the deuteron-energy range from 0.7 to 1.9 MeV, were investigated in works\(^{40}\) and \(^{41}\). Resonance values of the neutron yield were observed at deuteron energies equal to 0.91; 1.16; 1.30; 1.62; 1.76 MeV. In this case the cross sections of reaction (6) were respectively equal to 0.12; 0.14; 0.19 and 0.22 barn. The yield of neutrons at different angles proved to be sharply asymmetric (Fig. 7). Thus, for example, at a deuteron energy equal to 1.26 MeV, the neutron yield in the direction of the deuteron beam is five times smaller than the yield of neutrons at an angle of \(160^\circ\).

Recently the spectrum of neutrons arising in reaction (6) was investigated with the aid of photographic plates[^123]. The emulsion of the plates contained hydrogen (1.4% by weight), and this made it possible to register neutrons and determine their energy from the tracks of recoil protons. Three peaks were found in the neutron yield at neutron energies equal to 7.53, 5.40, and 4.30 MeV.

The first group of neutrons corresponds to the formation of \(N^{13}\) in the ground state.

The second and third make it possible to determine two excited levels of \(N^{13}\), situated at \(2.29 \pm 0.12\) MeV and \(3.48 \pm 0.12\) MeV, respectively.

It should be noted that these results agree with the results of determining the levels of \(N^{13}\) according to the data of[^122] for the reaction \(C^{12}(p,\gamma)N^{13}\) (see above).

When targets of lithium, boron, beryllium, and copper were bombarded with deuterons, fast neutrons were obtained[^42,^43], and the neutron energy in the bombardment of Li, B, and Be was respectively 14, 13, and 4.6 MeV; the energy of the neutrons obtained in the bombardment of copper was not determined.

Upon irradiating nitrogen with these neutrons, different yields of \(N^ {13}_{7}\), formed by the reaction

\[ N^{14}_{7} + n^{1}_{0} = N^{13}_{7} + 2n^{1}_{0} + Q. \tag{7} \]

were obtained. The dependence of the yield of \(N^{13}_{7}\) on the neutron energy is given in Table I.

Table I

Yield of \(N^{13}_{7}\) in reaction (7)

Neutron source	Li + \(d\)	B + \(d\)	Cu + \(d\)	Be + \(d\)
Neutron energy in MeV	14	13	—	4.6
Yield	100	27	18	0

The reaction energy, calculated from the masses of the particles participating in the reaction, is \(Q = -10.57\) MeV.

In the reaction

\[ N^{14}_{7} + H^{2}_{1} = N^{13}_{7} + H^{3}_{1} + Q, \tag{8} \]

observed in[^44,^45,^46], in addition to radioactive nitrogen \(N^{13}_{7}\), a radioactive isotope of hydrogen—tritium—is also obtained. The threshold of this reaction is \(6.8 \pm 0.1\) MeV. The particle-energy \(Q\) calculated from the masses is \(Q = -4.5 \pm 0.1\) MeV.

The nuclear photoeffect in nitrogen

\[ N^{14}_{7} + \gamma = N^{13}_{7} + n^{1}_{0} + Q. \tag{9} \]

observed in work \(^{47}\). The authors used very hard \(\gamma\)-rays with an energy of \(17\) MeV, obtained by bombarding lithium with protons.

They also observed a weaker effect when nitrogen was bombarded with \(\gamma\)-rays of energy \(12.8\) MeV, obtained from the reaction \(B^{11}(p,\gamma)C^{12}\) (see also \(^{48,49,50}\)).

By means of high-energy bremsstrahlung \(\gamma\)-radiation from a betatron, the threshold of the nuclear photoeffect in nitrogen was determined \(^{51}\), and was found to be
\[ E_\gamma = 11.1 \pm 0.5\ \text{MeV}. \]

In another work \(^{52}\), the value of the photoeffect threshold was found to be
\[ E_\gamma = 10.65 \pm 0.20\ \text{MeV}. \]

It should be noted that the effect threshold, calculated from the masses of the particles participating in reaction (9), is
\[ E_\gamma = 10.58\ \text{MeV}. \]

When nitrogen was bombarded with \(\gamma\)-quanta of energy \(E_\gamma = 100\) MeV \(^{53}\), strong positron activity with a period of \(9.96\) min. was again detected, which the authors attributed to the formation of \(N_7^{13}\) by the reaction \(N^{14}(\gamma,n)N^{13}\).

Measurements indicate that the yield of \(N_7^{13}\) is the same for bremsstrahlung \(\gamma\)-quanta with energies of \(50\) MeV and \(100\) MeV \(^{54}\).

In practice, to obtain \(N_7^{13}\) in the overwhelming majority of cases, the reaction \(C^{12}(d,n)N^{13}\) is used.

Carbon, usually in the form of graphite powder, graphite plates, or soot, is bombarded with deuterons with energies of several hundred thousand or, better, millions of electron-volts. The activated graphite powder (or soot) thus obtained is introduced directly into the object for measurements—the spectrometer, Wilson chamber, etc. The cross section of this reaction is so large (of the order of tenths of a barn) that the yield of \(N_7^{13}\) when using deuterons from a Van de Graaff generator or cyclotron is quite sufficient for all possible applications. Moreover, as Pollard and Davidson indicate \(^{55}\), if special careful measures are not taken to remove carbon from various targets, then when they are bombarded with deuterons a strong admixture of \(N_7^{13}\) is always added to the desired radioactive products.

The influence of the nature of the target on the chemical state of radionitrogen \(N_7^{13}\), obtained by bombarding carbon-containing targets with deuterons, was considered in work \(^{128}\).

Recently, in connection with the production of elementary particles with energies of hundreds of MeV, reactions of an entirely new type have been studied, in which whole complexes of elementary particles, as well as large fragments of nuclei, are split off from the bombarded nucleus. This opens up new possibilities for obtaining all kinds of radioactive substances, since during bombardment

when bombarded by ultra-fast particles, almost all elements lying closer to the beginning of the periodic system than the initial product arise. Thus, for example, reports\(^{56,57}\) describe experiments on the bombardment of carbon, nitrogen, oxygen, and fluorine with very fast neutrons of energy 90 MeV.

In this case, under bombardment of nitrogen, oxygen, and fluorine, a yield of \(N_7^{13}\) was observed. It is quite probable that a whole series of reactions with ultra-fast particles will yet be found in which the radio-nitrogen \(N_7^{13}\) is obtained.

2. Character of the \(\beta\)-spectrum

The nitrogen \(N^{13}\) obtained in reactions (3)—(9) is unstable and decays with a half-life of about 9.9 min, emitting a positron (and neutrino) and transforming into the stable isotope of carbon \(C_6^{13}\):

\[ N_7^{13}=C_6^{13}+\beta^+ + \nu + E_\beta . \]

However, although \(N_7^{13}\) was among the first three artificially obtained radioactive elements, the form of its \(\beta\)-spectrum has still not yet been definitively established. One group of researchers believes that the positron spectrum is complex and consists of two components: one with maximum energy of about 1.2 MeV and another corresponding to a transition to the excited level of \(C_6^{13}\) at 280 keV, with subsequent emission of a \(\gamma\)-quantum of energy 280 keV. This conception found its reflection also in the well-known isotope tables of Seaborg and Mattauch.

Another group regards the spectrum of \(N_7^{13}\) as simple, consisting of one component with limiting energy of the order of 1.2 MeV. Studies carried out in recent years seem to confirm the second point of view.

Among the first to measure the spectrum of positrons from the decay were the Soviet physicists Alikhanov, Alikhanyan, and Dzhelepov\(^{5,6,7}\). Using for this purpose an original apparatus with two coincidence counters in a magnetic field\(^{58}\), they recorded the positron spectrum and found their limiting energy to be \(E_\beta = 1.3\) MeV.

Later it was found\(^{59,60}\) that in the decay of \(N_7^{13}\), in addition to \(\gamma\)-quanta with energy 510 keV, produced in the process of positron annihilation, there is \(\gamma\)-radiation with energy 280 keV and intensity equal to 0.4 \(\gamma\)-quantum per one act of decay.

\(N_7^{13}\) was obtained by bombarding carbon with deuterons of energy 6 MeV. With the aid of a Wilson chamber, the spectrum of secondary electrons arising in lead under the action of \(\gamma\)-radiation from \(N_7^{13}\) was recorded. To eliminate the possibility of the appearance of secondary electro-

…from the walls of the Wilson chamber, the flux of γ-quanta from the source passed through a lead channel and struck a lead plate, which served as a source of secondary electrons. To eliminate positrons from decay, the source was surrounded by a layer of inactive carbon. The resulting energy distribution of the secondary electrons indicates the presence of γ-radiation with an energy of 280 kev.

Lyman $^{61,62}$, who investigated the decay of $\mathrm{N}_7^{13}$ by means of a magnetic spectrometer with high resolving power, also considers the spectrum complex. The spectrum obtained by him can be decomposed into two components with maximum energies of 1.20 Mev and 1.98 Mev and with a positron distribution in each component corresponding to Fermi’s theory. In this case the ratio of the intensities of these components proves to be $4:1$. Thus, transitions to the excited level occur four times less often than transitions to the ground level of $\mathrm{C}_6^{13}$. If in fact the emission of a positron belonging to the second component leads to the subsequent emission of a γ-quantum, then $(\beta-\gamma)$ or $(\gamma-\gamma)$ coincidences can be detected. The latter must occur because of the presence of positron-annihilation radiation. $(\gamma-\gamma)$ coincidences were indeed observed by Lyman with the aid of two coincidence counters registering γ-quanta, the angle between whose directions of flight was less than $90^\circ$ (the two γ-quanta of annihilation radiation fly apart at an angle equal to $180^\circ$ $^{63}$).

The presence of γ-radiation with an energy of about 280 kev is also confirmed in work $^{64}$, in which a broad peak of secondary electrons produced in lead under the action of this radiation was found. The β-spectra of $\mathrm{N}_7^{13}$, taken with the aid of a magnetic spectrometer $^{65}$, reveal at small positron energies a considerable deviation from the straight line obtained when applying Fermi’s theory in the form proposed in $^{66}$. In $^{65}$ this deviation is explained by the presence of a component of low energy and by partially scattered electrons.

The existence of γ-rays with energy $\sim 280$ kev was also confirmed in work $^{67}$. The authors placed graphite irradiated by deuterons with energy 5 Mev in a lead block. In one of its walls there was a channel which could be completely closed (Fig. 8). Owing to the thick walls of the block, radiation with an energy of 280 kev was strongly absorbed in them, whereas annihilation radiation was absorbed considerably more weakly. With the channel open, γ-rays from the source and annihilation radiation fell on a lead plate, which served as a source of secondary electrons. The energy of the secondary electrons was measured by means of a magnetic spectrometer. To prevent positrons from entering the channel, a thin (2.5 mm) aluminum pla-

wall. Two spectra were taken, with the channel open and with it closed. The difference of these values gives the effect caused by γ-radiation whose energy is less than the energy of annihilation radiation. Fig. 9 shows the curve obtained in this way. The annihilation γ-radiation gives three peaks, located on the right and caused by Compton electrons and photoelectrons from the \(K\)- and \(L\)-shells in lead (\(GKL\) are the theoretical values for these groups). The fourth peak gives \(K\)-photoelectrons of the softer γ-radiation. If

Fig. 8. Arrangement for investigating the presence of γ-radiation in the decay of \(N_7^{13}\).

Legend in Fig. 8: Lead; Brass; Aluminum; Source of secondary electrons; \(S\) source; Channel closed; Channel open.

Fig. 9. Spectrum of secondary electrons arising in lead under the action of γ-radiation \(^{67}\).

Axis labels and marks in Fig. 9: \(E\), MeV; \(0.2\), \(0.4\), \(0.6\); \(1000\), \(2000\), \(G\), \(K\), \(L\), \(3000\), \(H\rho\).

one adds the binding energy of an electron in the \(K\)-shell in lead (88 kev) to the energy of the electrons of this peak, then the energy of the γ-radiation is found to be \(E_\gamma = 285 \pm 10\) kev.

The authors estimated the intensity of this radiation, obtaining a value of 0.2 γ-quanta with energy 285 kev per decay event. However, the measurement results of a number of other authors contradict the above data on the presence of γ-radiation with energy 280 kev.

Thus, according to \(^{68}\), there are not more than 0.05 γ-quanta with energy about 280 kev per decay event.

Similar indications are also found in \(^{69}\).

Measurements made with \(N_7^{13}\) obtained by bombarding C with deuterons \(^{70}\) also indicate the simple character of the β-spectrum of \(N_7^{13}\). The curve obtained by the author with the aid of a magnetic spectrometer is shown in Fig. 10. For comparison, there is also shown

the Fermi–Curie plot, constructed in the coordinates \(\left(\frac{N}{f}\right)^{\frac{1}{2}}\), \(E+1\), from which it follows that the \(\beta\)-spectrum of \(N^{13}_{7}\) is simple.

In 1945 an attempt was made\(^{71,72}\) to verify the data on the presence of \(\gamma\)-radiation with an energy of 280 keV. For this purpose, graphite irradiated with deuterons of energy 5 MeV was introduced into a \(\beta\)-spectrometer. As in \(^{59,60,67}\), the spectrum of secondary electrons arising in lead under the action of \(\gamma\)-radiation from the source was investigated.

Fig. 10. Positron spectrum of \(N^{13}_{7}\) (1) and Curie–Fermi plot (2).

The spectrum obtained is shown in Fig. 11. On the curve the maxima from Compton electrons and \(K\)- and \(L\)-photoelectrons caused by annihilation radiation are visible. As can be seen from the figure, no other maxima were found here that could be associated with a definite \(\gamma\)-radiation.

An attempt was also made\(^{73}\) to confirm the presence of \((\gamma-\gamma)\)-coincidences in the radiation of \(N^{13}_{7}\), which had been reported by Lyman\(^{62}\). The apparatus (Fig. 12) consisted of two ordinary-type \(\gamma\)-counters placed at a distance of 2 cm from one another. Between the counters was placed a wide lead plate 15 mm thick. This prevented false coincidences that could be caused by one and the same secondary electron passing through both counters.

The positron source (graphite irradiated with deuterons) was placed on a thin foil. The distance between it and the counters could be varied, thereby making it possible to observe \(\gamma\)-radiation at different angles.

RADIOACTIVE ISOTOPES OF NITROGEN

To check the results, coincidence measurements were also carried out with radioactive carbon \(C_6^{11}\), which has positron activity but no \(\gamma\)-radiation.

The results obtained under these conditions indicate that the number of coincidences for \(N_7^{13}\), within the limits of statistical accuracy, is equal to the number of coincidences with \(C_6^{11}\) and corresponds to the theoretical number of accidental coincidences. This testifies in favor of the simple character of the decay of \(N_7^{13}\).

In 1947 the \(\gamma\)-radiation of \(N_7^{13}\) was studied\(^{74}\) by means of a magnetic spectrometer with high resolving power and small scattering and background\(^{75}\). A spectrum of secondary electrons in lead was taken. The results obtained agree with those of Refs. \(^{71,72}\). The authors conclude that there are no more than \(0.002\) \(\gamma\)-quanta with energy of the order of \(280\) kev per one decay event of \(N_7^{13}\).

Fig. 11. Spectrum of secondary electrons\(^{72}\) arising in lead under the action of the \(\gamma\)-radiation of \(N_7^{13}\).

Fig. 12. Arrangement for the study of \((\beta-\gamma)\)-coincidences in the decay of \(N_7^{13}\). 1—sample; 2—foil; 3 and 4—Geiger counters; 5—lead plate.

Dzhelepov\(^{76,77}\) indicated that the \(\beta\)-transformations of radioactive nuclei of the type \(M_Z^{2Z-1}\), to which \(N_7^{13}\) also belongs, should be allowed. Indeed, in the positron decay of such a nucleus only the replacement of a proton by a neutron occurs, while the residual nucleus contains an equal number of protons and neutrons.

In this case the wave functions of the nucleus before and after the transformation should be very close, which corresponds to reality, since all nuclei of this type have a very small product \(fT_{1/2}\), where \(T_{1/2}\) is the half-life and \(f\) is the well-known integral entering into Fermi’s theory of \(\beta\)-decay\(^{78}\). Therefore, in these nuclei it is difficult to expect the presence of several components of the \(\beta\)-spectrum, since

In this case a certain rearrangement of the nucleus would take place, and this would lead to a change in the wave function of the nucleus after the β-transformation.

The data presented above do not, however, allow the question of the existence of γ-radiation with an energy of 280 kev to be regarded as settled, since the conditions under which the experiments leading to contradictory results were carried out differed somewhat. Nevertheless, in our opinion, it is more probable that the β-spectrum of \(N_7^{13}\) has a simple character.

3. Boundary of the β-spectrum

Numerous measurements of the boundary of the β-spectrum of \(N_7^{13}\) are in the main in agreement with one another. Direct observations of the boundary of the β-spectrum give, for the maximum positron energy, a value of about 1.2 Mev. The same values are given by extrapolation of the Curie–Paxton graph \(^{66}\), constructed on the basis of Fermi’s theory. Extrapolation of this graph, constructed in accordance with the Konopinski–Uhlenbeck theory, gives considerably higher values—approximately 1.45 Mev. Almost all investigators come to the conclusion that the β-decay of \(N_7^{13}\), in the region of medium and high positron energies, exactly follows

Fig. 13. Curie–Fermi plot for \(N_7^{13}\).

...of the original Fermi theory, whereas the Konopinski-Uhlenbeck theory gives completely incorrect results.

For the maximum energy of the β-spectrum, the value \(E_\beta = 1.24 \pm 0.02\) MeV was obtained \(^{70}\). Measurements carried out more recently \(^{79}\) give \(E_\beta = 1.25 \pm 0.03\) MeV. Fig. 13 shows the Fermi-Kurie plot \(^{79}\). The experimental points lie on the theoretical straight line up to \(E + 1 = 1.35\, m_0c^2\). The value \(E_\beta = 1.25 \pm 0.03\) MeV was obtained by extrapolating this plot. Jelley \(^{77}\), who calculated theoretically the energy of the β-spectrum of \(N_7^{13}\), obtained for it the value \(E_\beta = 1.26\) MeV.

4. Half-life

The most careful measurements of the half-life were carried out in work \(^{80}\). \(N_7^{13}\) was obtained by bombarding carbon in vacuum with deuterons of energy 900 keV at a current of 50 μA. The irradiated carbon was placed in an aluminum cassette, which absorbed positrons, at a distance of 10 cm from an ionization chamber filled with argon to a pressure of 8 atmospheres. The ionization current in the chamber, caused by annihilation radiation from \(N_7^{13}\) positrons, was measured.

The decrease in activity, observed over the course of 11 half-lives, followed the exponential law with a very high degree of accuracy. The half-life of \(N_7^{13}\) found is

\[ T_{1/2} = 9.93 \pm 0.03 \text{ min.} \]

Measurements of the half-life carried out from the decrease in the number of particles from \(N_7^{13}\) with the aid of a Geiger-Müller counter usually give somewhat higher values (for example, \(10.3 \pm 0.8\) min. \(^{81}\); \(10.13 \pm 0.10\) min. \(^{71}\)).

It is possible that this is connected with the presence of radioactive impurities, which in measurements of activity by γ-radiation have a smaller effect, since the γ-radiation of sources with positron emission is usually stronger than the γ-radiation of sources emitting electrons.

5. Mass, spin, and magnetic moment

Taking the endpoint of the β-spectrum of \(N_7^{13}\) to be \(1.24 \pm 0.02\) MeV and the rest mass of the positron as \(0.511\) MeV, and the mass of the neutrino as zero, we find for the mass difference \(N^{13} - C^{13}\):

\[ N^{13} - C^{13} = 2.261 \pm 0.02 \text{ MeV} = 0.00243 \pm 0.00003 \, MU. \]

Taking from the tables of C. Petrovich\(^{25}\) the value \(M_{\mathrm{C}^{13}} = 13.00751\) for the mass of \(\mathrm{C}^{13}_6\), we find:

\[ M_{\mathrm{N}^{13}} = 13.00994 \pm 0.00003\, MU. \]

Theoretically\(^{78}\), the ground state and the first excited level of \(\mathrm{C}^{13}\) and \(\mathrm{N}^{13}\) should be members of the doublet \({}^{2}P_{1/2,\,3/2}\), the ground state having the character \(2P_{1/2}\). If the Fermi selection rules are applicable to the \(\beta\)-decay of \(\mathrm{N}^{13}\), it follows from this that the spin of \(\mathrm{N}^{13}\) is equal to \(\frac{1}{2}\) and that the spectrum has a simple character. If the Teller rules are valid, then transition is allowed to both levels of \(\mathrm{C}^{13}\), regardless of whether the spin of \(\mathrm{N}^{13}\) is one half or three halves. Accordingly, the spectrum should have a complex character and consist of two components. As has already been pointed out, the experimental investigations of the character of the \(\beta\)-spectrum of \(\mathrm{N}^{13}\) are contradictory.

However, relying on the most recent works, one may consider that the \(\beta\)-spectrum of \(\mathrm{N}^{13}\) consists of one component. In that case its spin should be equal to \(\frac{1}{2}\). The magnetic moment of \(\mathrm{N}^{13}\) has not been measured up to the present time.

III. ISOTOPE \(\mathrm{N}^{16}_{7}\). \(T_{1/2} = 7.3\) sec.

At the end of 1933, when a gas in a Wilson chamber (\(30\%\ \mathrm{CCl}_2\mathrm{F}_2\) and \(70\%\ \mathrm{He}\)) was irradiated with fast neutrons, in 10 out of 3200 photographs taken typical pictures of a nuclear reaction were found\(^{82}\). The authors proposed the following scheme for the reaction taking place here:

\[ \mathrm{F}^{19}_{9} + n^{1}_{0} = \mathrm{N}^{16}_{7} + \mathrm{He}^{4}_{2}, \tag{10} \]

and expressed the supposition that the nitrogen isotope \(\mathrm{N}^{16}_{7}\) formed in the reaction may be unstable and, emitting an electron, transform into the stable oxygen isotope \(\mathrm{O}^{16}_{8}\).

In 1934 it was discovered\(^{83,84}\) that, when fluorine is bombarded with neutrons, a radioactive element is formed which emits electrons and decays with a period of about 10 sec. Both groups of authors supposed that the active element is nitrogen \(\mathrm{N}^{16}_{7}\), formed according to reaction (10) and transforming after emission of an electron into the stable oxygen isotope—\(\mathrm{O}^{16}\). The same conclusion was reached in the same year by Kurchatov, Shchepkin, and Vibe\(^{85}\), who studied this reaction. They indicated that the electrons of the decay of \(\mathrm{N}^{16}\) must possess high energy. In 1936 analogous activity was found when nitrogen was bombarded with deuterons\(^{86}\), and the existence of the reaction was proposed

\[ \mathrm{N}^{15}_{7} + \mathrm{H}^{2}_{1} = \mathrm{N}^{16}_{7} + \mathrm{H}^{1}_{1}. \tag{11} \]

According to photographs taken in a Wilson chamber, the authors concluded that the upper limit of the \(\beta\)-spectrum lies near \(6\) MeV. A study of the activity obtained by reaction (10), carried out at the same time\(^ {87}\), gave for it a half-life value

\[ T_{1/2}=8.4\pm0.1\ \text{sec}. \]

When nitrogen was irradiated with neutrons from Ra + Be, capture of these neutrons by the nucleus \(N_7^{15}\) was observed\(^ {88}\), leading to the formation of \(N_7^{16}\) by the reaction

\[ N_7^{15}+n_0^1=N_7^{16}. \tag{12} \]

According to\(^ {89}\), the cross section of this reaction is \(\sigma_c<0.01\) barn.

From photographs in a Wilson chamber it was found\(^ {90}\) that the maximum energy of the electrons emitted by \(N_7^{16}\), obtained by irradiating fluorine with neutrons, is \(6.5\text{--}7.0\) MeV. The tail extending into the high-energy region was attributed by the authors to systematic errors in observing the curvature of the trajectories, which occurred, in their opinion, because of the smallness of the magnetic field (1000 oersteds). In 1937 the reaction\(^ {91}\)

\[ O_8^{16}+n_0^1=N_7^{16}+H_1^1, \tag{13} \]

was discovered, as a result of which radioactive nitrogen \(N_7^{16}\) is also formed. The authors obtained considerable activity, with a period of 8 sec, when oxygen was bombarded with neutrons from Li + \(d\) (neutron energy 14 MeV). Bombardment of oxygen with neutrons from Be + \(d\) (neutron energy 4.6 MeV) did not lead to the appearance of activity. This indicates that very fast neutrons are needed for reaction (13) to take place.

The chemical identification of the nitrogen isotope \(N_7^{16}\) was carried out in 1937 by Polessitskii\(^ {92}\) in Leningrad.

A solution of \(NH_4F\) was placed in a glass vessel 1 (Fig. 14), in which there were a plate of porous material 2 and a tube 3 containing several hundred millicuries of a Ra + Be mixture and serving as the neutron source. The radioactive product formed in 1 was carried in a stream of gas, blown in the direction indicated in the figure by an arrow, into vessel 4, in which there was a Geiger–Müller counter. If desired, various substances 5, interacting with the transported activity, could be placed between vessel 1 and the counter.

At first blowing air through, Polessitskii found that the activity from 1 was carried to the counter. When a stream of hydrogen was blown through, a trap with liquid air was placed at 5. In this case the activity continued to be transported completely. However, when the stream of hydrogen was passed through charcoal at the temperature of liquid air, a sharp decrease in the number of counts in 4 was observed.

The only gases that are adsorbed by charcoal kept at the temperature of liquid air, but pass through an empty trap with liquid air, are oxygen and nitrogen. Polesskii passed a stream of hydrogen through a platinum catalyst followed by passage through CaCl₂ in one experiment and through a trap with liquid air in another. On the platinum catalyst the oxygen should then have combined with hydrogen, forming water vapor and consequently being absorbed in both cases, whereas nitrogen, forming ammonia NH₃ upon combination with hydrogen, only in the second.

Fig. 14. Polesskii’s apparatus for studying the activity obtained by neutron bombardment of fluorine.

The data obtained by Polesskii convincingly show that the active substance formed when fluorine is irradiated (here in the form of NH₄F) with neutrons is the nitrogen isotope \(N^{16}_{7}\), formed by reaction (10).

Bete and Livingston\(^{93}\), using data\(^{94}\) from measurements (by means of photographs in a Wilson chamber) of the energy of reaction (10), calculated for the mass of \(N^{16}_{7}\) the value \(16.011 \pm 0.002\).

A close value of the mass of \(N^{16}_{7}\), equal to 16.0114, was obtained theoretically in\(^{95}\). In another work\(^{96}\) calculation of the mass of \(N^{16}_{7}\) led to the value \(16.0102 \pm 0.0007\) MU. These data indicate that the decay energy of \(N^{16}_{7}\) should be of the order of 10 MeV, whereas direct measurements of the upper limit of the β-spectrum gave for it values of about 6 MeV. It should be noted that, as will be seen below, this erroneous value of the β-spectrum limit is given in the tables of Seaborg and Mattauch. In 1946 it was shown\(^{97,98,99}\) that the β-spectrum of \(N^{16}_{7}\) is complex and consists of several components, the electron energy of the hardest component being approximately 10 MeV. In work\(^{97}\)

oxygen (in the form of $\mathrm{Be(OH)_2}$) was irradiated with fast neutrons obtained in a cyclotron. The target was placed in a hermetic chamber of volume $350\ \mathrm{cm}^3$, connected by means of a vacuum transmission line, with an internal diameter of about $2\ \mathrm{mm}$, to a receiving chamber located in the cyclotron control room at a distance of $40\ \mathrm{m}$ from it. Because of the strong background from $\gamma$-radiation during operation of the cyclotron, the latter had to be switched off for the duration of the measurements. Consequently, it was impossible to transfer the activity in a continuous gas stream into the receiving chamber, since the lifetime of $\mathrm{N}^{16}$ is very short.

The chamber in which the sample was located was filled with hydrogen at a pressure of $2\ \mathrm{atm}$. The transmission line and the receiving chamber were then under vacuum. After the bombardment was completed, a valve was opened connecting the sample with the transmission line, and the hydrogen carried the activity into the receiving chamber. Measurements could be begun as early as $2.5\ \mathrm{sec}$ after the end of the bombardment.

Careful measurements of the half-life of the activity obtained by bombarding oxygen with neutrons gave for it the value $T_{1/2}=7.3\pm0.3\ \mathrm{sec}$. In Fig. 15 the decay curve obtained for $\mathrm{N}^{16}_7$ is shown.

Fig. 15. Decay of $\mathrm{N}^{16}_7$.

The experiments showed that in the decay of $\mathrm{N}^{16}_7$, in addition to decay electrons, there is also hard $\gamma$-radiation. The authors measured up to 125 tracks of secondary electrons in a Wilson chamber with a magnetic field. The sample was in a glass tube $6\ \mathrm{mm}$ thick. The introduction of an additional $6$-mm copper shutter did not eliminate the tracks, which indicates that they arise under the action of secondary particles. Analysis of the tracks indicates the presence of fairly homogeneous $\gamma$-radiation with an energy of $5$–$6\ \mathrm{MeV}$. A rough statistical analysis of the number of tracks obtained on different photographs taken at time intervals of about $20\ \mathrm{sec}$ shows that the activity has the required half-life.

The absorption curve of the activity in aluminum also indicates the presence of $\gamma$-radiation with energy $\approx 6\ \mathrm{MeV}$.

From an analysis of this curve the authors conclude that the $\beta$-spectrum of $\mathrm{N}^{16}_7$ is complex and consists of two components—one

with energy 9.5 MeV and the second—with energy 3.5 MeV, the second component being approximately 3 times more intense than the first. After the β-transition belonging to the second component, the excited nucleus emits a γ-quantum with energy 6 MeV.

The authors also measured the threshold of reaction (13) and found that the neutrons participating in the reaction have a threshold energy somewhat less than 12.6 MeV. In this case, for the mass difference \({}_{7}\mathrm{N}^{16} - {}_{8}\mathrm{O}^{16}\) one obtains approximately the value \(\lesssim 12.6\) MeV.

The height of the potential barrier of nitrogen for emission of the proton leaving in reaction (13) is equal to

\[ E=\frac{Ze^{3}}{r_{0}A^{\frac{1}{3}}}. \]

Thus, in order for the probability of overcoming the internal potential barrier of \(\mathrm{O}^{16}\) to be large, the proton in the nucleus must have an energy of the order of 2.5–3 MeV. These considerations make it possible to conclude that the mass difference of \({}_{7}\mathrm{N}^{16}\) and \({}_{8}\mathrm{O}^{16}\) should be approximately 10 MeV.

The results of measurements in works \(^{98}\) and \(^{99}\) agree, on the whole, with the results described. In these experiments \({}_{7}\mathrm{N}^{16}\) was obtained by irradiating oxygen (in the form of \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{H}_{3}\mathrm{BO}_{3}\)) with fast neutrons. From the decay of the activity of irradiated cylinders of \(\mathrm{H}_{3}\mathrm{BO}_{3}\), observed over eight periods, the half-life of \({}_{7}\mathrm{N}^{16}\) was determined, equal to \(T_{1/2}=7.35\pm0.05\) sec. (In \(^{97}\) the value \(7.3\pm0.3\) sec is given.) This value is apparently the most accurate.

The absorption curve obtained by the authors for the activity they investigated is shown in Fig. 16 (curve 1). From it one can see the presence of hard γ-radiation emitted by the sample. The points on curve 2 were obtained by subtracting this radiation from curve 1. Curve 2 indicates the complex character of the β-spectrum and can be resolved into two components. One of them—the hard component (\(\simeq 18\%\) of the total intensity), corresponding to the transition to the ground level of \(\mathrm{O}^{16}\), has a maximum energy of approximately 10.5 MeV. The second—the soft component—has a limit in the region of 4 MeV. Since the β-transitions belonging to the soft component must be accompanied by γ-radiation, the electron distribution curve of this component can be obtained by observing the absorption of \((\beta-\gamma)\)-coincidences. The curve obtained by the authors for the decrease in the number of \((\beta-\gamma)\)-coincidences as a function of the thickness of the aluminum layer placed between the Geiger counters (Fig. 17) agrees very well with the absorption curve of the soft component (curve 5, Fig. 16).

The energy of the $\gamma$-rays emitted in the decay of $\mathrm{N}^{16}_{7}$ was measured from the absorption in aluminum of the secondary electrons arising under the action of these rays. The energy of the $\gamma$-rays was also measured from the curvature of the trajectories of secondary electrons photographed by means of a Wilson chamber placed in a magnetic field.

Analysis of the results of these measurements indicates the presence of $\gamma$-quanta with an energy of $6.2$ Mev (the excited level of $\mathrm{O}^{16}$, known from the reaction $\mathrm{F}^{19}(p,\alpha)\mathrm{O}^{16}$, see $^{10}$) and quanta with an energy of $6.7$ Mev.

Fig. 16. Absorption curve of electrons emitted by the radioisotope $\mathrm{N}^{16}_{7}$. 1 — total activity ($\beta+\gamma$); 2 — $\beta$-activity; 3 — $\gamma$-activity; 4 — hard component of the $\beta$-spectrum ($E_\beta=10.5$ Mev); 5 — soft component of the $\beta$-spectrum ($E_\beta=4$ Mev).

Fig. 17. Curve of the decrease in the number of coincidences when electrons pass through a layer of aluminum. 1 — coincidences plus background; 2 — background; 3 — coincidences without background.

There is also some probability of the existence of $\gamma$-radiation with an energy of $5.1$ Mev.

To check the results obtained, the authors investigated the energy of the decay electrons of $\mathrm{N}^{16}_{7}$ by photographing their trajectories in a Wilson chamber with a magnetic field. In these experiments, neutron-irradiated water flowed through a copper tube (wall thickness $d=0.1$ mm) into the expansion space of the Wilson chamber and was led outside. In this way the decay electrons entered the Wilson chamber without substantial loss of energy. In order to obtain reliable results, both at large and at small energies of the decay electrons, the measurements

were carried out at two different values of the magnetic field: \(H=500\) oersted and \(H=1000\) oersted. In Fig. 18 is shown the \(\beta\)-spectrum of \(N^{16}_{7}\), taken with a Wilson chamber and corrected for absorption of electrons in the layer of water and in copper, as well as for the geometry of the apparatus. From it one sees the presence of a component of high energy, for which extrapolation of the Kurie–Fermi graph gives a limiting energy of \(10\pm 1\) MeV. Its intensity is \(22\pm 5\%\). For the remaining part of the spectrum, extrapolation gives a value of the maximum energy of approximately \(4.6\) MeV.

In the Wilson photographs a certain number of positron tracks was also found. It is known\(^{100}\) that oxygen \(O^{16}\) has an excited level at \(6.0\pm 0.2\) MeV, the energy of which is released not in the form of a \(\gamma\)-quantum, but in the form of an electron–positron pair. The smallness of the number of positrons found in the photographs indicates that the transition to this level is forbidden.

Comparing the results of their measurements, the authors come to the conclusion that the decay energy of \(N^{16}\) is \(Q=10.3\pm 0.7\) MeV.

Table II gives data characterizing the radioactive decay of \(N^{16}_{7}\). (In parentheses are given the values obtained if it is assumed that the above-mentioned level at \(5.1\) MeV exists.)

Table II

Characteristics of the decay of \(N^{16}_{7}\)

Maximum energy of the spectrum component (in MeV)	Intensity (in %)	Character of the transition	Excitation energy	Dennison type
10.5 (10.1)	20 (20)	Forbidden	0	\(0^{+}\)
— (5.0)	(15)		5.1	\(3^{-}\)
4.5 (4.1)	2	Forbidden	6.0	\(0^{+}\)
4.3 (3.9)	40 (25)	Allowed	6.2	\(2^{-}\)
3.8 (3.4)	40 (40)	Allowed	6.7	1

It is known that the ground state of \(O^{16}\) is even and has spin equal to zero (\(0^{+}\) in the table). From the selection rules for \(\beta\)-transition\(^{78}\) we find that the ground state of \(N^{16}_{7}\) is odd and has spin, if Dennison’s scheme\(^{101}\) is adopted, 1 or 2.

The energy of the reaction \(F^{19}(n,\alpha)N^{16}\) was investigated in 1947.\(^{102}\) An ionization chamber, filled once with \(CF_{4}\) and the other time (for control) with \(SiF_{4}\), was irradiated with monochromatic neutrons of energy \(3.68\) MeV. The pulses arising in the chamber were amplified

linear amplifier and were recorded with an oscillograph. Knowing the amount of energy expended by the $\alpha$-particle, produced in the reaction $\mathrm{F}^{19}(n,\alpha)\mathrm{N}^{16}$, on the formation of one ion pair in the corresponding gas, and the magnitude of the pulse in the chamber, it was possible to determine the energy of the $\alpha$-particles. Hence the reaction energy was found to be $Q=-0.73\pm0.25$ MeV. In this case the mass of $\mathrm{N}^{16}_{7}$ is obtained as

Fig. 18. $\beta$-spectrum of $\mathrm{N}^{16}_{7}$, taken with a Wilson chamber with a magnetic field. 1—$H=1000$ oersted; 2—$H=500$ oersted.

equal to $M_{\mathrm{N}^{16}}=16.0104\pm0.00029$, and the maximum energy of the decay electrons is $E_{\beta}=9.68\pm0.28$ MeV.

In 1949, determinations of the reaction energy $\mathrm{N}^{15}(d,p)\mathrm{N}^{16}$ were carried out$^{124,125}$. Ammonia, whose nitrogen had been enriched to 61.5% with the isotope $\mathrm{N}^{15}$, was bombarded with deuterons, and the distribution of protons by energy was studied. The reaction energy proved to be $0.23\pm0.15$ MeV. For the mass of $\mathrm{N}^{16}$ this gives the value $16.01121\pm0.00023$.

On the basis of the works set out above one may draw the following conclusions: the radioactive isotope of nitrogen $\mathrm{N}^{16}_{7}$ is produced as a result of reactions 10, 11, 12, and 13. At present the principal method for obtaining $\mathrm{N}^{16}$ is the bombardment of oxygen with fast ($E>12$ MeV) neutrons, i.e. reaction (13). If the available source gives neutrons of lower energy, reaction (10) is used. The half-life of nitrogen $\mathrm{N}^{16}_{7}$ obtained in reactions (10)—(13) is $T_{1/2}=7.35$ sec. The $\beta$-spectrum obtained in the decay of nitrogen $\mathrm{N}^{16}$ is complex. Its hard component has a very high energy, equal to

$$ E_{\beta}=10.2\pm0.6\ \text{MeV}. $$

Computed from the energy of the β-spectrum and the mass of \(O_8^{16}\), the mass of the atom \(N_7^{16}\) turns out to be

\[ M_{N^{16}}=16.011\pm 0.0008\,MU. \]

The soft component of the β-spectrum has an energy equal to \(E_{\beta}'=4.3\,Mev\), and it is possible that it, in turn, consists of several components. The ground state of \(N_7^{16}\) is odd. Its spin must be an integer different from zero.

IV. ISOTOPE \(N_7^{17}\). \(T_{1/2}=4.1\) sec.

At the end of 1948, brief reports appeared on the discovery of artificial radioactivity of a new type, similar to the previously observed natural radioactivity of products arising as a result of the fission of heavy nuclei \(^{103}\). Here, following the β-decay of the initial product, the excited nucleus emits a neutron:

\[ M_{Z-1}^{A+1}=M_Z^A+n+\beta^-+\nu. \]

The neutrons formed in this process are not monochromatic, which would correspond to the existence of a single component of the β-spectrum, but apparently have a continuous spectrum extending approximately up to \(2\,Mev\). This type of radioactivity was obtained by bombarding oxygen and a number of other elements with deuterons of energy \(195\,Mev\). After the bombardment ended, the neutron yield did not cease immediately, but decreased exponentially with a period of about four seconds. With the aid of a proportional counter filled with \(BI_3\), the half-life of the neutron activity obtained upon deuteron bombardment of all elements—from oxygen through potassium inclusive—was measured. It turned out that this period is the same for all these elements and is equal to

\[ T_{1/2}=4.14\pm 0.04\ \text{sec}. \]

Joint observations of the intensity of the γ-radiation and of the yield of these “delayed” neutrons showed that this phenomenon cannot be explained on the assumption of the presence of any \((\gamma,n)\)-reaction.

Experiments showed \(^{104,105,106}\) that the element possessing such complex \((n\text{-}\beta)\)-radioactivity is the nitrogen isotope \(N_7^{17}\).

The formation of \(N^{17}\) was also observed \(^{126}\) when carbon was bombarded by α-particles with energy \(>30\,Mev\):

\[ C_6^{14}+He_2^4=N_7^{17}+H_1^1+Q. \]

The threshold of this reaction is less than \(16\,Mev\), and the cross section for α-particles with energy \(28\,Mev\) is \(0.06\) barn. Interesting data

relative to the production of \(N^{17}\) upon bombardment of oxygen isotopes with fast neutrons are given in another paper\(^{127}\). The authors bombarded with neutrons water containing \(O^{16}\), \(O^{17}\), and \(O^{18}\) in their natural ratio. A yield of delayed neutrons with a period \(\simeq 4.5\) sec was found, which indicated the formation of \(N^{17}\) by the reaction

\[ O^{17}_{8}+n^{1}_{0}=N^{17}_{7}+H^{1}_{1}+Q . \]

The energy of this reaction is approximately \(8\) Mev. The reaction cross section is, in order of magnitude, \(10^{-2}\) barn.

The features of \(N^{17}\) noted above make it a very convenient indicator for investigating a number of nuclear reactions occurring under bombardment by very fast particles, since it is the only one of the light elements that emits delayed neutrons. This circumstance makes it easy to distinguish it from other elements. The yield of delayed neutrons upon bombardment of a number of elements by very fast deuterons was studied in paper\(^{107}\). The deuteron beam passed through a thin lithium fluoride crystal, which served to monitor the irradiation dose received by the sample, and struck a thin sample of known surface density. After a 30-second irradiation the sample was placed inside a chamber filled with \(BF_{3}\), and the LiF crystal next to a filled \(BF_{3}\) counter; for thirty seconds simultaneous measurements were made of the activity of the sample and of the crystal. To reduce the background, the chamber and the counter were shielded by a thick layer of paraffin. Measurements of two kinds were carried out: 1) comparison of the relative yield per atom for various elements, and 2) recording excitation curves of the activity for deuterons of various energies. The change in deuteron energy was achieved by slowing them in a layer of copper placed between the lithium fluoride crystal and the sample. As can be seen from the table given below (Table III), the relative yield decreases approximately exponentially with increasing number of particles emitted from the nucleus.

Table III

Relative yield of \(N^{17}\) upon bombardment of light elements with deuterons

O	F	Na	Mg	Al	Si	P	S	Cl	K
100 000	8400	2600	840	840	230	230	70	78	25

From the table it is seen that the yield of \(N^{17}\) upon bombardment of oxygen with deuterons, apart from \(O^{16}\), exceeds the yield in potassium by 4000 times.

This indicates that, when oxygen is bombarded, the yield of \(N^{17}\) is very large. Measurements also show\({}^{107}\) that, for the given element, the yield of \(N^{17}\) increases monotonically with energy. The reaction threshold increases with increasing nuclear charge.

In work\({}^{108}\) rather rough measurements were presented of the spectrum of neutrons formed in the decay of \(N^{17}\). The measurements were made from recoil protons in a Wilson chamber filled with hydrogen. From these measurements it follows that the energy distribution of the neutrons has a maximum at \(E_n = 1\) MeV. Its width is \(\Delta E_n = 0.6\) MeV.

At one time, upon the discovery of neutron radioactivity among certain fragments of the fission of heavy nuclei, Bohr and Wheeler\({}^{103}\) put forward the hypothesis that the emission of a neutron from the nucleus occurs after the usual \(\beta\)-decay. The strongly excited nucleus remaining after \(\beta\)-decay very rapidly, in a time of the order of \(10^{-13}\)—\(10^{-15}\) sec, gives up its excitation energy, but not in the form of a \(\gamma\)-quantum, rather in the form of a neutron. Naturally, with such lifetimes of the excited nucleus this process will be recorded as a simultaneous \((n-\beta)\)-decay.

Fig. 19. Neutron spectrum in the decay of \(N_7^{17}\).

In the case of \(N^{17}\) this means that after \(\beta\)-decay an excited nucleus \(O^{17}\) must be formed, in turn transforming into \(O^{16} + n\). In this process the \(O^{16}\) nucleus and the neutron must have equal and oppositely directed momenta. Consequently, the neutrons will have an energy 16 times greater than that of the \(O^{16}\) nuclei.

This provides a very convenient possibility for measuring the spectrum of delayed neutrons from \(N^{17}\) by means of the recoil nuclei \(O^{16}\). A solution of \(NH_4F\) was irradiated\({}^{106}\) with deuterons of energy 190 MeV. The activity obtained as a result of the bombardment was carried in a stream of helium, flowing at atmospheric pressure, through a proportional Geiger counter. The pulses arising in the counter with a period of 4.1 sec exceeded the background pulses from \(\beta\)- and \(\gamma\)-radiation, which made it possible to carry out measurements continuously, with the cyclotron operating. In addition, the system operated on \((\beta-\gamma)\)-coincidences, which also reduced the number of false counts. The neutron spectrum obtained in this way is shown in Fig. 19.

As the author points out, in reality the width of the resonance curve should be still narrower.

Measurements by means of coincidences made it possible, in the presence of a very strong background of \(\beta\)-rays from \(\mathrm{N}^{17}\), to isolate those corresponding to the transition to the excited level of \(\mathrm{O}^{17}\). Their maximum energy, determined from absorption in a layer of material, proved to be \(3.7 \pm 0.2\) MeV. Preliminary measurements also show that the sum of the maximum energy of the \(\beta\)-transition and the neutron energy in each decay event remains constant, i.e. that the fastest neutrons are accompanied by the softest \(\beta\)-radiation, and conversely. This interesting question, however, is at present far from complete clarity. It is probable that, in addition to the \(\beta\)-transition to the excited level of \(\mathrm{O}^{17}\), there is also a component of the \(\beta\)-spectrum corresponding to the transition to the ground level of \(\mathrm{O}^{17}\). According to\(^ {106}\) its intensity should amount to less than 5% of the intensity of the soft component.

Therefore \(\beta\)-transitions to the ground level, not being accompanied by neutron coincidences, are indistinguishable because of the strong background from \(\mathrm{N}^{16}_{7}\), which is usually formed together with \(\mathrm{N}^{17}_{7}\). As is not difficult to calculate from the atomic masses and the energies of the \(\beta\)-particles and neutrons, the maximum energy of the transition to the ground level of \(\mathrm{O}^{17}\) should be

\[ E_{\beta}=8.7\ \text{MeV}. \]

In conclusion of this part of the survey we give a table of the main characteristics of the nitrogen isotopes.

Table IV

Characteristics of nitrogen isotopes

Isotope	%	Mass	Spin	Decay character	\(E_{\beta}\) in MeV	\(T_{1/2}\)
\(\mathrm{N}^{12}\)	—	\(12.0228 \pm 0.00015\)	0	\(\pi^+\)	16.6	0.0125 sec.
\(\mathrm{N}^{13}\)	—	\(13.0099 \pm 0.00003\)	\(1/3\)	\(\beta^+\)	\(1.24 \pm 0.02\)	9.93 min.
\(\mathrm{N}^{14}\)	99.6	14.00751	1
\(\mathrm{N}^{15}\)	0.4	15.00489	\(1/2\)
\(\mathrm{N}^{16}\)	—	\(16.0111 \pm 0.0008\)	integer distinct from zero	\(\beta^-\)	\(10.2 \pm 0.6\)	7.3 sec.
\(\mathrm{N}^{17}\)	—	17.0138		\(\beta^-\), \(n\)	8.7; 2.0 (for \(n\))	4.1 sec.

V. APPLICATION OF \(N_7^{13}\) IN CHEMISTRY AND BIOLOGY

The application of radioactive and stable isotopes to the solution of a number of scientific and scientific-technical problems has in recent years crystallized into a large independent field of research. Many hundreds of papers, a number of monographs, and review articles have been devoted to this question \(^{1a-114}\).

All four radioactive isotopes of nitrogen have relatively short half-lives, which makes it difficult to use them as labeled atoms. Therefore, in the majority of work with labeled nitrogen the stable isotope of the latter has been used, namely \(N_7^{15}\). However, the radioactive isotope \(N_7^{13}\), with a half-life \(T_{1/2}=9.9\) min, has also been used in a number of chemical and biological studies.

Recently \(N_7^{13}\) was used to confirm the correctness of a hypothetical mechanism of the thermal decomposition of nitrogen pentoxide \(^{115}\). The result of kinetic experiments can be explained by assuming the following decomposition scheme:

\[ \mathrm{N_2O_5} \xrightarrow{k_1} \mathrm{NO_3} + \mathrm{NO_2}, \]

\[ \mathrm{NO_3} + \mathrm{NO_2} \xrightarrow{k_2} \mathrm{N_2O_5}, \]

\[ \mathrm{NO_3} + \mathrm{NO_2} \xrightarrow{k_3} \mathrm{NO_2} + \mathrm{NO} + \mathrm{O_2}, \]

\[ \mathrm{NO} + \mathrm{N_2O_5} \xrightarrow[\;k_3 \ll k_2\;]{\text{rapid}} 3\mathrm{NO_2}. \]

If the constants \(k_1\) and \(k_2\) have comparatively large values, then, if the scheme is valid, rapid nitrogen exchange between the molecules \(\mathrm{N_2O_5}\) and \(\mathrm{NO_2}\) should be observed.

To test this conclusion, the radioactive isotope of nitrogen \(N_7^{13}\), obtained by bombarding a graphite disk with deuterons in a cyclotron, was used. The disk was burned in dry oxygen, the products being condensed at the temperature of liquid air in the presence of \(\mathrm{NO_2}\) as carrier. These products were then dissolved in carbon tetrachloride, into which the activity passed almost quantitatively. Nitrogen pentoxide was prepared in an all-glass apparatus by oxidation of \(\mathrm{NO_2}\) with ozone. The resulting \(\mathrm{N_2O_5}\) was likewise dissolved in \(\mathrm{CCl_4}\). To reduce thermal decomposition to a minimum, the experiments were carried out at a temperature of about \(10^\circ\mathrm{C}\).

Solutions of \(\mathrm{N_2O_5}\) and \(N^{13}\mathrm{O_2}\) were mixed; after a short time an aqueous solution of \(\mathrm{Ba(OH)_2}\) was added, whereby \(\mathrm{N_2O_5}\) passed into the sparingly soluble precipitate \(\mathrm{Ba(NO_3)_2}\). After removal of this precipitate, the excess \(\mathrm{Ba(OH)_2}\) was precipitated with carbon dioxide, and \(\mathrm{BaCO_3}\) was filtered off. To determine the amount of \(N^{13}\mathrm{O_2}\),

remaining in the solution; equimolecular amounts of \(AgNO_3\) and \(NaNO_2\) were added to the latter. A precipitate was formed, containing \(AgN^{13}O_2\). This precipitate was filtered off, washed, and its activity was determined with a Geiger–Müller counter.

In the control experiments, solutions of \(N_2O_5\) were first extracted with aqueous \(Ba(OH)_2\), and only then was the active solution of \(N^{13}O_2\) added. All operations were carried out rapidly, with the calculation that the duration of the entire experiment should be no more than 1 hour.

If the presumed exchange was present, the activity of the \(AgNO_2\) precipitates in the main experiments should be lower than in the control experiments. The experiments made it possible to draw an unambiguous conclusion concerning the presence of nitrogen exchange between \(N_2O_5\) and \(NO_2\), which at \(10^\circ C\) proceeds comparatively rapidly. For solutions of 0.1 N \(N_2O_5\) and 0.05 N \(N^{13}O_2\), the half-conversion time proved to be approximately 2 minutes. For comparison, we note that the half-conversion time for the thermal decomposition reaction of \(N_2O_5\) at \(10^\circ C\) is 60 hours.

The radioactivity of nitrogen was also used for rapid determination of the carbon content in iron \(^{116}\). The iron samples to be analyzed were irradiated with a beam of fast protons or deuterons, and \(N^{13}_7\) was obtained by reactions (4)—(6).

The surface layer of iron activated by protons had a thickness of the order of \(10^{-3}\) mm. This layer practically did not absorb positrons from the decay of \(N^{13}_7\). The irradiated samples were measured on a Geiger–Müller counter, and their activity was compared with the activity of an SiC standard with a known carbon content. This method makes it possible to determine hundredths of a percent of carbon in iron within 5–10 min.

Already in 1922, some authors expressed the supposition that barley can fix small amounts of free nitrogen \(^{117}\).

For testing this hypothesis, the \(N^{13}_7\) isotope was used \(^{118}\). Barley with cut roots, in the presence of control specimens killed by boiling in water, was exposed for 20 min. in an atmosphere containing \(N^{13}_7\).

Table V

Fixation of free nitrogen by barley

	Duration of exposure in an atmosphere containing \(N^{13}_7\)	Activity (pulses per minute)
Living barley . . .	20 min.	\(200 \pm 4\)
Killed barley . .	20 min.	\(2 \pm 2\)

Special measures were taken to remove \(CN\), \(NH_3\), \(NO\), and other nitrogen compounds. After the end of the exposure, the plants were treated with 80% ethyl

stratum, and the extracted active bound nitrogen was determined with the aid of a counter. It turned out that the live barley contained compounds of the radioactive isotope of nitrogen with a half-life of 10 min. The results of one of the experiments are given in Table V.

As can be seen from the table, the live barley assimilated appreciable amounts of free nitrogen (about 0.01 ml), whereas the killed barley, within the limits of error, contains no activity at all and, consequently, does not fix nitrogen.

\(N^{13}_{7}\) was also used in studies of gas exchange\({}^{119}\) and for measuring the rate of blood flow in humans\({}^{120}\).

In work\({}^{120}\), people inhaled air containing radioactive nitrogen \(N^{13}_{7}\), and the rate at which the latter entered the bloodstream from the lungs was determined from the \(\gamma\)-radiation by means of external Geiger–Müller counters applied to the chest, thigh, and fingers.

The use of radioactive isotopes of nitrogen in chemical and biological experiments is hardly likely to have broad prospects, since they have been practically displaced by the stable isotope \(N^{15}_{7}\). However, for solving a number of physical problems (for example, for detecting the neutrino), short-lived radioactive isotopes of nitrogen will probably find wide application.

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