EMISSION OF DELAYED NEUTRONS BY THE ISOTOPE N$^{17}$

In the fission of uranium and plutonium nuclei, along with the emission of prompt neutrons, the emission of delayed neutrons was already discovered in the first experiments. In accordance with the hypothesis of Bohr and Wheeler¹, the emission of delayed neutrons is associated with preceding $\beta$-decay, whose period determines the period of the neutron activity. Neutron emission will occur if the excitation energy of the nucleus formed as a result of $\beta$-decay proves to be greater than the binding energy of the neutron in the nucleus.

Among the fission fragments of \(U^{235}\), 6 different periods associated with the emission of delayed neutrons were found\(^2\). It also proved possible to isolate chemically 56-second and 22-second activities and to identify them with \(Br^{87}\) and \(I^{137}\)\(^3\), thereby confirming the Bohr–Wheeler hypothesis.

In 1948, Lawrence and co-workers\(^4\) irradiated various targets with deuterons of energy 195 MeV in the 184-inch cyclotron. After irradiation the targets were sent to a proportional boron counter for recording delayed neutrons. Delayed neutrons with a period of \(4.14 \pm 0.04\) sec were found when targets containing oxygen or neighboring elements were irradiated. It was shown that the isotope responsible for this activity has \(Z \leq 8\) and \(A - Z = 10\). Alvarez\(^5\) identified the above-mentioned period with the isotope nitrogen \(N^{17}\).

The delayed neutrons found in targets of elements with large \(Z\) (Ce, Gd, Pt, Au, Hg, Tl, Pb, and Bi) apparently arose from fission fragments of these nuclei.

Goward\(^6\) investigated the energy distribution of delayed neutrons of \(N^{17}\). After irradiation with deuterons in the cyclotron, an LiF crystal was placed in a Wilson chamber filled with hydrogen. From the tracks of recoil protons the energy of the neutrons emerging from the crystal was determined. The energy distribution of the delayed neutrons, obtained by processing 391 tracks, is shown in Fig. 1. The maximum in the neutron distribution lies near an energy of 1 MeV. The width of the proton spectrum is 0.6 MeV. The author points to two possible explanations for the diffuse character of the neutron spectrum: either there are several closely spaced levels of the excited \(O^{17*}\) nucleus, or the level of this nucleus is broad.

Vertical axis: Intensity
Horizontal axis: Neutron energy in MeV

Fig. 1

The most thorough work on the question under review is that of Alvarez\(^7\). A solution of \(NH_4F\) was irradiated with deuterons from Lawrence’s cyclotron. The gaseous products of the nuclear reaction were carried out of the solution by a stream of helium. Delayed neutrons with a period of 4.2 sec were then detected. The neutron activity did not change when the helium was passed through absorbers of CO, \(CO_2\), and O. When the helium was cooled to a temperature below \(90^\circ K\), the delayed neutrons disappeared. The totality of these experiments, as well as the results of work\(^4\), made it possible to conclude that the delayed neutrons are associated with the \(\beta\)-decay of \(N^{17}\).

In subsequent experiments the helium was passed through a proportional counter in order to record the recoil pulses of the \(O^{16}\) nucleus. Thus, if a neutron of energy 1 MeV is emitted, the \(O^{16}\) nucleus acquires an energy of 60 keV, which, taking into account the gas amplification coefficient of the proportional counter (50–200), can readily be detected. In more refined exper-

Thus coincidences of pulses in the proportional counter with pulses in the β-counters surrounding the first (Fig. 2) were measured, i.e., (β—n)-coincidences. It was thereby established, with an accuracy up to 10%, that every β-decay of N¹⁷ corresponds to the emission of a neutron. The neutron spectrum obtained in the Alvarez experiments (Fig. 3) has a characteristic maximum at an energy of \(920 \pm 70\) keV and a half-width of \(0.6\) MeV. The region of low energies was not recorded because of the background in the proportional counter from β- and γ-radiation.

Fig. 2.

The broadened spectrum of neutrons (Figs. 1 and 3) is partly explained by instrumental causes and therefore gives only an upper limit of the true width of the level O¹⁷*.

The absorption curve of β-electrons from N¹⁷ was measured from the decrease in the number of (β—n)-coincidences with increasing thickness of aluminum surrounding the proportional counter (Fig. 2). By this absorption method the upper boundary of the β-spectrum was found to be \(3.7 \pm 0.2\) MeV. Gamma radiation was not detected.

The decay scheme of N¹⁷ is presented in Fig. 4. A β-transition of N¹⁷ directly to the ground level of O¹⁷ is possible, but such a transition will be forbidden and may occur in approximately 0.05 of the cases.

Fig. 3.

Experimentally, the transitions \( \mathrm{N}^{17} \xrightarrow{\beta} \mathrm{O}^{17} \) were not observed. The masses of O¹⁶ and O¹⁷ are well known, and from Fig. 4 it follows that the mass of N¹⁷ is equal to 17.01385 m.e.

In the experiments described, deuterons with an energy of 195 MeV were used to obtain N¹⁷. However, this isotope can be obtained using beams of particles of lower energy. Thus it was found⁸ that the threshold

reaction \( \mathrm{C}^{14}(\alpha,p)\mathrm{N}^{17} \) is equal to 16 MeV. At an \(\alpha\)-particle energy of 28 MeV, the formation cross section of \(\mathrm{N}^{17}\) is equal to 0.06 barn.

Subsequently, the formation of \(\mathrm{N}^{17}\) from oxygen was studied.^9 The reaction thresholds calculated from the isotope masses have the following values:
\(\mathrm{O}^{17}(n,p)\mathrm{N}^{17}\)—7.9 MeV,
\(\mathrm{O}^{18}(n,d)\mathrm{N}^{17}\)—13.7 MeV,
and \(\mathrm{O}^{18}(\gamma,p)\mathrm{N}^{17}\)—15.9 MeV.
It should be taken into account here that the natural mixture of oxygen isotopes contains \(\mathrm{O}^{17}\)—0.039%, and \(\mathrm{O}^{18}\)—0.204%. The neutrons used to irradiate 500 g of water were obtained by bombarding thick LiF and C targets with deuterons of energy 14 MeV. The formation of \(\mathrm{N}^{17}\) was detected from delayed neutrons. In the case of the more energetic neutrons from the LiF target, 15 times more delayed neutrons were observed than from the C target. This confirmed the calculated reaction thresholds, since in the first case the reaction occurred on \(\mathrm{O}^{17}\) and on \(\mathrm{O}^{18}\), while in the second case only on \(\mathrm{O}^{17}\). The reaction cross section is of the order of \(10^{-26}\ \mathrm{cm}^2\).

Fig. 4.

I. Estulin

References Cited

1. N. Bohr and J. A. Wheeler, Phys. Rev. 56, 426 (1939).
2. D. J. Hughes, J. Daltz, A. Cahn and D. Hall, Phys. Rev. 73, 111 (1948). N. Sugarman, J. Chem. Phys. 15, 644 (1947).
3. A. H. Snell, J. S. Levinger, E. P. Meiners, M. B. Sampson and R. C. Willienson, Phys. Rev. 72, 545 (1947).
4. N. Knable, E. O. Lawrence, C. E. Leith, B. J. Moyer and R. L. Thornton, Phys. Rev. 74, 1217 (1948).
5. L. W. Alvarez, Phys. Rev. 74, 1217 (1948).
6. E. Hayward, Phys. Rev. 75, 917 (1949).
7. L. W. Alvarez, Phys. Rev. 75, 1127 (1949).
8. K. H. Sun, B. Jennings, W. E. Shouff, Phys. Rev. 75, 1302 (1949).
9. K. A. Charpie, K. H. Sun and B. Jennings, Phys. Rev. 76, 1255 (1949).