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A NEW TYPE OF RADIOACTIVITY*)
Irène Curie and F. Joliot
We have recently shown, by means of the Wilson chamber method¹, that certain light elements (beryllium, boron, aluminum) emit positive electrons when bombarded with polonium $\alpha$-rays. According to our interpretation, the emission of positive electrons by beryllium must be ascribed to the internal materialization of $\gamma$-radiation, whereas the positive electrons emitted by B and Al are electrons arising in a nuclear reaction and accompanying the emission of neutrons.
In attempts to clarify the mechanism of these phenomena, we have found the following.
*) Comptes Rendus 198, 254–256 (1934).
IRÈNE CURIE AND F. JOLIOT
The emission of positive electrons by certain light elements, irradiated with the $\alpha$-rays of polonium, persists for some more or less prolonged time after removal of the source of $\alpha$-rays; in the case of boron this time exceeds half an hour. Aluminum foil is placed at a distance of $1\ \mathrm{mm}$ from the polonium source. After irradiation for approximately 10 min, the foil is placed above a Geiger–Müller counter with a window whose thickness is $7/100\ \mathrm{mm}$ of aluminum. The foil then emits radiation whose intensity decreases exponentially with time, with a period of 3 min 15 sec. Similar results were obtained with boron and magnesium, the half-life periods being different, namely: 14 min for boron and 2 min 30 sec for magnesium.
The intensity of the radiation (immediately after irradiation with $\alpha$-rays) increases with the duration of irradiation up to a limiting value. Thus, for B, Mg, Al, we obtain initial intensities of the same order, equal to approximately 150 pulses per min in the counter, when irradiated by a polonium source of 60 millicuries.
In the case of H, Li, Be, N, O, F, Na, Ca, Ni, Ag, no effect is observed*). For some of these elements the phenomenon apparently does not occur; for others, the half-life period may be too small.
Experiments carried out by the Wilson chamber method or by the trochoid method proposed by Thibaud showed that the radiation emitted by boron and aluminum consists of positive electrons. It is possible that the radiation of magnesium also consists of positive electrons.
When an absorber of copper was placed between the counter and the irradiated foil, it was found that the greater part of the radiation is absorbed in $0.88\ \mathrm{g/cm^2}$ for Al and $0.26\ \mathrm{g/cm^2}$ for B and Mg. If one assumes the same absorption law as for negative electrons, this corresponds to an energy of $2.2 \times 10^6\ \mathrm{eV}$ for Al and $0.7 \times 10^6\ \mathrm{eV}$ for B and Mg.
When the energy of the $\alpha$-rays irradiating aluminum is decreased, the number of positive electrons decreases, but the decay period apparently does not change. When the energy is reduced to $10^6\ \mathrm{eV}$, such electrons are almost not observed.
These experiments indicate the existence of a new type of radioactivity, accompanied by the emission of positive electrons. We believe that in the case of aluminum the reaction proceeds as follows:
\[ {}^{27}_{13}\mathrm{Al} + {}^{4}_{2}\mathrm{He} = {}^{30}_{15}\mathrm{P} + {}^{1}_{0}\mathrm{n}. \]
*) Consequently, this phenomenon cannot be ascribed to contamination by the polonium source.
A NEW TYPE OF RADIOACTIVITY
254
ACADEMY OF SCIENCES
NUCLEAR PHYSICS. — A new type of radioactivity.
Note by Mme Irène Curie and M. F. Joliot, presented by M. Jean Perrin.
We have recently shown, by Wilson’s method1, that certain light elements (glucinium, boron, aluminum) emit positive electrons when they are bombarded with the alpha rays of polonium. According to our interpretation, the emission of positive electrons from Be would be due only to the internal materialization of the $\gamma$ radiation, whereas the positive electrons emitted by B and Al would be transmutation electrons accompanying the emission of neutrons.
In seeking to clarify the mechanism of these emissions, we have discovered the following phenomenon:
The emission of positive electrons by certain light elements irradiated by the alpha rays of polonium persists for a more or less long time, which may exceed half an hour in the case of boron, after removal of the source of alpha rays.
We place a sheet of aluminum at 1 cm from a polonium source. After the aluminum has been irradiated for about 10 minutes, we place it above a Geiger-Müller counter having an orifice closed by a screen of $7/100$ of a millimeter of aluminum. We observe that the sheet emits a radiation whose intensity decreases exponentially as a function of time with a period of 3 minutes 15 seconds. An analogous result is obtained with boron and magnesium, but the periods of decay are different: 14 minutes for boron and 2 minutes 30 seconds for magnesium.
The intensity of the radiation (immediately after exposure to the alpha rays) increases with the time of irradiation up to a limiting value. The initial intensities are then of the same order, for B, Mg, Al—about 150 pulses per minute in the counter when using a polonium source of 60 millicuries.
With the elements H, Li, C, Be, N, O, F, Na, Ca, Ni, Ag, no effect has been observed2. For some of these elements the phenomenon probably does not occur; for others the period of decay is perhaps too short.
The isotope \({}^{30}_{15}\mathrm{P}\) is radioactive, with a period of 3 min. 15 sec., and emits positive electrons according to the reaction
\[ {}^{30}_{15}\mathrm{P} = {}^{30}_{14}\mathrm{Si} + {}^{+}e . \]
One may imagine an analogous reaction for boron and magnesium, with the unstable nuclei being \({}^{13}_{7}\mathrm{N}\) and \({}^{27}_{14}\mathrm{Si}\). The isotopes \({}^{13}_{7}\mathrm{N}\), \({}^{27}_{14}\mathrm{Si}\), and \({}^{30}_{15}\mathrm{P}\) can exist only for a rather short time; therefore they are not observed in nature.
We consider unlikely an explanation according to which
\[ {}^{27}_{13}\mathrm{Al} + {}^{4}_{2}\mathrm{He} = {}^{30}_{14}\mathrm{Si} + {}^{1}_{1}\mathrm{H}, \]
\[ {}^{30}_{14}\mathrm{Si} = {}^{30}_{14}\mathrm{Si} + {}^{+}e + {}^{-}e; \]
since the isotope \({}^{30}_{14}\mathrm{Si}\) is excited and can, with time, become de-excited, the radiation materializes, forming an electron pair. The emission of negative electrons is not observed, and theoretically it is very unlikely that the energy difference between the electrons would be sufficient for the negative electrons not to be observed2. On the other hand, this process presupposes an extraordinary duration of the excited state, with an internal materialization coefficient equal to unity.
As a result, in the present work it has for the first time proved possible, with the aid of an external action, to induce in certain atomic nuclei a radioactivity capable of existing for a measurable time in the absence of the exciting cause.
Long-lived radioactivities analogous to those observed by us are undoubtedly possible also in the case of bombardment by other particles. Undoubtedly, identical radioactive atoms may be obtained in various nuclear reactions. For example, the nucleus \({}^{13}_{7}\mathrm{N}\), which according to our hypothesis is radioactive, could be obtained by bombarding carbon with deuterons after the emission of one neutron.