The PRODUCTION of ARTIFICIAL RADIOACTIVITY FRANK BRESCIA
AND
ROBERT ROSENTHAL
The College of the City of New York
T
HE PHENOMENON of spontaneous disintegration or radioactivity has been known since 1896, when it was accidentally discovered by Becquerel. All attempts to alter the velocity of the disintegration process have been unsuccessful. It is entirely unaffected by temperature changes of as much as 1000°C., whereas it is empirically known that the velocity of most chemical reactions is doubled or tripled by an increment of 10'. Similarly, extremes of pressure are without effect. Very recently, however, artificial radioactivity has been produced. This was accomplished by Professor F. Joliot and his wife, Mme. I r h e Curie-Jo1iot.l They irradiated an aluminum foil with alpha particles and found that the emission of positrons from the foil continued even after the source of the radiation was removed. Boron and magnesium acted in a similar manner. The half-lives of the activities were: aluminum-3'/4 min., boron-14 min., magne~ium-Zl/~ min. Negative results were obtained with the elements H, Li, Be, C, N, 0 , F, Na, Ca, Ni, Ag. These results have been verified in various other laboratories. Lauritsen and Crane,% a t the California Institute of Technology, bombarded targets of LiF, Be, Bz03, C, Mg, and A1 with deutons. In the cases of carbon and boron, positive electron (positron) tracks were detected in a Wilson cloud chamber after the bombardment had been stopped. The half-lives of these activities were respectivdy 10 minutes and 20 minutes. The other substances mentioned above gave much smaller effects but it is possible that these results may have been due to a slight contamination of the targets with carbon. The same experiments were repeated using protons instead of deutons to bombard the target. Identical results were obtained, as measured by the half-lives of the activities. Cockcroft, Gilbert, and W a l t ~ n a, ~t the Cavendish Laboratory, have likewise produced artificial radioactivity by bombarding graphite with high-velocity protons. They found the half-life of the activity to be lo1/%minutes. To explain these nuclear reactions, Joliot and Curie' suggest that they consist of a disintegration with capture of the exciting particle, the emission of a neu1 CURIEAND JOLIOT, "A new type of radioactivity." Compt. rend., 198, 254 (1934). LAWRITSEN AND CRANE."Radioactivitv from C and BIOI bombarded with deutons and the conversio~of positrons to diation," Phys. Rev.. 45, 430 (1934). * Cocxcnom, GILBERT, AND WALTON, "Production of induced radioactivity by high velocity protons," Nature. 133, 328 (1934).
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tron, and the formation of a radioactive nucleus. This nucleus then disintegrates with the emission of a positron, forming finally a stable nucleus of another element. In the case of the disintegration of boron induced by alpha-particle irradiation, this may be represented as follows4: aBIO 2He4 = 7N13 oneutron', where the subscript is the atomic number; the superscript, the atomic weight. The isotope of nitrogen that is formed, being radioactive, is appropriately called "radio-nitrogen." This nucleus subsequently disintegrates as follows: 7 i i L 3 = gCI3 lpositronO
+
+
+
In the case of aluminum, the transformations may be represented by the analogous equations:
+ %He4= upa0+
uAlZ7
neutron'
+ ,positron0
eP30 = 1rSi30 For magnesium :
The general process may be represented by the following symbolic equations :
where M, N, and 0 are the three elements involved in the transformations, z is the atomic number of the parent element, and a is its atomic weight. Curie and Joliot5 announced that they had chemically identified the intermediate elements in the boron and aluminum series. They predicted that the elements radio-nitrogen, radio-silicon, and radio-phosphoms may possibly be formed in different nuclear reactions with other bombarding particles such as protons, deutons, and neutrons. Their prediction has been fulfilled by the experiments mentioned above that were carried out a t the California Institute of Technology and the Cavendish Laboratory. The reactions involving the deuton may be pictured thus:
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DABROW, "Transmufation by ~a l ~ h aDarticleS." . .Re". Sci. Indrumsnts, 5, 66 (1934). C m r ~AND JOLIOT,"Chemical separation of new radioelements emitting positive electrons," Compt. rand., 198, 559 11934).
+
+
aC12 ,Hz = 1N13 ~nentronl ,positrono 7N1a= ,Cia
+
It seems, however, that when carbon is bombarded with deutons, only about 1% of the transformations yield radio-nitrogen; while 99% give the isotope of carbon. It is believed that the latter transformation takes place according to the following scheme:
&12
+ ,Hz =
+ ,H1 + gamma rays
The reason for this statement is that the amount of neutron and positron emission is very small. of induced radioactivity by highThe speed protons may be pictured as: &IZ IHI = ,NI3 ,NLS= 6ClS l p ~ s i t r ~ n "
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