PHYSICAL EXPERIMENTS PERFORMED ON THE BETATRON AND SYNCHROTRON

The practical realization of powerful electron accelerators—the betatron and the synchrotron—has given physicists an artificial source of high-energy γ rays and has made possible a number of physical experiments with photons of high energies, which until now we could observe only in cosmic rays at significantly lower intensities than those obtained today in the betatron or synchrotron.

The study of nuclear reactions caused by photons with energies of approximately \(100\ \mathrm{MeV}\) is of interest in that such an energy is sufficient to knock several particles out of a nucleus, which leads to a great variety of nuclear reactions at these energies. Indeed, one of the first works carried out on the \(100\ \mathrm{MeV}\) betatron of the General Electric company[^1] led to the discovery, alongside the previously known reactions \((\gamma,n)\), \((\gamma,p)\), of a large number of reactions of the type \((\gamma,pn)\), \((\gamma,2n)\), \((\gamma,2p)\), \((\gamma,2pn)\), \((\gamma,p2n)\), \((\gamma,3pn)\), \((\gamma,\alpha n)\). Both in this work and in subsequent works devoted to a more detailed study of the observed reactions, the principal methods used were photography in a Wilson chamber and measurement of the radioactivity of the fission products.

Several works are devoted to studying the relative yield of various nuclear reactions as a function of the bombarded element and the energy of the γ-quanta[^2][^3][^4]. To interpret the results obtained it is necessary to know the spectral composition of the X-ray radiation. The authors of these works adopted the theoretical spectrum of bremsstrahlung. In the recently published work of Lawson[^5] it is indicated that the experimental results agree with the theoretical ones.

Chronologically the first is the work of Baldwin and Klaiber[^2] on photofission of nuclei under the action of γ-quanta from a betatron at 100 MeV. Curves were obtained for the dependence of the number of fission events (per roentgen of the intensity of the radiation) on the maximum energy of the γ-rays. In this experiment fission events were recorded with a differential ionization chamber, and the intensity of the X-ray radiation was determined with a standard “finger” Victoreen chamber surrounded by 3 mm of lead. For U and Th the yield rises rapidly up to a certain maximum energy (18 MeV for U and 20 MeV for Th), and then decreases slowly.

The authors calculated, on the basis of the data obtained, the effective cross section for photofission of U and Th, proceeding from the spectrum of bremsstrahlung in thin targets (according to Heitler). On the basis of such a spectrum and a simplified scheme for the formation of secondary particles in the lead surrounding the “finger” chamber, the efficiency of the chamber was calculated; knowledge of this efficiency is necessary for calculating the effective cross section. The calculated cross section passes through a maximum (18 MeV for U and 20 for Th) and falls practically to zero at energies \(> 30\) MeV. The maximum cross section is \(\sim 5 \cdot 10^{-26}\) for U and \(2.5 \cdot 10^{-26}\) for Th.

Bi, Pb, Tl, Au, W, and Sm were also investigated; however, fission of these elements was not observed, i.e. the photofission cross section is less than \(10^{-29}\).

Another work by the same authors[^3] is devoted to the investigation of the reactions \( \mathrm{C}^{12}(\gamma,n)\mathrm{C}^{11} \) and \( \mathrm{Cu}^{63}(\gamma,n)\mathrm{Cu}^{62} \).

The dependence of the reaction yield on the maximum γ-ray energy was determined. The intensity of the γ-radiation was measured, as in the preceding work, and the reaction yield was determined from the radioactivity of the reaction products (the samples used were a sheet of polyethylene or copper foil 0.075 mm thick) after irradiation of the samples at the center of the γ-ray beam 170 cm from the betatron. The dependence of the yield on energy has the same character as for photofission. The calculated effective cross section has a maximum at 22 MeV for Cu and 30 MeV for C and rapidly falls practically to zero. The rapid decrease of the cross section with increasing energy in these reactions and in photofission is explained by the authors as due to the appearance of competing reactions at these energies.

Perlman and Friedlander[^4] investigated a whole series of nuclear reactions, irradiating various samples with γ-rays from a betatron and a synchrotron at two different energy values (50 MeV and 100 MeV). The reaction yield was determined by measuring the radioactivity of the samples after their irradiation. The results of the experiments are summarized in the following table (the yield of the reaction \( \mathrm{N}^{14}(\gamma,n)\mathrm{N}^{13} \) is taken as 1).

The approximately identical dependence of the yields of the reactions \((\gamma,n)\) and \((\gamma,p)\) on the mass number at both energy values is noteworthy. It also supports the conclusion, made in works[^2][^5], that the effective cross section of the reactions \((\gamma,p)\) and \((\gamma,n)\) is negligibly small for high energies. The reason for the jump in the yield of the reactions \((\gamma,n)\) near \(A \simeq 60\) is not clear to the authors.

It should be noted once again that a careful investigation of γ-radiation from a betatron at 100 MeV, carried out with the aim of confirming or refuting the results of the experiments of Shein, Gardner, and Klaiber[^6], who supposedly detected the reaction \((\gamma,p)\), is of interest.

Klaiber, Lovke, and Baldwin[^7] carried out careful experiments with a Wilson chamber placed in a magnetic field of 1.2650 gauss. The chamber was arranged next to aluminum foils of increasing thickness (from 0.1 to

2 mm). The momentum and energy of the particles were measured: the momentum was determined from the curvature of the track in its initial part, and the energy from the number of the foil in which the particle came to rest. From the momentum and the energy one can determine the mass of the particle. In all cases it proved to be equal to the proton mass. Only in two cases did it prove to be less than 150 electron masses, but these cases can easily be explained by the apparent curvature of the tracks caused by multiple scattering. Experiments specially carried out by the authors\(^{8}\) with a Wilson chamber without the magnetic field switched on (on the same apparatus) confirmed this explanation: of 129 proton tracks, 46 have an apparent curvature, with 21 tracks bent in one direction and 25 in the opposite direction.

Bonner, Friedlander, and others\(^{9}\) showed the absence of the \((\gamma,\mu)\) reaction by another method. The formation of \(Zn^{63}\) and \(Mg^{27}\) upon irradiation of copper and aluminum samples can be explained either by the reactions \(Cu^{63}(\gamma,\mu)Zn^{63}\), \(Al^{27}(\gamma,\mu+)Mg^{27}\), accompanied by the formation of mesons, or by reactions involving secondary neutrons and protons: \(Cu^{63}(p,n)Zn^{63}\), \(Al^{27}(n,p)Mg^{27}\). However, the experiments unambiguously show that the latter case occurs: 1) when the energy is decreased from 100 MeV to 50 MeV, the \((\gamma,p)\) reaction should cease; the experiment shows that \(Zn^{63}\) and \(Mg^{27}\) are formed even at an energy of 50 MeV; 2) splitting the thick target into several thin ones separated by air gaps should not affect the yield of a reaction of the \((\gamma,\mu)\) type, whereas the yield of the \((n,p)\) and \((p,n)\) reactions should decrease because part of the proton or neutron range falls in air; the experiment shows a decrease in the yield when the target is split into 79 sheets by a factor of 3.5–4.

Thus, the experiments refute the data on obtaining mesons by means of \(\gamma\)-quanta with an energy of 100 MeV.

So far, only a single paper\(^{10}\) has been published in which an electron beam extracted directly from the betatron (at 22 MeV) is used.

and not bremsstrahlung \(\gamma\)-radiation. The beam was directed onto a series of thin, identical foils of \(\mathrm{Cu}^{63}\), \(\mathrm{Ag}^{107}\), and \(\mathrm{Ag}^{109}\), producing nuclear disintegrations in them. The number of disintegrations caused directly by electrons is the same in all foils, whereas for photons it decreases linearly with the foil number. This makes it possible to distinguish the two kinds of disintegration and to find their effective cross sections. For electrons with an energy of 16 MeV the effective cross sections are \(1.6\cdot10^{-28}\) for \(\mathrm{Cu}^{63}\), \(5.4\cdot10^{-28}\) for \(\mathrm{Ag}^{107}\), and \(7.9\cdot10^{-28}\) for \(\mathrm{Ag}^{109}\). For photodisintegration the cross section is 400 times larger.

E. L. Burshtein

CITED LITERATURE

1. G. K. Boldwin and G. S. Klaiber, Phys. Rev. 70, 259—270 (1948); see the abstract by E. Shpolsky, UFN 34, issue 3 (1948).
2. G. K. Boldwin and G. S. Klaiber, Phys. Rev. 71, 3—10 (1947).
3. G. K. Boldwin and G. S. Klaiber, Phys. Rev. 73, 1156—1163 (1948).
4. M. L. Perlman and G. Friedlander, Phys. Rev. 74, 442—448 (1948).
5. J. L. Lawson, Gen. El. Rev. 51, No. 10, 47—50 (1948).
6. M. Schein, A. J. Hartzler and G. S. Klaiber, Phys. Rev. 70, 435 (1946).
7. G. S. Klaiber, E. A. Lubke and G. C. Boldwin, Phys. Rev. 70, 789 (1946).
8. E. A. Lubke, G. S. Klaiber and G. C. Boldwin, Phys. Rev. 71, 657—660 (1947).
9. N. A. Bonner, G. Friedlander et al., Phys. Rev. 71, 511—520 (1947).
10. L. S. Skagge, T. S. Laughlin et al., Phys. Rev. 73, 420 (1948).