STUDY OF FAST PROTONS PRODUCED IN THE PHOTODISINTEGRATION OF NUCLEI

It has already been reported in UFN[^1] that the energy and angular distribution of photoprotons with energies up to 70 MeV, produced by irradiating a number of elements with a γ-ray beam whose maximum spectral energy was \(E_{\gamma m}=320\) MeV, have been observed. Studies[^2][^3] carried out on a synchrotron with \(E_{\gamma m}=300\) MeV in the region of higher-energy protons have yielded additional interesting results. In the work with photographic plates,[^2] the energy of fast protons produced in a carbon target was determined from their absorption in aluminum. The energy distribution obtained for photoprotons with energy \(E_p\) up to 124 MeV, emitted at angles of 135°, 90°, 60°, and 30° to the γ-ray beam with \(E_{\gamma m}=195\) MeV, corresponds to an exponential \(E^{-n}\), where \(n\) is close to 4. The value of \(n\) found is a minimum because of the projection of tracks of low-energy protons. The dependence of the differential cross section on the proton-emission angle for \(E_p=70\) MeV and 90 MeV is shown in Fig. 1.

Fig. 1. Angular distribution of protons produced by a γ-ray beam with \(E_{\gamma m}=195\) MeV.

Fig. 2. Differential energy spectrum of protons at an angle of 67.5°, produced by a γ-ray beam with \(E_{\gamma m}=300\) MeV.

In the work[^3], protons were recorded by two NaJ(Tl) scintillation counters connected in coincidence. Measurements with “slow phosphors” NaJ (resolving time of the circuit 1 μsec) proved possible because the γ-ray beam was extended in time to 2000 μsec. The energy interval of the particles counted was determined by setting the response threshold of the rear counter.

An absorber was placed in front of both counters, its thickness corresponding to the minimum energy of the particles being registered. The distribution with respect to amplitudes of coincident pulses in the front counter, measured with a differential discriminator, made it possible to separate mesons, protons, and deuterons having the same residual range. The accuracy of the experiment was determined mainly by the stability of the high-voltage source feeding the photo-

multiplier; thus, to obtain constancy in the counting rate within 10%, the voltage on the photomultiplier must be stabilized to 0.5%.

The differential energy spectrum of protons emitted from carbon and cadmium at an angle of 67.5° to the beam direction at \(E_{\gamma m}=300\) MeV (Fig. 2) was measured with the telescope counter described above with a copper absorber and partly with the aid of a Terenin counter with a lead absorber. In the region \(E_p<100\) MeV the course of the energy distribution agrees with the data of works \(^{1,2}\), but near the point corresponding to \(E_p=130\) MeV a break is observed in the spectrum, after which the number of protons falls considerably more rapidly (as \(E^{-(5\div 7)}\)). The presence in the energy spectrum of protons, at

\[ E_p=\frac{1}{2}E_{\gamma m}, \]

of a break is convincingly explained by the assumption that a high-energy photon interacts not with the whole nucleus, but only with a pair of nucleons. In this case the recoil particle in the formation of the proton is an individual nucleon (deuteron model) \(^{4,5}\). The existence of energetic protons with higher energy may be caused by the motion of the two-nucleon system in the nucleus \(^{4}\). The angular distribution of photoprotons from carbon with energies 100, 130, and 175 MeV, obtained in this work, like that measured previously for \(E_p=40\) MeV and 70 MeV (Fig. 1), has a noticeable forward directionality, which also confirms the considerations set forth above.

Fig. 3. Relative proton yield per nucleus at an angle of 67.5°, produced by \(\gamma\)-rays with \(E_{\gamma m}=300\) MeV.

Table I gives a comparison of the experimental cross sections for protons with \(E_p=70\) MeV emitted at an angle of 90° to the beam direction.

The closeness of the cross sections for \(\gamma\)-spectra with maximum energies of 200 MeV and 300 MeV indicates an insignificant contribution of photons with energies from 200 to 300 MeV, which agrees with the idea of direct photonuclear interaction. On the contrary, in most processes associated with meson production, the role of these photons should be very significant.

Table I

Cross sections for protons with \(E_p = 70\ \mathrm{MeV}\) at \(90^\circ\)

The dependence of the proton yield per nucleus \((Y)\) on \(Z\) of the target nucleus was also studied for Be, C, Al, Cu, Cd, and Pb (Fig. 3); it is well approximated by the curve

\[ Y \sim \left(\frac{NZ}{A}\right)\cdot A^{-1/4}, \]

which agrees with the theoretical data \(^{6}\).

B. R.

Cited Literature

1. Uspekhi Fizicheskikh Nauk, vol. XLV, 638 (1951); C. Levinthal and A. A. Silverman, Phys. Rev., 82, 822 (1951).
2. D. Walker, Phys. Rev., 81, 634 (1951).
3. J. C. Keck, Phys. Rev., 85, 410 (1951).
4. J. S. Levinger, Phys. Rev., 84, 43 (1951).
5. J. Heidmann, Phys. Rev., 80, 171 (1950).
6. J. S. Levinger and H. A. Bethe, Phys. Rev., 78, 115 (1951).