From Current Literature

Excitation of Heavy Nuclei by the Electric Field of Low-Energy Protons

In a paper¹ it is reported that, when targets of Ta, Pt, Au, Tl, and Bi were irradiated with protons of energy from 1.4 to 2.6 MeV, intense γ radiation was observed. The protons were accelerated in a Van de Graaff generator, and the γ-radiation spectrum was measured with a scintillation counter; NaI(Tl) crystals were placed near the target. For tantalum, the measured pulse distribution reveals a maximum corresponding to the γ line \((0.138 \pm 4\%)\) MeV. This line arises in the \(M1\) transition from the first excited state \(g_{7/2}\) to the ground state \(g_{7/2}\) of the nucleus Ta\(^{181}\). For platinum a line of 0.126 MeV was found, corresponding to the transition from the state \(f_{5/2}\) to the ground state \(f_{1/2}\) of Pt\(^{195}\). Measurements with improved resolving power showed the presence, in the radiation of tantalum, of a second line with an energy of about 0.5 MeV and indicated radiation corresponding to the energy difference of both lines. In the radiation of silver, two lines with energies of about 0.25 and 0.45 MeV were found. These examples, whose number can be increased, indicate that the radiation is emitted by nuclei that have entered an excited state under the action of protons with energies of 1–2 MeV. Meanwhile, the probability of forming a compound nucleus as a result of the penetration of such protons through the potential barrier of a heavy nucleus is very small. Thus, for example, for tantalum and protons of energy 1.5 MeV the corresponding effective cross section is of the order of \(10^{-39}\ \mathrm{cm}^2\). The probability of inelastic scattering of a proton by the nucleus is still smaller. Moreover, such scattering cannot lead to the formation of the lines obtained. Meanwhile, the effective cross section for the observed process is close to \(\sigma = 0.4 \times 10^{-27}\ \mathrm{cm}^2\), even if one assumes the absence of internal conversion, and is equal to \(\sigma = 8 \times 10^{-27}\ \mathrm{cm}^2\) for 80% internal conversion. Such a large value of \(\sigma\) forces one to suppose that the observed phenomenon consists in the direct excitation of the nucleus by the electric field of a proton that has not penetrated through the potential barrier. The existence of such a process was predicted theoretically.² ³ It should be noted that the observed phenomenon is easily observed and may be a convenient method for studying the lower excited levels of heavy nuclei: it occurs at low energies of the particles bombarding the target, when the accompanying nuclear phenomena have low intensity.

A. V.

Cited Literature

1. McClelland C., Goodman C., Phys. Rev. 91, No. 3, 760 (1953).
2. Ter-Martirosyan, ZhETF 22, 284 (1952).
3. Mullin H., Guth E., Phys. Rev. 82, 141 (1951).