STUDY OF THE ABSORPTION OF BREMSSTRAHLUNG RADIATION WITH MAXIMUM ENERGY UP TO 1000 MeV IN MATERIALS WITH DIFFERENT ATOMIC NUMBERS

V. A. VOROB'EV, A. V. PUSHKIN

Studies of the depth distribution of absorbed energy (DDAE) at bremsstrahlung energies up to 30 MeV [1, 2, 3] have shown that the distribution of absorbed energy over the depth of an absorber is nonuniform. At first an increase in the magnitude of the absorbed energy is observed as the absorber depth increases, up to a certain maximum value \(D_{\max}\), and then, with further increase in absorber depth, a decrease. Moreover, the area under the DDAE curve is proportional to the magnitude of the incident energy [4, 5]. It has been established that the course of the DDAE curves is strongly influenced by the energy of the bremsstrahlung radiation and the atomic number of the absorber, although the shape of the curves remains constant.

In the region of high bremsstrahlung energies, the problem of the distribution of absorbed energy has practically not been studied. Only three works are known [3, 4, 5] in which DDAE curves are presented for several materials at bremsstrahlung energies of 85–330 MeV.

We have carried out studies of the absorption of bremsstrahlung radiation with energy up to 1000 MeV in aluminum, copper, lead (of technical purity), and graphite with density \(1.7\ \mathrm{g/cm^3}\). The spatial distribution of absorbed energy was studied by the photographic method. The technique and conditions of the study are described in [6].

As the experimental results showed, the shape of the DDAE curve does not change even at high bremsstrahlung energies for all the materials studied (Fig. 1). For all materials, at bremsstrahlung energies from 250 to 1000 MeV, the DDAE curves begin not at the origin of coordinates, but at some finite value \(D_{\text{init}}\). They then increase with increasing absorber depth to a maximum value and then decrease. The value of \(D_{\text{init}}\) is due to backscattering of shower electrons. Its magnitude depends on the bremsstrahlung energy and the type of absorber. With increasing bremsstrahlung energy, \(D_{\text{init}}\) decreases, while with increasing atomic number of the absorber it increases. This conclusion is illustrated by Fig. 2, where the ordinate axis gives the ratio of \(D_{\text{init}}\) to \(D_{\max}\) in percent, and the abscissa axis gives the atomic number of the absorber. As can be seen, the value of \(D_{\text{init}}\) is rather considerable, especially for lead, and at high energies it is not clear why the authors of [5] were unable to detect it, although in constructing the curves the initial value adopted by them is not equal to zero. The values of \(D_{\text{init}}\) obtained at energies of 250–1000 MeV,

agree well with the data for paraffin at bremsstrahlung energies of 50–250 MeV [3].

Fig. 1. Depth distribution of absorbed energy in lead along the beam axis. 1—250 MeV, 2—500 MeV, 3—750 MeV, and 4—1000 MeV

Fig. 2. Experimental values of \(D_{\text{init.}}\)

The position of the maximum of the DEDC curve likewise depends on the bremsstrahlung energy and on the atomic number of the substance, Fig. 3. As the bremsstrahlung energy increases, the depth of the position of the maximum, \(t_{\max}\), increases. Moreover, at energies of 500–1000 MeV the value of \(t_{\max}\) is practically proportional to the bremsstrahlung energy for all the materials investigated. The influence of the atomic number of the absorber on the value of \(t_{\max}\) is manifested as follows. The higher the atomic number, the smaller the value of \(t_{\max}\) at one and the same bremsstrahlung energy.

Fig. 3. Dependence of \(t_{\max}\) on the bremsstrahlung energy. 1 — graphite, 2 — aluminum, 3 — copper, 4 — lead. □ — data from works [4, 5].

The leading and trailing edges of the DEDC curves, their rise and fall, also depend on the magnitude of the bremsstrahlung energy and on the atomic number of the absorber. The leading edge is steeper the lower the bremsstrahlung energy and the higher the atomic number of the absorber. The fall of the trailing edge down to a value of 10–15% of the maximum value obeys the same regularities.

A study of the radial distribution of the absorbed energy in the materials investigated showed that this distribution has the same form for all materials. The curves of the radial distribution of absorbed energy in copper at different depths of the absorber are given in [6].

Using the data obtained, one can, to a first approximation, construct DEDC curves for materials with another atomic number in the considered range of bremsstrahlung energies.

The authors express their gratitude to Professor Doctor Aleksandr Akimovich Vorob’ev for proposing the topic and for his attentive and constant guidance of this work.

References

1. V. A. Vorob’ev. Dissertation. Tomsk, TPI, 1965.
2. V. A. Vorob’ev, V. N. Rudenko. Izv. TPI, 140, 90 (1965).
3. B. I. Isaev, M. I. Shal’nov. Atomic Energy, 6, no. 1, 57 (1959).
4. S. P. Kruglov. ZhTF, XXXI, no. 9, 1092 (1961).
5. W. Blocker, R. M. Kenney, W. K. H. Panofsky. Phys. Rev., 79, 419, 1950.
6. V. A. Vorob’ev, A. V. Pushkin. Study of the depth distribution of the absorbed energy of high-energy bremsstrahlung radiation in copper (present collection).