Photo-protons from high-energy bremsstrahlung

Abstract

Abstract. Recent results on the yields of high-energy photo-protons from both bremsstrahlung and monochromatic photons incident mainly on carbon are compared with calculations using the Gabriel-Alsmiller model of high-energy photo-induced reactions. Good agreement is found for photons of energies up to 600 MeV.Recently, extensive sets of data on the cross-sections for nuclei from lithium to gold to produce high-energy photo-protons have become available [1,2]. In [1], data at an end-point energy of 400 MeV were given, while in [2], proton yields mainly from carbon irradiated with bremsstrah~ungofend-point energies between 125 and 1000MeV were listed, as were some data obtained by a "'photon difference" method. Some of the results of [1] have been analyzed in [3], in terms of the "quasi-deuteron" model as formulated by Matthews [4] and of the intra-nuclear cascade model (PICA) of Gabriel and Alsmiller [5]. In this paper, analysis of the data in [1] and [2] is extended, using the latter model. Three specific matters are examined:I. The earlier analysis indicated that, to obtain agreement with experiment, it was necessary to renormalize that part of the flux in the Gabriel-Alsmiller model which originates via the quasi-deuteron mechanism. The values of the quasi-deuteron constant, L, given by fits to the experimental data were _~ 6 for C, ~-8 for Cu and ___13 for Au. Although there thus appears to be a tendency for the value of L to increase with increasing atomic number, this conclusion depends largely on the value for gold. This is, however, the least well-determined of the values, partly due to the fact that the statistical accuracy of the calculations was not good and partly to the fact that the results for gold are not particularly sensitive to the exact value of quasi-deuteron constant used. (Few of the particles photo-produced in gold are protons.) Additional calculations have now been performed for Be and Ag, for comparison with the data of [I]. The calculated values have, as before, been fitted to experiment by treating the value of L as if it were a free parameter. The results are shown in Fig. 1. The values indicated by the comparison are L = 5 +1 for Be and L=10_+2 for Ag, (As in the earlier analysis, these values are determined by the value of L giving the best visual fit to the experimental data. The errors quoted in the values of L do not take into account any errors in the input data, e.g. in the values of the cross-sections for deuteron photo-disintegration. These are usually smaller than the statistical errors in P1CA calculations.) With the present results, PICA analyses have now been performed for six of the elements for which data were reported in [1] (A1 has been studied by Gabriel who finds L=7.2 [6]). All the analyses confirm the trend reported in [1], that, within the framework of the Gabriel-Alsmiller model, the value of quasi-

Photo-protons from beryllium irradiated with intermediate energy bremsstrahlung

“…No extensive experimental checks of the latter have yet been made. However, at higher photon energies, it has been shown in both [4] and [5] that the pion-connected and quasi-deuteron contributions must be present at about the strength required by the data presented here for the PICA predictions to give adequate fits. In any case, below 150 MeV, this cannot account for the discrepancy found and obviously further experimental study is needed to clarify the experimental situation.…”

“…In Figure 3, the results of calculations with L= 5.2 and 9.3 are also included, to show how the calculated values depend on L. From these fits (and those at other angles), it is concluded that the uncertainty in L is about one unit, so that L--7.2 • There is only one earlier PICA analysis of beryllium data, viz., of the 400 MeV data in [1], which are analyzed in [4]. There, the value of L was determined to be 5.2+_1.0, somewhat smaller than but not incompatible with that found here.…”

“…Since doublepion production (which is expected to become important about 400 MeV) is not included in the model, it is possible that the apparently satisfactory nature of the fit to the 600 MeV data is not so significant as might, at first sight, appear. On the hand, it should not be forgotten that data for carbon at two angles have been fitted for bremsstrahlung end-point energies up to 600 MeV with one and the same value of L [4]. The Mainz group has also analyzed its data between 50 and 100 Meu to obtain values of the quasi-deuteron constant.…”

“…In the last few years, measurements of the cross-sections for photo-proton production from a wide range of nuclei irradiated with bremsstrahlung of maximum energy 400MeV [-1] and of the yields of photo-protons from carbon irradiated with bremsstrahlung of maximum energies up to 1000 MeV [2] have been reported from this laboratory. These data have been analyzed [3][4][5] using the intra-nuclear cascade model PICA developed by Gabriel and Alsmiller [6], in which the basic processes of photon absorption (pion production and the "quasi-deuteron" effect) and secondary processes such as the scattering and absorption effects occurring during traversal of the residual nucleus are simulated using Monte Carlo methods. In these analyses, it has been found that good fits to experimental photo-proton data,can be obtained at E0=400MeV [-3, 5] and that, for carbon, data at different end-point energies can be fitted with one and the same value of L [4].…”

“…These data have been analyzed [3][4][5] using the intra-nuclear cascade model PICA developed by Gabriel and Alsmiller [6], in which the basic processes of photon absorption (pion production and the "quasi-deuteron" effect) and secondary processes such as the scattering and absorption effects occurring during traversal of the residual nucleus are simulated using Monte Carlo methods. In these analyses, it has been found that good fits to experimental photo-proton data,can be obtained at E0=400MeV [-3, 5] and that, for carbon, data at different end-point energies can be fitted with one and the same value of L [4]. Extensive as these data are, the flexibility of the model is such that they only permit a very limited test of its applicability.…”