Increasing Proton Energy Research Articles

Total-Reaction-Cross-Section Measurements for 30-60-MeV Protons and the Imaginary Optical Potential

We have measured total reaction cross sections for 30-, 40-, 49.5-, and 60.8-MeV protons incident on thin, separated isotopes covering the range from $^{12}\mathrm{C}$ to $^{208}\mathrm{Pb}$. Our results are consistent with previous data at 30 MeV, but disagree with earlier data at 60 MeV. We find a strong dependence of the reaction cross section on neutron excess for a series of Fe and Ni isotopes. Little, if any, such dependence is observed for the $N=28$ isotones. The data are well represented by the relation ${\ensuremath{\sigma}}_{\mathrm{R}}=\ensuremath{\pi}{({r}_{0}{A}^{\frac{1}{3}}+\overline{\ensuremath{\lambda}})}^{2}$ with ${r}_{0}=1.23\ifmmode\pm\else\textpm\fi{}0.01$ F. When analyzed with the conventional optical model, our data require the volume absorption to increase and the surface absorption to decrease with increasing proton energy ${E}_{p}$. The analysis reveals a striking $\frac{(N\ensuremath{-}Z)}{A}$ dependence for the product ${W}_{D}{a}^{\ensuremath{'}}$. Using the Oak Ridge parameters for the real and spin-orbit potentials, we arrive at the following parametrization for the imaginary potential: volume absorption potential, ${W}_{0}=(1.2\ifmmode\pm\else\textpm\fi{}0.09{E}_{p})$ MeV; surface absorption potential, ${W}_{D}=[4.2\ensuremath{-}0.05{E}_{p}+15.5\frac{(N\ensuremath{-}Z)}{A}]$ MeV; imaginary diffusivity, ${a}^{\ensuremath{'}}=[0.74\ensuremath{-}0.008{E}_{p}+1.0\ifmmode\times\else\texttimes\fi{}\frac{(N\ensuremath{-}Z)}{A}]$ F.

Isobaric yield distribution in the fission of uranium by medium-energy protons

The independent formation cross sections of 139Ba, 139Ce, 141Ce, and 143Ce, and the cumulative formation cross sections of 139Cs, 141Ba, and 143La, produced from the fission of natural uranium by protons of energies 20–85 MeV, have been measured radiochemically. The proton energies at which the excitation functions reach their maxima correlate well with the neutron-to-proton ratio of the fission product. Nuclear charge-dispersion curves exhibit, with increasing proton energy, the behavior previously found by other workers, i.e. broadening and a shift of the most probable nuclear charge towards stability.

Isomeric yield ratios and fractional chain yields for 117In and 117Cd produced by proton-induced fission of Bi, Th and U

The relative independent yields of the isomeric pair 117 g In- 117 m In produced by proton-induced nuclear fission have been measured as a function of energy from 15–157 MeV and target element. The high spin to low spin yield ratio is generally increasing with increasing proton energy, and below 115 MeV proton energy the three targets produce the same curve. Above this energy the ratio from uranium fission seems to level off while the other reactions do not show this effect. The ratios obtained at the lowest bombarding energies are as low or lower than known isomeric yield ratios from thermal neutron-induced fission. Comparison of the results with calculations based on statistical level density theory also show that at low proton energy the primary fission fragments are formed with a very low intrinsic angular momentum. The fractional chain yields of 117In is shown to be increasing with increasing bombarding energy for all targets, and to be much larger for reactions with bismuth target than for uranium and thorium. Lastly, the cumulative high spin to low spin isomeric yield ratio for the pair 117 m Cd- 117 g Cd has been obtained and is increasing with increasing proton energy.

Carrier Lifetime Studies in Electron-(10–55 MeV) and Proton-(25–110 MeV) Irradiated Germanium

Samples of single-crystal germanium were irradiated at room temperature with electrons of energies from 12 to 55 MeV and with protons of energies from 25 to 109 MeV. Measurements of the minority carrier lifetime as a function of flux and initial carrier concentration suggest that: (1) More than one recombination level, active at room temperature, is introduced in the forbidden gap by the defects created during electron and proton bombardment, with the multilevel effect being greater for the proton-irradiated specimens. (2) The carrier lifetime change with increasing proton energy does not decrease as sharply as predicted by Rutherford scattering and the Kinchin-Pease model. This indicates that nuclear elastic and inelastic processes must be included in computing the number of primary knockons created by protons of energies above at least 50 MeV. (3) The energy dependence of carrier lifetime change in the case of electron bombardment is complicated by a non-constant contribution of multiple levels to the recombination process in the electron energy range studied. At 12 MeV the behavior is adequately represented by single-level theory and a value of 15.4 displacements/cm is obtained.

Photoprotons from carbon

The angular and energy distributions of photoprotons from C 12 have been measured from 6.75 to 21 MeV proton energies with an E, d E/d x coincidence-counting spectrometer and with 65 MeV maximum gamma-ray energy. The spectrometer consisted of a CsI(Tl) crystal as the E detector and a silicon p-n junction diode as the d E/d x detector. The data were analysed in 1 MeV intervals, and the equation for electric dipole-electric quadrupole interference in a direct interaction − a + b sin 2 θ + A cos θ + B sin 2 θ cos θ — fitted to the data of each energy bin by the least-squares method. The results indicate that the direct interaction model with E1–E2 interference gives a good description of the interaction, with the magnitude of the interference increasing with increasing proton energy. The energy spectra show the existence of a small resonance near 13 MeV proton energy.

Relative yields of the pairs 96gNb 95mNb and 96Nb 95Nb in fission of Pb, Bi, Th and U induced by 15–157 MeV protons

The ratio between the fission yield of the high spin isomer and that of the low spin isomer has been measured for the isomeric pair 95 g Nb 95 m Nb a function of the irradiation energy and of the target used. For all the targets used, the isomeric yield ratio increases with increasing proton energy, and for given irradiation energy, the ratio is highest when lead is irradiated and lowest when uranium is irradiated. A simple statistical model calculation indicates that the spin of the primary fission fragment produced by the fission of uranium is very low for low bombarding energies. In the same experiments the ratio between the fission yields of 24 hr 96Nb and 95Nb has been measured.

NUCLEAR CHARGE DISPERSION IN THE HIGH-ENERGY FISSION OF URANIUM

The independent formation cross sections of Cs130, Cs132, Cs134, Cs136, Cs138, Rb86, Nb95m, Nb96, and Nb97 produced in the fission of natural uranium by protons have been measured radiochemically at incident proton energies ranging from 10 Mev to 85 Mev. The excitation functions correlate well with those obtained by Friedlander, Friedman, Gordon, and Yaffe (1963). Charge dispersion curves were constructed at 20, 36, 50, 65, and 80 Mev. The curves broaden with increasing proton energy and the most probable charge approaches the line of β-stability. An explanation for both of these effects is given in terms of spallation–fission competition.

The role of hyperons in extensive air showers and in other high-energy phenomena

Experimental results obtained at the Geneva proton synchrotron support the hypothesis that in high energy nuclear collisions the particle which carries away most of the energy is often a hyperon. In proton collisions at 25 GeV this occurs in about 20% of the cases. The fraction of hyperons appears to increase further with increasing proton energy and probably reaches a value close to 50% for energies above ∼ 1000 GeV. It is shown that in the atmosphere most pions, μ-mesons, and γ-rays in the energy interval from a few times 1011 eV to about 1014 eV are due to hyperon decay rather than to direct pion production and that a large fraction of the primary energy appearing in the electron-photon component is transmitted through this process. The presence of hyperons in the nuclear cascade in the core of air showers gives rise to a large number of characteristic phenomena, both in the atmosphere and underground, some of which may be related to phenomena recently reported in the literature. Since hyperon decays result in a large excess of negative over positive mesons, charged eterminations on μ-mesons underground may give information on the relative yield of nucleons, nuclear isobars, and of strangeness one and strangeness two hyperons at high energies. It is further shown that the simultaneous arrival of two or more μ-mesons at great depth below ground signals the arrival and interaction of a complex primary nucleus at the top of the atmosphere.

Angular Distributions of the Reactions Co59(p, α)Fe56 at 7.7∼14.1 Mev and Mn55(p, α)Cr52 at 7.6∼12.9 Mev

The angular distributions of the reactions Co 59 ( p , α)Fe 56 and Mn 55 ( p , α)Cr 52 resulting in the ground and first excited states of the residual nuclei have been measured at 7.7, 8.7, 9.8, 10.85, 11.9, 13.1 and 14.1 Mev and at 7.65, 8.85, 9.65, 10.7, 11.8 and 12.9 Mev of the incident proton energy, respectively, making use of proportional counters. Similar behaviour was obtained for these four reactions. For the reaction Co 59 ( p , α)Fe 56 the forward peak in the angular distribution is prominent at 14.1 and 13.1 Mev and disappears gradually with decreasing proton energy, and the angular distribution is approximately 90° symmetrical below about 11 Mev. For the reaction Mn 55 ( p , α)Cr 52 the angular distribution is considerably forward peaked at 12.9 and 11.8 Mev and it approximately 90° symmetrical below about 10 Mev. It was found that the total cross section integrated from the angular distributions for both reactions decrease smoothly with increasing proton energy. It is considered that the reactions Co 59 ( p , α)Fe 56 and Mn 55 ( p , α)Cr 52 resulting in the ground and first excited states of the residual nuclei occur through the similar reaction process, that is, in the lower energy region the compound nucleus process plays a predominant role, although some contribution from the direct interaction process cannot be neglected, and the contribution from the direct interaction process becomes gradually significant with increasing proton energy and is predominant above about 11 Mev.

Secondary nuclear reactions induced in tin by fast protons

Nuclear reactions in which spallation products are captured by the target nucleus have been studied by the radio-chemical method. The excitation functions of secondary reactions leading to product nuclei having charges 2 or 3 units greater than that of the target nuclei have been obtained from measurements of the yields of isotopes of tellurium (Z = 52) and iodine (Z = 53) formed in the 170–660 MeV proton bombardment of tin (Z = 50). The cross-sections of these reactions increase with increasing proton energy: for Ep = 170 MeV, σ(α, xn) and σ(Li, xn) are 18·5 ± 5 and 0·17 ± 0·1 μbarns, respectively; for Ep = 660 MeV, these cross-sections are 50 ± 6·5 and 1·6 ± 0·5 μbarns, respectively. The cross-sections for the capture of lithium nuclei by tin obtained in this work are in good agreement with the results for similar reactions with copper, tin, and lead over a comparable proton energy interval but they are 50 times less than the cross-sections found by Marquez and Perlman. The observed secondary reaction cross-sections for the capture of lithium nuclei can be accounted for only by supposing that the lithium nuclei are emitted with energies greater than they could acquire as a result of fission or evaporation of the target nucleus. Secondary reactions of the type (α, xn) can be accounted for by assuming that the m-particles are emitted as a result of the evaporation process.

Breakup of Deuterons by Protons

Results are given for the neutron spectra at zero degrees from the reaction $p+d\ensuremath{\rightarrow}2p+n$ for a range of incident proton energies from ${E}_{p}=4.0$ to ${E}_{p}=7.0$ Mev. Also given are the spectra as a function of angle for ${E}_{p}=6.5$ Mev. The observed shapes show systematic discrepancies from those given by the theory of Frank and Gammel; these discrepancies increase with increasing proton energy although the reaction yield is accurately given by the theory.

Formation ofBe7in Interactions of Various Nuclei with High-Energy Protons

Cross sections are reported for the formation of Be/sup 7/ in interactions of C, Al, Cu, Ag, and Au with protons of energies between 1 and 3 Bev. They are compared with previous data at 0.34-Bev proton energy. The carbon cross section is strikingly constant at 11 to 14 mb from 0.1 to 3.0 Bev. The other cross sections rise with increasing proton energy and are all in the neighborhood of 10 mb at 3 Bev. Beryllium-7 may be considered as a spallation residue from carbon, and the same mechanism probably accounts for at least a large part of its formation from Al. However, Monte Carlo calculations show that this mechanism predicts much too low cross sections from the higher Z targets. Other mechanisms must be invoked, the most promising being the evaporation of Be/ sup 7/ nuclei. (auth)

(p, pn) and (p, 2p) Reactions ofCe142at Proton Energies from 0.4 to 3.0 Bev

Cross sections are reported for ($p, \mathrm{pn}$) and ($p, 2p$) reactions of ${\mathrm{Ce}}^{142}$ with protons of 0.4, 1.0, 2.2, and 3.0 Bev. To evaluate the contribution of proton and neutron evaporation from excited ${\mathrm{Ce}}^{142}$ nuclei to the observed cross sections, this evaporation competition was studied by means of cross-section measurements on the reactions ${\mathrm{Ba}}^{138}(\ensuremath{\alpha}, n)$ and ${\mathrm{Ba}}^{138}(\ensuremath{\alpha}, p)$ with $\ensuremath{\alpha}$ particles of 19- to 40-Mev kinetic energy. The ratio $\frac{{\ensuremath{\sigma}}_{\ensuremath{\alpha},n}}{{\ensuremath{\sigma}}_{\ensuremath{\alpha},p}}$ is between 2.4 and 4.4 in this region as compared with a ratio of 1.58\ifmmode\pm\else\textpm\fi{}0.14 for $\frac{{\ensuremath{\sigma}}_{p,pn}}{{\ensuremath{\sigma}}_{p,2p}}$ at 0.4 Bev. It is concluded that at this energy the ($p, \mathrm{pn}$) and ($p, 2p$) reactions proceed at least partially by a pure knock-on mechanism. This conclusion is supported by comparison of the data with recent Monte Carlo calculations of intranuclear cascades. With increasing proton energy both ${\ensuremath{\sigma}}_{p,2p}$ and ${\ensuremath{\sigma}}_{p,pn}$ are found to decrease, the former more rapidly than the latter. This energy dependence is discussed in terms of the elementary nucleon-nucleon cross sections. The Monte Carlo calculations mentioned agree within their rather poor statistical accuracy with the measured cross-section ratios and with the energy dependence of the cross sections, but predict cross-section values about a factor of three too small.

Probabilities of Symmetric and Asymmetric Fission in the Proton Bombardment ofTh232

The ratio of symmetric to asymmetric fission in the proton bombardment of ${\mathrm{Th}}^{232}$ does not rise steadily with increasing proton energy; a periodic decrease is superposed upon the over-all increase. This is attributed to the changing pattern of various fission reactions, ($p, f$), ($p, \mathrm{nf}$), etc.

Polarization of Protons Elastically Scattered from Nuclei

The polarization of 130-Mev protons elastically scattered from Be, C, Al, and Fe has been calculated by means of the optical model of the nucleus, including a spin-orbit coupling term. The results are in reasonable agreement with the measurements of Dickson, Rose, and Salter. Calculations were also carried out for proton energies of 50 Mev and 90 Mev, in order to explain the rapid decrease of the polarization as the energy is decreased below 130 Mev. By using a central potential whose real part decreases with increasing proton energy between 50 Mev and 130 Mev, reasonably good agreement was obtained for the dependence of the polarization on energy for Be and C.

XXX.Special mechanisms in the reaction7Li(pγ)8Be

Abstract The course of the reaction 7Li(pγ)8Be is an unusual one: above the strong resonance at Ep =440 kev the cross section shows a background that, apart from weak resonances, increases steadily with increasing proton energy until at least Ep = 5·2 mev. It is suggested that this background has two origins: at the lower values of Ep it is due to direct radiative transitions between a free proton state and a bound state of 8Be—no compound nucleus is formed; at the higher values of Ep it is due to a process inverse to the familiar ‘giant resonances’ of photo-disintegration. The first contribution is calculated for a variety of conditions in LS and in jj coupling; the second is deduced from the known properties of neighbouring photodisintegration cross sections. The two processes together give a good account of the experimental results; there is some suggestion that the ‘radius’ of 8Be may be about 3·2 × 10−13 cm.

Proton Induced Reactions of Thorium—Fission Yield Curves

The fission yields of a number of nuclides produced in proton-induced fission of ${\mathrm{Th}}^{232}$ were determined at several proton energies in the range of 6.7 Mev to 21.1 Mev. At the same time, the relative cross sections of the ($p,n$) and ($p,3n$) reactions on ${\mathrm{Th}}^{232}$ were also determined. It was found that, although the fission reaction was predominant at proton energies greater than 8 Mev, competition from the ($p,xn$) type of reaction was considerable over all energies investigated.The trough in the fission yield curve becomes shallower with increasing proton energy and this change was quantitatively determined over the energy range investigated. A model is proposed to explain the change in shape of the fission yield curves.An approximate determination of the excitation function for the ($p,f$) reaction was made; the shape of the resulting curve was at least qualitatively in agreement with the excitation function predicted from existing theory.

Stars in Photographic Emulsions Initiated by Protons

A study has been made of stars in photographic emulsions initiated by protons from the 184-inch Berkeley cyclotron. Ilford G-5 emulsions were exposed to the deflected proton beam with various Al absorbers interposed to obtain protons of various energies. Both the average number of prongs per star and the cross section for star production were found to increase with increasing proton energy.

The Scattering of Protons by Protons

The cross section for scattering of protons by protons has been measured at angles from 15\ifmmode^\circ\else\textdegree\fi{} to 45\ifmmode^\circ\else\textdegree\fi{} for protons of energies 860, 1200, 1390, 2105 and 2392 kev. Observed scattering cross sections at 15\ifmmode^\circ\else\textdegree\fi{} are close to Mott values for all energies. Ratios of observed cross section to Mott cross section rise with increasing angle and with increasing proton energy. At 45\ifmmode^\circ\else\textdegree\fi{} the ratio rises from 3.90 at 860 kev to 42.9 at 2392 kev. At 1830 kev measurements were made at scattering angles up to 60\ifmmode^\circ\else\textdegree\fi{} and the scattering cross section showed the theoretically expected asymmetry ($cos\ensuremath{\theta}$) about 45\ifmmode^\circ\else\textdegree\fi{}. As a check on the proton-proton measurements the scattering of protons from argon and krypton was investigated with protons of energies between 850 kev and 2440 kev.