Medium Energy Proton Research Articles

Medium Energy Inelastic Hadron Scattering and Nuclear Structure

AbstractSome aspects of medium energy (∼ 150 MeV to 1 GeV) proton and pion scattering theory from complex nuclei are reviewed with emphasis on inelastic scattering as a tool to extract nuclear structure information.

Proton stopping powers: Binary encounter calculations based on accurate speed distributions for target electrons

Atomic and molecular electronic stopping powers for medium energy protons (≈ 10 keV-10 MeV) have been calculated using the binary-encounter approximation in conjunction with (1) either an energy or maximum impact parameter cut-off based on minimum excitation energies; and (2) ab initio electronic speed distributions. The maximum impact parameter approach yields good agreement with experiment for inert gases and closed-shell polyatomic molecules comprised of first-row atoms.

Identification and decay characteristics ofIm132

The 83.6-min isomer of $^{132}\mathrm{I}$ has been produced both by fission of uranium with medium-energy (30-85 MeV) protons and by the reaction $^{130}\mathrm{Te}(\ensuremath{\alpha},pn)^{132}\mathrm{I}$, using tellurium enriched in mass 130. The isomeric transition is by a 98-keV $\ensuremath{\gamma}$ ray (86%), the remainder decaying by ${\ensuremath{\beta}}^{\ensuremath{-}}$ emission to levels in $^{132}\mathrm{Xe}$. Three new levels, viz. 2650.1 \ifmmode\pm\else\textpm\fi{} 0.9, 2829.1 \ifmmode\pm\else\textpm\fi{} 0.9, and 2960.1 \ifmmode\pm\else\textpm\fi{} 1.2 keV in $^{132}\mathrm{Xe}$ have been found. A decay scheme for $^{132}\mathrm{I}^{m}$ is presented.

Nuclear charge dispersion in mass chains 130–135 from the fission of 238U by medium-energy protons

Independent formation cross-sections of 131mTe, 131gTe, 130I, 131I, 132I, 133I, 134I and 132Cs, and cumulative formation cross-sections of 131Sb, 132Te, 133mTe, 133gTe, 134Te and 135I resulting from the fission of 238U induced by protons of 30–85 MeV energy have been measured. From these data isobaric charge dispersion curves were constructed for mass chains A = 130–135 and were found to be well represented by Gaussian distributions. Within the mass region 130 < A < 135 and proton energy range 30 < E p < 85 MeV, Z A − Z p values for each mass chain approached stability with increasing energy and were independent of mass number. Values of Z p varied smoothly but nonlinearly with proton energy. The derivative ( ∂Z p / ∂A E was measured to be 0·38 ± 0·02, in good agreement with the assumption Z p parallels Z A . The charge dispersion widths were found to increase slightly with both increasing proton bombarding energy and mass number. Estimates of the number of neutrons emitted in fission as a function of average excitation energy were made using a Monte Carlo cascade model, the unchanged charge distribution (UCD) mechanism and the experimentally determined Z p values from this study.

Charge dispersion studies of heavy-mass elements in the fission of 232Th by protons of medium energy

Cross-sections for the independent formation of 139Ba, 139Ce, 140La, 141La, 141Ce and 143Ce and the cumulative formation of 139Cs, 140Ba, 141La, 141Ba, 141Ce, 143La and 144Ce have been determined in the fission of 232Th by protons of energies 28–83 MeV. Charge dispersion curves have been constructed from these data with slight corrections for the variation of N Z p as a function of A. Observations are made about the shapes of the curves obtained in the present work and previously if one plots, from the excitation functions, the energy at which a certain fission product will have the maximum yield against its value of N Z . This is related to the advent of symmetric fission. Observations are also made about charge dispersion curves as a function of single and multi-chance fission.

An optical potential for nuclei with collective states

In first approximat ion the optical potent ia l is s imply related to the target densi ty (x), and the elastic scat ter ing of medium-energy protons (2) and or-particles (8) can be of use in determining the size and shape of closed-shell nuclei. In contrast for nuclei which possess s t rongly excited low-lying states the corrections to the first-order optical potent ia l arising from the vi r tual excitat ion of these collective states may be important , and unless they are explici t ly included in a coupled-channel calculation the parameters of the result ing opt ical potent ia l are no longer s imply re la ted to the ta rge t densi ty and appear to be anomalous (4,~). The contributions from these collective states are reflected as differences in the optical potent ia ls employed in coupled.channel and DWBA calculations, and i t is of interest to systemat ical ly unders tand the connection between them (5). The determinat ion of a local optical potent ia l which reproduces the elastic pa r t of a given mult ichannel scattering problem is quite complicated, and the solution is nonunique (6). In this note we present a simple construction of a local optical potent ia l which uses the high-energy eikonal approximat ion as an in termediary (*). Although our approach is quite general , i t is best i l lus t ra ted in terms of a specific example. Fo r the sake of s implici ty the development is carried out for the generalized optical potent ia l int roduced for v ibra t ional nuclei by BOHR and MOTT~LSON (s):

Nuclear charge dispersion in the fission of 237Np by protons of medium energy

Independent formation cross-sections of 129Cs, 130Cs, 131Cs, 132Cs, 134 m Cs, 134 g Cs, 135 m Cs and 136Cs and the cumulative formation of 140Ba in the fission of 237Np by protons of energies 20–85 MeV have been determined. The most probable charges, Z p , at the various bombarding energies agree with predictions made. No correlation with nuclear charge postulates was found. An estimate has been made of the numbers of neutrons emitted at various bombarding energies.

Nuclear charge dispersion of light-mass elements in the fission of 232Th by protons of medium energy

The fission of 232Th by protons of energies 20–85 MeV has been studied. Independent formation cross sections of 90mY, 90m + gY, 91mY, 91m + gY, 92Y, 95 + gNb, 96Nb, 97Nb and 98Nb, and cumulative formation cross sections for 91Sr, 92Sr, 93Y, and 97Zr were measured. Charge dispersion curves were obtained. The full-width at half-maximum remains sensibly constant over this energy range, unlike products studied in the heavy-mass region. The value of Z A − Z p decreases, but does not show the same sensitivity to the N/ Z value of the target, as found in the heavy-mass region. Calculation shows that the data for Z p lie closer to the unchanged charge distribution than that suggested by the equal charge displacement hypothesis. An estimate was made of the number of neutrons emitted in fission. The light fragment was found to emit half the number of neutrons as the heavy fragment.

Nuclear interactions of 45Sc and 68Zn with protons of medium energy

Abstract Cross sections for the following reactions with 45 Sc (p, n), (p, 2n), (p, pn), (p, 3p), and (p, 3pn), and 68 Zn (p, n), (p, 2n), (p, 3n), (p, 2p) and (p, p3n)+(p, 4n) produced by protons of energy up to 85 MeV have been measured using radiochemical methods. Comparisons have been made with calculations based on the statistical and cascade-evaporation theories. It is shown that the nuclear reactions proceed via a compound nucleus mechanism at lower energies and by a combination of the compound nucleus and direct interaction mechanisms at higher energies.

Nuclear charge dispersion of light-mass fission products in the fission of 235U and 238U by medium-energy protons

Independent fission yields for 90Ym, 90Ym+g, 91Ym, 91Ym+g, and 92Y and cumulative yields for 91Sr, 92Sr, and 93Y in the fission of 235U and 238U by protons of energy from 20–85 MeV have been determined. Excitation functions and charge dispersion curves have been obtained. The most probable charge, Zp, moves closer to stability, ZA, with increasing energy. The values of ZA − Zp for the two fissioning systems are identical, unlike the situation in the heavy-mass region. The values of ZA − Zp for the light- and heavy-mass regions differ considerably, showing that the equal charge displacement hypothesis does not hold even at these moderate energies.

Charge dispersion studies in the fission of 239Pu by protons of medium energy

The independent formation cross sections of 129Cs, 130Cs, 131Cs, 132Cs, 134Cs m , 134Cs m+ g , 136Cs, and 138Cs and the cumulative yield of 138Xe produced in the fission of 239Pu induced by protons of energies 20–85 MeV have been determined by radiochemical methods. The excitation functions of the neutron-deficient products increase with increasing bombarding energy, whereas those of the neutron-rich products show maxima in this energy range. The cross sections have been used to construct charge dispersion curves. As the bombarding energy increases, the charge dispersion curve becomes broader and the most probable charge, Z p , shifts towards β-stability. The full-widths at half-maximum of the dispersion curves, and the displacement of Z p from β-stability ( Z A − Z p ) have been compared with the published data for other targets, and a correlation with the neutron-to-proton ratio in the target has been established. Calculations of Z p have been made, based on the unchanged charge distribution and equal charge displacement hypotheses. The results, although not definitive, favor the former hypothesis. The total number of neutrons, ν T , emitted in fission has been estimated.

A study of medium-energy proton radiative capture by deuterons

Experimental study of the D(p, γ) 3He reaction was carried out, from 17.5° to 150° (lab), with the 156 MeV Orsay synchrocyclotron proton beam. Results for the photon differential cross section are given and the experimental arrangement is described. The cross-section maximum occurs at 52° (c.m.), reaching a value of 0.21 μb sr . A theoretical analysis is performed in the framework of the direct reaction mechanism, assuming the validity of the plane wave Born approximation. The deuteron S- and D-states and 3He S, S' and three D-states are taken into account. It is found that distortion effects and reasonable admixture of S' and D-states cannot remediate to the observed discrepancy between experimental and theoretical results. Calculations made with five usual 3He wave functions lead to the conclusion that direct reaction mechanism is not sufficient to explain the experimental angular distribution.

The Transverse-Field Silicon Detector

It has been our experience during the past few years that LiD silicon detectors can be produced with depths up to not more than 7mm on a production basis and up to about double this depth as a laboratory project. With the installation of several machines producing protons of energies in excess of 50MeV, standard SiLiD detectors with depths to 5mm became unable to provide energy-linear signals for many experiments. Further, deeper detectors present problems due to their higher voltages of operation and to the long transit time for carriers produced by the ionizing radiation. This paper describes a new form of LiD silicon detector which combines the advantages of large sensitive depth (25mm), thin window (less than 0.5 micron) and rapid sweep-out (less than 50nS). Construction details are given. The device is arranged so that the direction of field produced by detector bias is at right angles to the line of flight of experimental particles. A protected thin wind. ow is obtained by a silicon oxide process; active areas up to 200mm2 are achieved by use of two wafers, produced by the diffused-contact process, and placed face-to-face. Resolutions for medium energy protons have now been achieved of small fractions of one per cent. The device gives total energy measurements on alphas up to 20nMeV, protons to 80MeV. The paper describes the significant process details together with some parameter measurements.

Slit scattering effects with medium energy alpha particles and protons

The slit scattering effect has been measured for medium energy protons and alpha particles on aluminum, brass and tantalum slits. A successful comparison with the theoretical treatment of Courant has enabled us to predict the slit scattering effect for our experimental conditions lending credence to the application of the Courant theory to other experimental conditions.

Nuclear charge dispersion in the fission of 233U by protons of medium energy

Independent yields and excitation functions have been determined for the formation of cesium isotopes with masses 129 ≤ A ≤ 138 in the fission of 233U with protons of energies between 20 and 80 MeV. The energies at which the various excitation functions reach their maxima have been compared with those obtained for the same isotopes from fissioning systems with targets of different neutron-to-proton (N/Z) ratios. The neutron-deficient species have a higher probability of being produced at lower energies from targets with lower N/Z ratios. Charge dispersion curves have been obtained and these show the familiar broadening and displacement of Zp, the most probable charge, towards stability with increase in bombardment energy. Values of ZA – Zp have been compared with those obtained from other fissioning systems, and predictions made for values of Zp in the fission of systems yet unstudied, e.g. 235U and 239Pu. In addition Zp values were calculated for 233U fission, assuming compound nucleus formation up to a proton energy of 50 MeV, assuming pre- and post-fission neutron evaporation on the equal charge displacement and unchanged charge division postulates. Experimental values were found to lie between the two predictions.

An energy-dependent calculation of the isomer ratios of 134Cs produced in the fission of 239Pu by medium-energy protons

The isomer ratios of 134Cs, produced in the fission of 239Pu induced by protons of energies 30–85 MeV, have been measured experimentally. Calculations based on a statistical theory of the spin distribution of the primary fragments have been made for these isomer ratios. Competition between fission and neutron emission has been taken into account in the calculation. The calculated isomer ratios have been weighted with the relative formation probabilities of the pertinent fragments resulting from different fissioning nuclei. Values of √ J 2, the root-mean-square angular momentum, which gave the best fit between the calculated and experimental isomer ratios, varied from 11ħ at 30 MeV to about 16ħ at 60–85 MeV. These large values of √ J 2, which cannot be accounted for by the initial angular momentum of the fissioning nucleus, indicate that angular momentum must be induced in the fission process. Moreover, these results also show that a fraction of the angular momentum of the fissioning nucleus appears as the intrinsic spin of the fragments.

Relativistic electrons from solar flares

Observations of interplanetary relativistic electrons from several solar-flare events monitored through 1964 to mid-1967 are presented. These are the first direct spectral measurements and time histories, made outside the magnetosphere, of solar-flare electrons having relativistic velocities. The 3- to 12-MeV electrons detected have kinetic energies about two orders of magnitude higher than those solar electrons previously studied in space, and measurements of both the time histories and energy spectra for a number of events in the present solar cycle were carried out. These measurements of interplanetary electrons are also directly compared with solar X-ray data and with measurements of related interplanetary solar protons.The time histories of at least four electron events show fits to the typical diffusion picture. A demonstrated similarity between the electron and the medium-energy proton fits for the event of 7 July, in particular, indicates that at these electron energies, but over several orders of magnitude of rigidity, whatever diffusion does take place is very nearly on a velocity, rather than a rigidity or an energy, basis. Diffusion-fit time histories varied as a function of T 0 also indicate that the electrons in certain flare events originate at times near the X-ray and microwave burst, establishing their likely identity as the same electrons which cause the impulsive radiations. Also, the energy spectra and total numbers of the interplanetary electrons, compared with those of the flare-site electrons calculated from X-ray and microwave measurements, indicate that probably a small fraction of flare electrons escape into interplanetary space.

Interplanetary and solar electrons of energy 3–12 MeV

This paper reviews two topics related to the low-energy relativistic electrons detected in interplanetary space with the satellites IMP-I, IMP-II, and IMP-III:1. The first observations of 3–12-MeV solar-flare electrons in interplanetary space are presented. The solar electrons detected have kinetic energies nearly two orders of magnitude higher than any previously studied; thus, although flare events with a detectable flux of such particles occur relatively rarely, their study provides a new parameter in solar physics. The 7 July and 14 September 1966 events are outlined in detail, having the greatest relativistic electron to medium-energy proton ratios of the events detected before 1967. These events contrast with the 28 August 1966 event, which was intense in nucleons but contained no detectable component of relativistic electrons. The electron time histories are shown to have delayed onsets, and to be similar in form to those of high-energy protons, and the energy spectra and other features are described.2. Progress in the study of the solar modulation of interplanetary 3–12-MeV electrons is reviewed. Characteristics of the electron-intensity time variations during parts of 1963–67 are outlined; they are shown to be consistent with the hypothesis of the primary cosmic-ray nature of these particles and with a strong dependence on the local field conditions.

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.

Multiple scattering of charged particles

Numerical calculations have been carried out for the multiple scattering of charged particles as described by the theories of Molière and of Nigam, Sundaresan and Wu (NSW). The NSW theory is the more accurate but the deviations from the older theory of Molière are not great. An approximation to the NSW theory is given which is shown to be essentially indistinguishable from the complete NSW theory for the scattering of medium energy protons and which allows the construction of “universal” angular distribution curves. A method is given which permits the rapid evaluation of the angle at which the angular distribution has decreased to 1/e of its value in the forward direction.