Thin Target Experiments Research Articles

The Xe136+Pt198 reaction: A test of models of multi-nucleon transfer reactions

The yields of 42 projectile-like fragments (PLFs) and fission fragments and 36 target-like fragments (TLFs) were measured using off-line $\ensuremath{\gamma}$-ray spectroscopy in a thin target experiment involving the $^{136}\mathrm{Xe}+^{198}\mathrm{Pt}$ reaction. The center of target beam energy was 760.5 MeV $({\mathrm{E}}_{\mathrm{c}.\mathrm{m}.}=450$ MeV). The reported yields are compared with those from previous measurements for this reaction and with predictions of the GRAZING, di-nuclear systems (DNS), and improved quantum molecular dynamics (ImQMD) models. The yields of the TLFs and PLFs are, in general, substantially smaller than those previously observed at a beam energy of 1085 MeV. Neither the GRAZING nor the DNS model correctly describes the measured TLF and PLF yields in this lower- energy reaction but the ImQMD model describes these yields adequately.

Reexamining the heavy-ion reactions238U+238U and238U+248Cm and actinide production close to the barrier

Recent theoretical work has renewed interest in radiochemically determined isotope distributions in reactions of ${}^{238}$U projectiles with heavy targets that had previously been published only in parts. These data are being reexamined. The cross sections \ensuremath{\sigma}($Z$) below the uranium target have been determined as a function of incident energy in thick-target bombardments. These are compared to predictions by a diffusion model whereby consistency with the experimental data is found in the energy intervals 7.65--8.30 MeV/u and 6.06--7.50 MeV/u. In the energy interval 6.06--6.49 MeV/u, the experimental data are lower by a factor of 5 compared to the diffusion model prediction indicating a threshold behavior for massive charge and mass transfer close to the barrier. For the intermediate energy interval, the missing mass between the primary fragment masses deduced from the generalized ${Q}_{\mathrm{gg}}$ systematics including neutron pair-breaking corrections and the centroid of the experimental isotope distributions as a function of $Z$ have been used to determine the average excitation energy as a function of $Z$. From this, the $Z$ dependence of the average total kinetic-energy loss ($\overline{\mathrm{TKEL}}$) has been determined. This is compared to that measured in a thin-target counter experiment at 7.42 MeV/u. For small charge transfers, the values of $\overline{\mathrm{TKEL}}$ of this work are typically about 30 MeV lower than in the thin-target experiment. This difference is decreasing with increasing charge transfer developing into even slightly larger values in the thick-target experiment for the largest charge transfers. This is the expected behavior which is also found in a comparison of the partial cross sections for quasielastic and deep-inelastic reactions in both experiments. The cross sections for surviving heavy actinides, e.g., ${}_{98}$Cf, ${}_{99}$Es, and ${}_{100}$Fm indicate that these are produced in the low-energy tails of the dissipated energy distributions, however, with a low-energy cutoff at about 35 MeV. Excitation functions show that identical isotope distributions are populated independent of the bombarding energy indicating that the same bins of excitation energy are responsible for the production of these fissile isotopes. A comparison of the survival probabilities of the residues of equal charge and neutron transfers in the reactions of ${}^{238}$U projectiles with either ${}^{238}$U or ${}^{248}$Cm targets is consistent with such a cutoff as evaporation calculations assign the surviving heavy actinides to the 3$n$ and/or 4$n$ evaporation channels.

Methodology for the neutron time of flight measurement of 120-GeV proton-induced reactions on a thick copper target

A methodology for the time-of-flight measurement of the neutron energy spectrum for a high-energy proton-beam-induced reaction was established at the Fermilab Test Beam Facility of the Fermi National Accelerator Laboratory. The 120-GeV proton beam with 3×105protons/spill was prepared for event-by-event counting of incident protons and emitted neutrons for time-of-flight energy determination. An NE213 organic liquid scintillator (12.7cm in diameter by 12.7cm in length) was employed with a veto plastic scintillator and a pulse-shape discrimination technique to identify neutrons. Raw waveforms of NE213, veto and beam detectors were recorded to discriminate the effects of multi-proton beam events by considering different time windows. The neutron energy spectrum ranging from 10 to 800MeV was obtained for a 60-cm-long copper target at 90° with respect to the beam axis. The obtained spectrum was consistent with that deduced employing the conventional unfolding technique as well as that obtained in a 40-GeV/c thin-target experiment.

Discovering new light states at neutrino experiments

Experiments designed to measure neutrino oscillations also provide major opportunities for discovering very weakly coupled states. In order to produce neutrinos, experiments such as LSND collide thousands of Coulombs of protons into fixed targets, while MINOS and MiniBooNE also focus and then dump beams of muons. The neutrino detectors beyond these beam dumps are therefore an excellent arena in which to look for long-lived pseudoscalars or for vector bosons that kinetically mix with the photon. We show that these experiments have significant sensitivity beyond previous beam dumps, and are able to partially close the gap between laboratory experiments and supernovae constraints on pseudoscalars. Future upgrades to the NuMI beamline and Project X will lead to even greater opportunities for discovery. We also discuss thin target experiments with muon beams, such as those available in COMPASS, and show that they constitute a powerful probe for leptophilic PNGBs.

Progress in hadronic physics modelling in Geant4

Geant4 offers a set of models to simulate hadronic showers in calorimeters. Recent improvements to several models relevant to the modelling of hadronic showers are discussed. These include improved cross sections, a revision of the FTF model, the addition of quasi-elastic scattering to the QGS model, and enhancements in the nuclear precompound and de-excitation models. The validation of physics models against thin target experiments has been extended especially in the energy region 10 GeV and below. Examples of new validation results are shown.

Testing suite for validation of Geant4 hadronic generators

The testing suite for validation of Geant4 hadronic generators with the data for thin target experiments has been updated. We automated and extended the suite providing a production mode of operation. The new data sets for incident proton beam momentum up to 12.9 GeV/c have been included. The results of comparisons with the inclusive neutron and pion production data for different Geant4 hadronic generators are shown.

γ-ray spectroscopy of the neutron-rich nucleiRb89,Y92, andY93with multinucleon transfer reactions

The positive-parity yrast states in the ^{89}Rb, ^{92}Y, and ^{93}Y nuclei were studied using $\gamma$-ray spectroscopy with heavy-ion induced reactions. In the multinucleon transfer reactions ^{208}Pb+^{90}Zr (590 MeV) and ^{238}U+^{82}Se (505 MeV), several $\gamma$-ray transitions were identified in these nuclei by means of coincidences between recoiling ions identified with the PRISMA spectrometer and $\gamma$ rays detected with the CLARA $\gamma$-ray array in thin target experiments. Level schemes were subsequently determined from triple- $\gamma$ coincidences recorded with the GASP array in a thick target experiment, in the reactions produced by a 470 MeV ^{82}Se beam with a ^{192}Os target. The observed level schemes are compared to shell-model calculations.

Observation of states beyond band termination in 156, 157, 158Er and strongly deformed structures in 173, 174, 175Hf

High-spin terminating bands in heavy nuclei were first identified in nuclei around 158Er90. While examples of terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work, the high-spin structure of 156, 157, 158Er has been studied using the Gammasphere spectrometer. The subject of triaxial superdeformation and ‘wobbling’ modes in Lu nuclei has rightly attracted a great deal of attention. Very recently four strongly or superdeformed (SD) sequences have been observed in 174Hf, and cranking calculations using the Ultimate Cranker code predict that such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment took place to search for linking transitions between the SD bands, possible wobbling modes, and new SD band structures.

Beyond band termination in 157Er and the search for wobbling excitations in strongly deformed 174Hf

High-spin terminating bands in heavy nuclei were first identified in nuclei around 158Er90. While examples of special terminating states have been identified in a number of erbium isotopes, almost nothing is known about the states lying beyond band termination. In the present work the high-spin structure of 157Er has been studied using the Gammasphere spectrometer.The subject of triaxial superdeformation and ‘wobbling’ modes in Lu nuclei has rightly attracted a great deal of attention. Very recently, four strongly or superdeformed (SD) sequences have been observed in 174Hf and ultimate cranker calculations predict such structures may have significant triaxial deformation. We have performed two experiments in an attempt to verify the possible triaxial nature of these bands. A lifetime measurement was performed to confirm the large (and similar) deformation of the bands. In addition, a high-statistics, thin-target experiment was run to search for linking transitions between the SD bands and possible wobbling modes.

Spectroscopy in theZ=49108,110Inisotopes: Lifetime measurements in shears bands

Excited states have been populated in ${}^{108}\mathrm{In}$ and ${}^{110}\mathrm{In}$ in complementary backed- and thin-target experiments using the Stony Brook and the $8\ensuremath{\pi}\mathrm{Ge}$-detector arrays. The level schemes for both isotopes have been extended and modified, including the first observation of $\ensuremath{\Delta}I=2$ bands in ${}^{110}\mathrm{In}.$ Lifetimes of states in four $\ensuremath{\Delta}I=1$ bands and one $\ensuremath{\Delta}I=2$ band have been measured using the Doppler-shift attenuation method. Experimental total angular momenta and reduced transition strengths for the $\ensuremath{\Delta}I=1$ bands have been compared with tilted axis cranking predictions for shears bands with configurations involving one proton ${g}_{9/2}$ hole and one or three valence quasineutrons from the ${h}_{11/2}$ and ${g}_{7/2}{/d}_{5/2}$ orbitals. The $\ensuremath{\Delta}I=2$ bands have been compared with principal axis cranking predictions for configurations with two ${g}_{9/2}$ proton holes and a ${g}_{7/2}$ or ${d}_{5/2}$ proton and one- or three-quasineutron configurations involving the ${h}_{11/2}$ and ${g}_{7/2}{/d}_{5/2}$ orbitals. In general, there is good overall agreement for both the angular momenta and reduced transition strengths. The $\ensuremath{\Delta}I=1$ and $\ensuremath{\Delta}I=2$ bands have large ${\mathcal{J}}^{(2)}/B(E2)$ ratios as expected for the shears mechanism. The $B(M1)$ strengths deduced for the $\ensuremath{\Delta}I=1$ bands show a decreasing trend as a function of spin, which is also a feature of the shears mechanism. Configuration assignments have been made for most observed bands based on comparisons with theory and systematics of neighboring nuclei.

Bremsstrahlung: an experimentalist’s personal perspective on the postmodern era

In this brief review I will discuss the recent experimental work on the doubly differential cross section, i.e. the photon energy and angular distribution, for electron bremsstrahlung from thin solid film and gas targets. Since the beginning of the modern era in the study of bremsstrahlung with the publication of the 1971 paper by Tseng and Pratt, Professor Pratt has been the dominant influence in bremsstrahlung research. Most, if not all, experimental research during the modern era has been motivated by the interest in comparing data with the theory of Pratt and his coworkers. As bremsstrahlung research has moved into its postmodern era, new experiments with increasing precision are concentrating on determining under what conditions ordinary bremsstrahlung theory needs to be supplemented by a contribution from polarization bremsstrahlung. Efforts to improve the comparison of thin-target experiment with theory have also led to new experimental and modeling work on bremsstrahlung from thick solid targets. Thick-target bremsstrahlung is interesting in its own right, but we also want to understand it better since it is the ever-present background in the thin-target experiments and the limiting factor in the effort to distinguish the polarization contribution to the total bremsstrahlung spectrum. Professor Pratt ushered in the modern era in bremsstrahlung research and has recently guided the transition into the postmodern era. It can be expected that he will continue to have a formative influence on the developments of bremsstrahlung research into the foreseeable future.

High-spin study of111I

A level scheme is presented for 53111I, populated by the 58Ni(58Ni,αp) reaction. A backed-target experiment, at a low beam energy of 210 MeV, was performed using the JUROSPHERE spectrometer, while a thin-target experiment at 250 MeV was performed using the GAMMASPHERE spectrometer in conjunction with the MICROBALL charged-particle detector array. The new level scheme fits well the systematics of light iodine nuclei and provides evidence for a terminating band at the highest spins. Low-lying transitions previously assigned to 111I could not be confirmed.Received 20 January 2000DOI:https://doi.org/10.1103/PhysRevC.61.064320©2000 American Physical Society

γ-ray spectroscopy in111Te

Excited states in ${}^{111}\mathrm{Te}$ have been studied using the $4pn$ evaporation channel from the ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Ni}$ reaction at a beam energy of 250 MeV. A thin-target experiment was carried out using GAMMASPHERE coupled with the MICROBALL charged-particle detector array and an array of neutron detectors. A complementary thick-target experiment was performed with the Stony Brook six-Ge-detector array in order to measure the lifetime of an ${11/2}^{\ensuremath{-}}$ isomer observed in the GAMMASPHERE data. The structure of ${}^{111}\mathrm{Te}$ at low spin is dominated by the yrast $\ensuremath{\nu}{h}_{11/2}$ band, although a band assigned to the $\ensuremath{\nu}{(g}_{7/2}{d}_{5/2})$ configuration is also observed. Four collective bands have been found at higher spins. Experimental results are compared to the odd-$A$ Te systematics and to core-quasiparticle and total Routhian surface calculations.

Shape competition and shape coexistence in75Br

High-spin states in ${}^{75}\mathrm{Br}$ were populated to investigate shape competition and shape coexistence phenomena in the single and three-quasiparticle regions. The thin-target experiment ${}^{48}\mathrm{Ti}{(}^{30}\mathrm{Si}{,p2n)}^{75}\mathrm{Br}$ at 90 MeV was performed using the FSU Tandem-LINAC accelerator facility, and the FSU-PITT array of 10 Compton-suppressed Ge detectors for the collection of $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence data. Construction of level schemes and assignments of decays, excitation energies, spins, and parities were achieved through examination of coincidence data, relative intensity measurements, and angular correlation analysis. A rotational sequence with band-head spin-parity ${J}^{\ensuremath{\pi}}={\frac{9}{2}}^{+}$ was found and is compared to a similar band in ${}^{77}\mathrm{Br}.$ A negative-parity three-quasiparticle band exhibiting rigid rotation was also discovered. In total, 65 new transitions and 36 new excitation states were added to the level scheme. Cranked-shell, Hartree-Fock-Bogoliubov, and particle-rotor coupling models are used in the interpretation.

Production of stable and radioactive nuclides in thick stony targets ( R = 15 and 25 cm) isotropically irradiated with 600 MeV protons and simulation of the production of cosmogenic nuclides in meteorites

Two artificial meteoroids made out of gabbro with a density of 3 g cm −3 with radii of 15 and 25 cm were isotropically irradiated with 600 MeV protons in order to simulate the production in meteoroids of cosmogonie nuclides by galactic cosmic ray protons. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, Lu, and Au was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si as well as from degassed meteoritic material was determined. Together with earlier results on an artificial meteoroid with a radius of 5 cm, and with data derived from thin-target experiments, the depth dependence of production rates is investigated for radii from 0 to 75 g cm −2. 60Co from Co shows the strongest size dependence; the center production rates differ by a factor of 100 for radii of 5 and 25 cm. Other low-energy products, like 58Co from Co and 24Na produced from Al, increase only up to a factor of 3.5 over the entire range of radii. For extreme high-energy products, in contrast, the center production rates decrease by up to a factor of 10. The observed depth profiles show a wide varity of shapes. Low-energy products have pronounced maxima in the center, high-energy products exhibit strong decreases from surface to center and, in between, essentially flat profiles are seen as well as such with a transition maximum. The spectra of primary protons and of secondary protons and neutrons in the artificial meteoroids were calculated using Monte Carlo techniques. The fluxes of secondary protons and neutrons depend strongly on depth and size, the spectral shapes being different for protons and neutrons. Calculating also the nucleon spectra which result from irradiation with real GCR p-spectra, the differences between simulation experiments and cosmic irradiation conditions are quantitatively described. On the basis of all these spectra and of thin-target excitation functions, production rates were calculated and compared with the experimental ones. The theoretical depth profiles allow to distinguish the different contributions of primary and secondary particles and to unravel the various production modes of cosmogenic nuclides in meteoroids. Our investigation shows that it is possible to model the production of residual nuclides in artificial meteoroids with excellent accuracy by thin-target calculations, provided that reliable thin-target excitation functions are at hand.

The excitation functions of Ba (p, X) M Xe (M — 124-136) in the energy range 38-600 MeV; the use of ‘cosmogenic’ xenon for estimating ‘burial’ depths and ‘real’ exposure ages

Thin target experiments were performed to obtain the excitation functions of the reactions Ba (p, X) MX.e. The abundances of all stable Xe isotopes and of the radionuclides 127Xe and 131Ba were determined by means of rare gas mass spectroscopy and y -counting, respectively. The excitation functions show marked characteristics leading to strong variations in the proton-induced Xe-ratios as functions of energy. The 131Xe/126Xe ratio - the special lunar anomaly-was found to vary from 1.14 ±0.4 (600 MeV) to 248.8 ± 4.0 (75 MeV). The Reproduction rates and the MXe/126Xe ratios as functions of depths were estimated for 271 geometry utilizing the depth dependent galactic-cosmic-ray (g.c.r.) fluxes of Reedy & Arnold (1972). Substantial isotopic variations for all of the proton-induced Xe ratios were found, sufficient to explain most of the cosmogenic Xe ratios (exceptions are 130Xe, and 132Xe) measured yet in lunar samples with proton-induced reactions on Ba at different depths in the Moon. Actual lunar samples are used to check the validity of the results.

Recoil studies of the interaction of 48Ca with 18–36 MeV 4He ions and 5–43 MeV 3He ions

Thin-target thick-catcher recoil experiments are described for the (α, αn), ( 3He, α) and ( 3He, p2n) reactions on neutron-excess 48Ca with 18–36 MeV 4He ions and 5–43 3He ions. Product distributions between the target and catcher foils are compared with Monte Carlo calculations assuming compound nucleus and direct interaction mechanisms. Using these comparisons the (α, αn) and ( 3He, α) excitation functions are broken down into their direct interaction and compound nucleus components. The direct interaction contribution to the (α, αn) reaction at 36 MeV is in the range of 20 % to 40 %. It is found that the thin-target experiments probably afford better differentiation between direct interaction and compound nucleus mechanisms than the comparable thick-target experiments.

Der einquantenzerfall von positonen

Single-quantum annihilation of positions in the field of high- Z nuclei has been studied with the aid of a β-ray spectrometer and a counter-telescope. Performing a thin-target experiment, mono-energetic γ-quanta due to single-quantum annihilation have been observed. The total cross section of this decay mode, measured in the energy range from 260 to 1400 keV, has been found to be in agreement with calculations by Jaeger and Hulme. According to these measurements, a positon of 1500 keV kinetic energy, stopped in a lead absorber, has a probability of 0.17% to annihilate by the emission of a single γ-quantum.