Gunter H. R. Kegel

Prompt fission neutron energy spectra induced by fast neutrons

Abstract Prompt fission neutron energy spectra for 235 U and 239 Pu have been measured for fission neutron energies greater than the energy of the incident neutrons inducing fission. The measurements were undertaken to investigate the shape dependence of the fission neutron spectra upon both the incident neutron energy and the mass of the nucleus undergoing fission. Measurements were made for both nuclides at incident neutron energies of 0.50, 1.50, 2.50 and 3.50 MeV. The data are presented either as relative yields or as ratios of measured spectra to that of 235 U at 0.50 MeV. Incident neutrons were produced by the 7 Li(p,n) 7 Be reaction using a pulsed, bunched proton beam from the 5.5 MV Vand de Graaff accelerator at the University of Massachusetts Lowell Radiation Laboratory. Fission neutrons were detected by a thin liquid scintillator with good time resolution capabilities. Neutron energies were determined by time-of-flight techniques with pulse-shape discrimination to reduce gamma-ray background levels. The measurements are compared to calculations using the Los Alamos model of Madland and Nix to test the predictive capabilities of the model. The calculations were performed prior to the measurements. The data are fit by the Watt equation to determine the mean energy of the spectra and facilitate comparison of the results to previous measurements.

Fast neutron generation with a type CN Van de Graaff accelerator

Abstract We have used the University of Lowell accelerator to generate fast neutrons by irradiating a thick metallic lithium target with protons. The neutron radiation field resembles that of a point source. A computer code has been written to obtain neutron energy spectra and fluences as functions of position. At the highest fluence point 10 14 neutrons/cm 2 is obtained in 90 min with a 4-MeV, 20-μA proton beam. This is not a large fluence rate when compared to that of a research reactor but our gamma intensity is much lower and slow neutrons are virtually absent so that irradiated materials show little induced radioactivity. The design of our station will be described and its use for the irradiation of electronic devices and other objects will be reviewed.

Fluences and spectra of accelerator generated fast neutrons

A fast neutron irradiation station has been designed at the University of Massachusetts at Lowell. The fluence delivered by this station is less than that of a nuclear reactor, but it is essentially free of slow neutrons and the admixture of gamma rays is small. The station, dosimetry methods, a computer code, verification techniques, and applications are described. The information provided should enable others to construct a similar station on type CN or tandem accelerators. >

Neutron irradiation of insulated gate field effect transistors

Insulated gate field effect transistors and polysilicon-gated capacitors were irradiated with fast (10 keV <E < 2 MeV) neutrons. As expected, damage to the bulk silicon was detected as a degradation in the minority carrier lifetime. Optically assisted electron injection was employed for the first time to examine neutral electron trap and fixed positive charge generation in the gate insulator of the devices. While fixed positive charge densities of ≤6 x 1010 cm−2 were detected, little or no neutral electron trap generation was observed. The small density of coulombic defects observed in the insulator could be accounted for fully by the known flux of gamma rays associated with the neutron irradiation process. This indicates that fast neutrons passing through a thin gate oxide do not produce significant amounts of damage in the oxide. Somewhat surprisingly, it was found that 1.5 keV X-rays created similar lifetime degradation effects in the bulk silicon, as did fast neutrons, even though this photon energy is not believed to be capable of producing bulk damage in the form of atom displacement in either the semiconductor or the insulator. The minority carrier lifetime of the silicon could be restored to initial values following either neutron or x-ray exposure by annealing in H2 for 30 min at 400° C.

Charged particle induced X-ray analysis: A new tool in forensic science

The application of the charged particle X-ray excitation technique (CPXE) to the problem of determination of residues from gun firing is described. Preliminary results indicate that certain elements, including S, Ba, Fe, and Pb can be detected in statistically significant larger amounts on firing hands than on nonfiring hands with 90 to 98% confidence limits. Results for other elements including K, Ca, Sb, Cr, Mn, Ni, Cu, are also discussed. The simplicity on the technique, together with the number of elements detected, offers advantages over other techniques for firearm discharge residue detection.

Precipitation of Lithium in Germanium during Ion Drift

The precipitation of lithium in germanium has been investigated by Morin and Reiss and by Swalin and Weltzin. The theories developed by these investigators have been used to calculate the rate of lithium precipitation in a typical lithium-drifted germanium detector. It is found that precipitation occurs predominantly in the n+ - region of the detector. Precipitation in the intrinsic layer can be avoided, in general, by maintaining the lithium solubility at a high value in this layer. This will be the case if the electron concentration in the intrinsic layer is kept at a small value at all times.

Studies of a Poenitz-type black neutron detector as a neutron flux monitor

Abstract A black neutron detector of novel design has been constructed and tested as a neutron flux monitor. The neutron time-of-flight (TOF) technique was used to measure the zero degree neutron spectrum from a subnanosecond pulsed proton beam on a thick metallic lithium target. We describe the detector, discuss the shielding arrangement and electronics, determine the low-energy cut-off. We calculate the efficiency, show spectra and compare measured zero degree differential cross-section of the 7 Li(p, n 0 ) 7 Be reaction with recommended published cross-sections.

Neutron scattering angular distributions in 239Pu at 570 and 700 keV

Neutron elastic and inelastic scattering in 239 Pu have been studied via the time-of-flight technique. Neutrons were generated by the 7 Li(p, n) 7 Be reaction at the University of Massachusetts Lowell 5.5-MV Van de Graaff Accelerator Laboratory. Angular distributions were measured at 570 and 700 keV for two level groups, ground state plus first excited state, and second plus third excited state. The plutonium sample was disk-shaped with a mass of 28. 7 g. The angle-integrated cross sections obtained for the two scattered neutron groups, one corresponding to the elastic plus 7.9 keV level and the other corresponding to the inelastic 57 keV plus 76 keV levels, were 5864 ± 264 mb and 570 ± 42 mb, respectively, for 570-keV incident neutrons and 5060 ± 308 mb and 518 ± 62 mb, respectively, for 700-ke V incident neutrons. The results are compared with ENDF/B-VI and with the measurement of Haouat et al. at 700 keV.

Measurement of the lithium isotopic concentration

Abstract A method for determining the atomic proportions of 6 Li and 7 Li isotopes in enriched 6 LiF, 7 LiF and lithium metallic samples is presented. This study is based on measuring the energy spectra of alpha particles from proton-induced 7 Li(p,α) 4 He and 19 F(p,α) 16 O nuclear reactions and those of backscattered protons. The enriched LiF targets were obtained by thermal evaporation of thermoluminescent dosimetry chips, TLD-700 ( 7 LiF) and TLD-600 ( 6 LiF), onto self-supporting carbon substrates. Targets of metallic lithium were prepared in the same way. The 6 Li enriched TLD-600 sample was found to contain 4.78 at.% of 7 Li; however, no significant amount of 6 Li was found in the TLD-700 sample. A 7.57 at.% concentration of 6 Li in lithium metallic target was determined by using the TLD-700 as a 7 Li isotope content standard. This value is consistent with the reported 7.5 at.% natural abundance of 6 Li.

Tuning of the Schottky barrier height using bi‐metallic layered structures

Bi‐metallic Schottky contacts of Cr‐Al on p‐type Si using a layered structure have been investigated. In these contacts, the thickness of the inner layer in contact with Si was varied, while that of the outer metal layer was kept constant. Our studies indicate that the barrier height changes with the thickness of the inner metal layer. Furthermore, the morphology of our samples was examined with a transmission electron microscope (TEM) which indicates the presence of inhomogeneous mixing of Cr and Al.