J.F. Harmon

New positron annihilation spectroscopy techniques for thick materials

Abstract The Idaho Accelerator Center (IAC) has developed new techniques for positron annihilation spectroscopy (PAS) by using highly penetrating γ -rays to create positrons inside the material via pair production. Two sources of γ -rays have been employed. Bremsstrahlung beams from small-electron linacs (6xa0MeV) were used to generate positrons inside the material to perform Doppler-broadening spectroscopy. A 2xa0MeV proton beam was used to obtain coincident γ -rays from 27 Al target and enable lifetime and Doppler-broadening spectroscopy. This technique successfully measured stress/strain in thick samples, and showed promise to extend PAS into a variety of applications.

Doppler broadening measurements of positron annihilation using bremsstrahlung radiation

Abstract The first system to measure Doppler broadening of positron annihilation based on during electron-pulse bremsstrahlung radiation has been constructed and demonstrated. No photon-induced activation or positron emitters are involved in the process. The collimated bremsstrahlung radiation from a small electron accelerator, which exhibits excellent penetrability, is used to generate positrons inside the sample via pair production. The annihilation photons are recorded by a HPGe detector. The line-shape parameters of Doppler broadening can be used to identify defects in pure metals and alloys. The dependence of these parameters on different elements has been measured and shows promise as a probe of momentum of electronic wave-functions in pure and composite materials. This method also shows promise as an additional tool for measuring elemental composition, when used in conjunction with accelerator-based X-ray fluorescence.

Non-Rutherford elastic scattering in fluorine

Abstract Nuclear scattering of protons by light elements may give rise to cross sections that are significantly greater than Rutherford scattering. We present graphical and tabular cross section data for proton scattering from fluorine for proton energies of 0.85 MeV to 1.01 MeV (ΘLAB = 165°) and 1.00 MeV to 1.875 MeV (ΘLAB = 153°).

Non-Rutherford proton scattering in RBS spectra

It has long been known that it is possible to take advantage of non-Rutherford scattering to enhance the sensitivity of backseattering spectrometry for light elements. We have utilized recently published proton cross section data to demonstrate the utility of this phenomenon for enhanced sensitivity for carbon and other light elements. RBS spectra illustrating the enhancement for C and F and Si in a number of materials are presented.

Cross sections and astrophysical S̃ factor for the 7Li(p, α) reaction at low energies

Abstract The energy dependence of the total cross section and the astrophysical S factor down to 20 keV are presented for the 7Li(p, α) reaction. The results were obtained by using the known S factor for the 6Li(p, α) reaction and the slopes of thick target yield vs energy for the two reactions. This relative method avoids several major difficulties with thick target measurements at low energies. Good agreement is obtained with the higher energy results of Spinka, Tombrello, and Winkler and the results of Rolfs and Kavanagh.

Positron lifetime measurements by proton capture

A positron lifetime spectroscopy (PLS) technique was developed using coincident γ rays induced by proton capture. Proton capture in some light elements induce coincident MeV γ rays, allowing positron lifetime to be measured. One γ quantum provides a start signal for the positron lifetime spectrometer, whereas the other γ quantum bombards the sample under investigation, generating a positron inside it through pair production. The stop signal is obtained from the detection of one of the two 511keV photons emitted from positron annihilation with the sample electrons. This new technique can extend PLS, which is a powerful tool to identify the size and concentration of defects, to thick materials and a broad range of applications. It also eliminates the source contribution from the measured spectra, which may lead to the identification of more defect types in a sample.

INEL and ISU BNCT research using a 2 MeV RFQ-based neutron source

Abstract A radio frequency quadrupole (RFQ) proton linear accelerator manufactured by AccSys Corp. was purchased by the U.S. Department of Energy and was installed in the Particle Beam Laboratory at Idaho State University (ISU). It is available for physics studies consistent with the INEL mission such as those related to accelerator produced neutron sources for boron neutron capture therapy (BNCT) and waste interrogation. It is an AccSys model PL-1 and is designed to produce 2 MeV protons at an average current of 150 μA. The overall objective of the INEL BNCT/ISU collaborative program is to evaluate neutron filter design concepts which use a 2 MeV proton accelerator with a lithium target as the neutron source. This paper will discuss the overall plan of INEL/ISU collaborative program and how it relates to other university and government laboratory studies, the methods being employed in this study and results of neutron spectra and angular distribution measurements for different lithium target configurations.

Use of the 9Be(α, n)12C reaction for detection of beryllium

Abstract A strong, sharp resonance in the cross section of the 9 Be (α, n ) 12 C reaction gives the possibility of profiling beryllium. The resonance occurs near 620 keV, an energy around the maximum of the stopping power of the α-particles in beryllium. The excitation function of the reaction was measured with a 4 π neutron detector using thin Be films on Ta in the energy range of 480–740 keV. The peak value of the cross section is about 0.21 mb and its width is less than 6 keV. Particular interest is in the possibility of characterizing 9Be layer on the plasma facing surfaces of tokamak materials.