Reaction Gamma-rays Research Articles

Fast-ion energy resolution by one-step reaction gamma-ray spectrometry

The spectral broadening of γ-rays from fusion plasmas can be measured in high-resolution gamma-ray spectrometry (GRS). We derive weight functions that determine the observable velocity space and quantify the velocity-space sensitivity of one-step reaction high-resolution GRS measurements in magnetized fusion plasmas. The weight functions suggest that GRS resolves the energies of fast ions directly without the need for tomographic inversion for selected one-step reactions at moderate plasma temperatures. The D(p,γ)3He reaction allows the best direct fast-ion energy resolution. We illustrate our general formalism using reactions with and without intrinsic broadening of the γ-rays for the GRS diagnostic at JET.

Velocity-space observation regions of high-resolution two-step reaction gamma-ray spectroscopy

High-resolution γ-ray spectroscopy (GRS) measurements resolve spectral shapes of Doppler-broadened γ-rays. We calculate weight functions describing velocity-space sensitivities of any two-step reaction GRS measurements in magnetized plasmas using the resonant nuclear reaction 9Be(α, nγ)12C as an example. The energy-dependent cross sections of this reaction suggest that GRS is sensitive to alpha particles above about 1.7 MeV and highly sensitive to alpha particles at the resonance energies of the reaction. Here we demonstrate that high-resolution two-step reaction GRS measurements are not only selective in energy but also in pitch angle. They can be highly sensitive in particular pitch angle ranges and completely insensitive in others. Moreover, GRS weight functions allow rapid calculation of γ-ray energy spectra from fast-ion distribution functions, additionally revealing how many photons any given alpha-particle velocity-space region contributes to the measurements in each γ-ray energy bin.

Alpha particle induced gamma yields in uranium hexafluoride

Fluorine has a relatively large (α,n) production cross-section in the MeV range, the energy range of interest for special nuclear materials. In the uranium fuel cycle enriched UF6 in particular is a reasonably prolific source of (α,n) neutrons because along with 235U, 234U becomes enriched and it has a relatively short half-life. This enables the mass content of storage cylinders containing UF6 to be verified by neutron counting methods.In association with such measurements high resolution gamma-ray spectrometry (HRGS) measurements using a high-purity Ge detector are often undertaken to determine the 235U enrichment based off the intensity of the direct 186keV line. The specific (α,n) neutron production, neutrons per second per gram of U, is sensitive to the relative isotopic composition, particularly the 234U concentration, and the traditional gross neutron counting approach is needed to quantitatively interpret the data.In addition to F(α,n) neutrons, α-induced reaction γ-rays are generated, notably at 110, 197, 582, 891, 1236 and 1275keV. If one could observe 19F(α,xγ) gamma-lines in the HRGS spectra the thought was that perhaps the α-activity could be estimated directly, and in turn the 234U abundance obtained. For example, by utilizing the ratio of the detected 197–186keV full energy peaks. However, until now there has been no readily available estimate of the expected strength of the reaction gamma-rays nor any serious consideration as to whether they might be diagnostic or not.In this work we compute the thick target yields of the chief reaction gamma-rays in UF6 using published thin target data. Comparisons are made to the neutron production rates to obtain γ/n estimates, and also to the 235U decay line at 186keV which we take as a fiducial line. It is shown that the reaction gamma-rays are produced but are far too weak for practical safeguards purposes.Now that the underlying numerical data is readily available however, it can be used to support neutron and gamma production calculations in other fluorine compounds, for example impure plutonium reference materials where fluorine may be present only at the parts per million by weight level yet still present a serious nuisance addition to the neutron production rate.

Electromagnetic transition rates in 100,101Pd using the Recoil Doppler Shift Technique

The quadrupole deformations for the low-lying states in the transitional nuclei 100,101Pd have been deduced through the measurement of their electric quadrupole transition probabilities using the Recoil Distance Doppler Shift Method. The nuclei were studied using a 268MeV 80Se beam impinging on a thin, self-supporting 24Mg target. States in 100Pd and 101Pd populated by the four and three neutron evaporation channels respectively, with reaction gamma-rays detected using the SPEEDY gamma-ray detection array. The recoiling nuclei were stopped in a copper foil and gamma-ray coincidence data taken at 10 separate target-stopper distances between 35μm and 750μm. The mean-lifetimes for the lowest lying 2+ (in 100Pd) and 15/2− (in 101Pd) states were measured to be 13.3(9)ps and 10.8(8)ps respectively. These data are compared with predictions from nuclear Total Routhian Surface calculations, which are found to agree with the experimentally deduced values to within 10%.

Photofission in uranium by nuclear reaction gamma-rays

The ideal photon source for active interrogation of fissile materials would use monoenergetic photons to minimize radiation dose to surroundings. The photon energy would be high enough to produce relatively large photofission signals, but below the photoneutron threshold for common cargo materials in order to reduce background levels. To develop such a source, we are investigating the use of low-energy, proton-induced nuclear reactions to generate monochromatic, MeV-energy gamma-rays. Of particular interest are the nuclear resonances at 163keV for the 11B(p,γ)12C reaction producing 11.7MeV gamma-rays, 340keV for the 19F(p,αγ)16O reaction producing 6.13MeV photons, and 441keV for the 7Li(p,γ)8Be reaction producing 14.8 and 17.7MeV photons. A 700keV Van de Graaff ion accelerator was used to test several potential (p,γ) materials and the gamma-ray yields from these targets were measured with a 5″×5″ NaI detector. A pulsed proton beam from the accelerator was used to induce prompt (neutron) and delayed (neutron and gamma-ray) photofission signals in uranium which were measured with 3He and NaI detectors. We show that the accelerator data is in good agreement with Monte Carlo radiation transport calculations and published results.

High resolution gamma-ray spectroscopy with a radioactive beam

The gamma-rays de-exciting the yrast states in neutron deficient nuclei around A-55 have been observed from fusione-vaporation reactions induced by an intense radioactive Ne-19 beam on a Ca-40 target. An array of escape suppressed germanium detectors recorded reaction gamma-rays. Background subtraction of radiation from the decay of the beam was achieved using the timing properties of the pulsed beam and through particle detection using the LEDA charged particle array. These data constitute the first use of an on-line separated and accelerated radioactive beam for studies of near yrast nuclear states using fusion-evaporation reactions. Information on the gamma-ray yields from evaporation residues, relative to yields with a stable beam, has been obtained and the problems associated with performing such experiments in a highly radioactive background environment have been addressed.

Neutron Tomography: A Survey and Some Recent Applications

ABSTRACTA survey is given of recent developments in selected areas of neutron tomography, within the context of several applications Argonne is involved in, including high penetration of reactor-fuel bundles in thick containers (involving TREAT and NRAD facilities), dual-energy hydrogen imaging (performed at IPNS), dynamic coarse-resolution emission tomography of reactor fuel under test (a proposed modification to the TREAT hodoscope), and an associated-particle system that uses neutron flight-time to electronically collimate transmitted neutrons and to tomographically image nuclides identified by reaction gamma-rays.

A high-energy gamma-ray detector system for fast neutron-induced reactions

We have developed a gamma-ray detector system which is used in conjunction with a pulsed spallation neutron source to study fast neutron induced reactions. The detector system consists of five 7.6− × 7.6-cm bismuth germanate crystals that span an angular range from 45° to 140° in the reaction plane. The sample is located 7.2 m from the neutron-production target. With this apparatus it is possible to simultaneously measure the excitation functions and angular distributions of reaction gamma rays in the neutron energy range from 1 to 100 MeV. Results on the 12C(n,n′ γ = 4.44 MeV) and 40Ca(n, γ 0) reactions show the unique capabilities of this system.

A cryopumped transmission gas target for nuclear strucuture studies

A differentially pumped transmission gas target suitable for study of capture reaction gamma rays, elastic scattering and particle reactions has been developed for use with the Queen's University 4 MV Van de Graaff. The primary pumping stage is cryogenic and the target gas may be recovered which makes use of separated isotopes practical.

An optimization technique applied to experiments measuring reaction gamma-ray angular distributions

A general mathematical technique is presented for optimization of any experiment whose data is fit with a model. As an example, this technique is applied to the optimization of experiments measuring the angular distribution of reaction gamma-rays.

Pair Measurement of Gamma-Rays with a Lens Spectrometer

A conventional lens spectrometer has been adapted to the measurement of high energy gamma-rays by detecting positron-electron pairs in coincidence. The source and converter are centered on the optic axis and the baffle system selects pairs of nearly equal energy. The detection method consists of dividing the focal circle of confusion in half and observing coincidences between particles entering these two regions. Statistically the components strike different areas in half the cases. The detectors are semicircular anthracene crystals connected by light pipes to 1P21 photomultiplier tubes. Pulses from these are fed through amplifiers to a coincidence circuit of 7×10−8 sec resolving time. The method has been tested by observing ``pair peaks'' due to the 2.76- and 2.62-Mev gamma-rays of Na24 and ThC″, respectively. The Po–Be reaction gamma-ray has been measured at 7.5 percent resolution and found to have an energy of 4.47±0.04 Mev. Using a proton beam from the Brookhaven electrostatic accelerator gamma-rays of 6.19±0.08 Mev and 7.13±0.12 Mev occurring in the F19(p,α)O16 reaction have been observed with this apparatus. The results on both Po–Be and the fluorine reaction are in agreement with previous measurements by other methods.

Gamma-Ray Absorption Measurements

Gamma-ray absorption measurements have been made using a NaI energy selective scintillation counter with radioactive source gamma-rays of 0.411, 0.664, 1.33, and 2.62 Mev energy. A range of absorbers from hydrogen to uranium was used. The final absorption coefficient was determined to \ifmmode\pm\else\textpm\fi{}0.2 percent rootmean-square statistical accuracy. Additional absorption measurements were made with nuclear reaction gamma-rays at the energies 4.47, 6.13, and 17.6 Mev with the same absorbers. A statistical accuracy of \ifmmode\pm\else\textpm\fi{}2 percent was attained. A comparison of theory to experiment is presented and no radical differences are observed.