Gamma-ray and neutron spectrometers

A neutron spectrometer separates detected neutrons into different energy ranges (thermal, epithermal, fast). Proportional counters containing 3He are commonly used for 

There are a number of possible sensor options for a y-ray spectrometer. These include a germanium sensor or scintillators made of various synthetic materials. Elements that are routinely analyzed with y-rays include silicon, iron, titanium, magnesium, calcium, and aluminum, plus the radioactive elements potassium and thorium (uranium concentrations are usually too low). 

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Usui, T. and McSween, H. Y. (2007) Geochemistry of 4 Vesta based on HED meteorites prospective study for interpretation of gamma-ray and neutron spectrometer for the Dawn mission. Meteoritics and Planetary Science, 42, 255-269. 

The Lunar Prospector orbiter carried a gamma-ray/neutron spectrometer (GRS) that made precise measurements of the concentration and distribution of thorium (Lawrence et al., 1998) and hydrogen (Feldman et al., 2001). Subsequent spectral deconvolutions (Prettyman et al., 2006) have produced analyses of iron, titanium, potassium, magnesium, aluminum, calcium, and silicon. The principles of these analytical techniques are explained in Box 13.1. 

Comparison of titanium contents of lunar mare basalts, as measured from neutrons by the Lunar Prospector gamma-ray spectrometer and from reflectance spectra by the Clementine spacecraft. After Prettyman et al. (2006). 

Neutron activation has been used to determine the concentrations of trace elements in polyethylene. A procedure has been optimized which involves three irradiations with a SICWFOKE nuclear reactor and four counts with a gamma-ray spectrometer. Phosphorus is determined with beta-ray spectrometry. The detection limits, most of which are below one ppm, have been determined for 42 elements. The merits of the method are discussed in terms of sensitivity, accuracy, ease of use, interferences, and freedom from contamination. 

Lunar Prospector s two most important geochemical mapping sensors were designed for gamma-ray spectrometry (GRS) and neutron spectrometry. The neutron spectrometer, a matched pair of He gas proportional counters, was mainly designed to map the global... 

In practice, nuclear activation analysis is carried out by placing thesample to be examined in a nuclear reactor. The neutrons available in the reactor bring about the n/c (neutron/gamma ray) reactions described above. The radioactive sample is then removed from the reactor and examined with a gamma ray spectrometer. This device measures the type and intensity of radiation released by the sample. These data can then be compared to standard tables to determine which elements and the amounts of each are present in the sample. 

The first identification of solar neutrons at Earth took place on June 3,1982 (Fig. 2a), by neutron monitor measurements at Jungfraujoch, Lomnicky Stit, and Rome [Debrunner et al., 1983 Chupp et al., 1987], This was two years after the discovery of solar neutrons in near-Earth space by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) satellite [ Chupp et al., 1982], Thereafter, standardized neutron monitors were set up at favorable observational locations at Earth, such as Haleakala, Hawaii [Pyle and Simpson, 1991], Additionally, new ground-based detectors with enhanced sensitivity to solar neutrons were developed [e.g. Shibata et al., 1991 Muraki et al., 1993]. 

Making an Activation-Analysis Measurement. The most prominent technique in nuclear analytical chemistry is Instrumental neutron activation (INAA), in which thermal neutrons from a nuclear reactor are used to irradiate the sample and the induced radionuclides are measured nondestructively with a germanium gamma-ray spectrometer. Sensitivity may be enhanced by chemically separating the elements of interest before radionuclide assay. 

Table III gives the conditions, data and reactions for the long irradiations of iridium, osmium and ruthenium. Samples and standards were irradiated in the University of London Reactor under a maximum thermal neutron flux of 1.4 x 10 n cm" s". The samples were counted using a lithium drifted germanium detector (Ortec Inc) linked to a computer based gamma ray spectrometer (Nuclear Data Inc. 6620 Multichannel Analyser). A general Neutron Activation Package written in FORTRAN IV was employed to run a peak search and calculate PGM concentrations in the plant samples. The irradiation of platinum is a special case and details are given in Table IV. Various nuclides emit y-rays of similar energy, and in INAA these become a serious interference in the determination of platinum in biological samples.

The study of nuclear gamma-ray lines that are produced by isomeric transitions in Ge detectors offers the possibility of estimating the neutron spectrum inside the shield. This process constitutes an important source of background in gamma-ray spectrometers flown in the upper atmosphere and in earth orbit. 

Bertrand GHV, Hamel M, Normand S, Sguerra F (2015) Pulse shape discrimination between (fast or thermal) neutrons and gamma rays with plastic scintillators state of the art. Nucl Instmm Methods Phys Res Sect A Accelerators Spectrometers Detectors Assoc Equip 776 114-128... 

The experiments were conducted using trace elements solution containing different concentrations. Each treatment was replicated three times. The rhizome and frond were analyzed for inorganic arsenic. Total arsenic contents in soil and plant were determined by instrumental neutron activation analyses (INAA), Nuclear Reactor lEA-Rlm. The sample irradiation was carried out at the nuclear reactor for 8 h and under thermal neutron flux of 2.7 X 10 ncm s subsequently, a decay time of 3 days and 3 h counting time were employed. The radionuclide activities were measured using a gamma-ray spectrometer comprising a hyper-pure GE detector and associated electronic system. The spectra of the samples were measured under the same experimental conditions of the standard photopeak of 657 keV of As. The amount of anion absorbed was calculated as the difference between the anion concentration before and after reaction with the sod-plant systems. 

A high resolution and high throughput gamma ray spectrometer with a Ge detector is required. A computer based multi-channel analyzer (MCA) with software to control data acquisition and to analyze spectra is also required. The detector assembly must be well shielded to minimize neutron activation of the detector itself and to reduce the background components. 

Beads of mass 5-10 mg were first cleaned ultrasonically, as required. They were stored individually in 1.2 mL polyethylene vials, were irradiated serially for five minutes at a neutron flux of 1.0 10 neutrons.cm. sec. Five to seven minutes after irradiation, the induced radioactivity was counted for five minutes using a hyper-pure germanium detector-based gamma-ray spectrometer. This produced analytical concentration data for cobalt (Co), tin (Sn), copper (Cu), sodium (Na), aluminium (Al), manganese (Mn), chlorine (Cl) and calcium (Ca). The samples were recounted for five to thirty-three minutes the next day to measure the concentrations of the longer-lived radioisotopes of Na, arsenic (As),... 

In the area of planetary studies, NASA s Mars Odyssey mission is also scheduled for launch in 2001. Among its suite of instruments are a gamma-ray spectrometer and two neutron detectors. These will be used to fully map the Martian surface and determine its elemental composition. The neutron and gamma-ray measurements in combination will also be used to obtain an estimate of the water content of the Martian near-surface. 

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