Compton Suppression Research Articles

An evaluation of atmospheric deposition of trace elements into the Great Lakes.

High-volume air samplers were used to collect aerosol samples on Whatman 41 air filters at the Canadian air sampling stations Burnt Island, Egbert, and Point Petre. Once collected, the samples were analyzed for trace elements by neutron activation analysis. Air concentrations of over 30 trace elements were determined. A special focus was made to utilize Compton suppression gamma-ray spectroscopy and epithermal irradiations to enhance the detection limits of neutron activation analysis. These techniques allowed for the determination of trace elements at very low levels. Results of the study of the trace-metal dry deposition into Lakes Huron and Ontario indicated that the majority of the total deposition resulted from crustal materials. However, dry deposition is also a significant pathway for many toxic anthropogenic trace metals into the Great Lakes.

An experimental method for the optimization of anti-Compton spectrometer

Abstract The reduction of the Compton continuum can be achieved using a Compton suppression shield. For the first time, an experimental method is purposed for estimating the optimum dimensions of such a shield. The method can also provide information on the effect of the air gap, source geometry, gamma-ray energy, etc., on the optimum dimension of the active shield. The method employs the measurements of the Compton suppression efficiency in two dimensions using small size scintillation detectors. Two types of scintillation materials; NaI(Tl) and NE-102A plastic scintillators, were examined. The effect of γ-ray energy and source geometry were also investigated using 137 Cs and 60 Co sources with three geometries; point, cylindrical, and Marinelli shapes. The results indicate the importance of both NaI(Tl) and NE-102A guard detectors in surrounding the main detector rather than the distance above it. The ratio between the part of the guard detector above the surface of the main detector to that below it should be about 1:3 for NaI and 1:4 for NE-102A. The optimum dimensions for NaI(Tl) detector were found to be 12.5–20 cm for the height and 15–20 cm for the thickness. The corresponding values for NE-102A are 17.5–25 cm for the height and 25–30 cm for the thickness.

Quantitative Compton suppression spectrometry at elevated counting rates

For quantitative Compton suppression spectrometry the decrease of coincidence efficiency with counting rate should be made negligible to avoid a virtual increase of relative peak areas of coincident isomeric transitions with counting rate. To that aim, a separate amplifier and discriminator has been used for each of the eight segments of the active shield of a new well-type Compton suppression spectrometer, together with an optimized, minimum dead-time design of the anticoincidence logic circuitry. Chance coincidence losses in the Compton suppression spectrometer are corrected instrumentally by comparing the chance coincidence rate to the counting rate of the germanium detector in a pulse-counting Busy circuit (G.P. Westphal, J. Rad. Chem. 179 (1994) 55) which is combined with the spectrometer’s LFC counting loss correction system. The normally not observable chance coincidence rate is reconstructed from the rates of germanium detector and scintillation detector in an auxiliary coincidence unit, after the destruction of true coincidence by delaying one of the coincidence partners. Quantitative system response has been tested in two-source measurements with a fixed reference source of 60Co of 14 kc/s, and various samples of 137Cs, up to aggregate counting rates of 180 kc/s for the well-type detector, and more than 1400 kc/s for the BGO shield. In these measurements, the net peak areas of the 1173.3 keV line of 60Co remained constant at typical values of 37 000 with and 95 000 without Compton suppression, with maximum deviations from the average of less than 1.5%.

Spatial resolution attainable in germanium detectors by pulse shape analysis

There are several applications for which it is desirable to calculate the locations and energies of individual gamma-ray interactions within a high purity germanium (HPGe) detector. These include gamma-ray imaging and Compton suppression. With a segmented detector this can be accomplished by analyzing the pulse shapes of the signals from the various segments. We examine the fundamental limits to the spatial resolution attainable with this approach. The primary source of error is the series noise of the field effect transistors (FETs) at the inputs of the charge amplifiers. We show how to calculate the noise spectral density at the output of the charge amplifiers due to an optimally selected FET. This calculation is based only on the detector capacitance and a noise constant for the FET technology. We show how to use this spectral density to calculate the uncertainties in parameters, such as interaction locations and energies, that are derived from pulse shape analysis using maximum likelihood estimation (MLE) applied to filtered and digitized recordings of the charge signals. Example calculations are given to illustrate our approach. Experimental results are given that demonstrate that one can construct complete systems, from detector through data analysis, that come near the theoretical limits.

Portable LN 2 gamma-ray spectrometer system

The purpose of this program is to design and demonstrate a highly sensitive, portable spectrometer for detecting and identifying trace amounts of nuclear materials in a broad range of nonproliferation scenarios. A high-efficiency 100% n-type germanium (Ge) detector is combined with a special liquid nitrogen dewar designed to accommodate an active Compton suppression shield. The added collimation capability is required in order to be able to identify localized sources that may be obscured by background radiation. These components are complemented by the addition of a newly developed state-of-the-art multichannel analyzer (MCA) capable of 64×16 000 channel acquisition and electronic Compton suppression. The entire system fits into a watertight case that could be stored below the seat of commercial airliners. The unit has 2.74 keV resolution at 1.33 MeV, 8.5° angular resolution, low-power, >25 h full-time battery life, and operations between 35 keV< E<3 MeV.

HPGe Compton suppression using pulse shape analysis

We present a new technique for High Purity Germanium (HPGe) Compton suppression using pulse shape analysis (PSA). The novel aspect of our approach involves a complete unfolding of the charge pulse shape into discrete components associated with individual γ-ray interactions. The information thus acquired is then used to favorably reject Compton escape events. The advantage of the current PSA approach, as compared with other recent approaches, is the potential to reject not only single-site escape events, but also multiple-site escape events. Here we discuss the details of our algorithm, and present experimental results from a real-time implementation on a 5 cm×5 cm HPGe. An experimental comparison with a standard BGO suppressor is also shown. In addition, we discuss the improvements to the current PSA approach which are possible if one can highly segment the outer contact of the HPGe.

Development of large volume organic scintillators for use in the MASCO telescope

The development and construction of plastic scintillators of large volume, to be used as Compton suppressors in the MASCO telescope, are described. We also outline some characteristics of MASCO, which is a coded-mask gamma-ray imaging telescope with an angular resolution of 14 in a 14° (diameter, conical) field-of-view with a sensitivity of 3.92, 1.33 and 1.18×10 −5 photons cm −2 s −1 keV −1 in 0.05, 0.30 and 1 MeV respectively, for an integration time of 6 h and a residual atmosphere of 3.5 g cm −2. The Compton supressor described contains 12 modules of plastic scintillators, each one built as a trapezoidal prism (1 m of length) with a cross section with sides of 14×22 cm 2 and 15 cm thick (27 l of volume). Measurements of transmittance, luminescence, X-ray fluorescence and light output were carried out with the purpose of characterizing the detector. We present the estimates of the imaging sensitivity and the sensitivity for pulsed sources of the MASCO instrument.

Study on nuclear activities by the measurement of residual radioactivity

Caesium-137, caesium-134, cobalt-60, and some other radioisotopes were introduced into the atmosphere as a result of nuclear activities such as nuclear test explosions, the failure of the nuclear power plant, and the leakage of the radioactive waste. In fact test explosions taking place in various areas of the world mostly occurred in the fifties and sixties. It can be assumed that the resulting total fallout of137Cs into the Earth surface has been almost uniformly even over large regions. On the contrary the fallout resulting from a single power plant accident or nuclear test explosion differed, greatly depending on the migration of the radioactive cloud in the subsequent days after the nuclear accident and occurrence of rainfalls or aerosols fallout introducing the radioisotopes directly onto the Earth surface. Preliminary results of the research program in the Radioanalysis Research Laboratory are presented. About 300 samples of plants, soil, water, and fertilizer were analyzed for isotopes of137Cs,134Cs,60Co, and40K using a single γ-ray spectrometer employing 10% HPGe detector and the Compton suppression spectrometer. Large differences in the concentrations of137Cs and60Co have been observed. The results gave us information on the nuclear activities in various localities.

Real-time correction of chance coincidence losses in high-rate Compton suppression spectrometry

Chance coincidence losses in a Compton suppression spectrometer are instrumentally corrected by comparing the chance coincidence rate to the counting rate of the Germanium detector. The normally not observable chance coincidence rate is reconstructed from the rates of Germanium detector and scintillation detector in any auxiliary coincidence unit, after the destruction of true coincidence by sufficiently delaying one of the coincidence partners.

Compton suppression gamma-spectroscopy in the analysis of radium and lead isotopes in ocean sediments

Compton suppression gamma-spectroscopy provides an efficient method for measuring the concentration of226Ra,228Ra, and210Pb in ocean sediments. The background reduction realized from the suppressed mode of operation provides an “active shield” to allow for the low-level counting required for these measurements. We have used the method to study the possible accumulation of radium from produced waters in sediments from Gulf of Mexico sites of varied distances from drill platforms. In addition,210Pb levels were determined to evaluate the use of our spectrometer for this purpose. Baseline undisturbed concentrations have been obtained from the study of subsurface core material which predates drilling activities. Correlations of radium isotope concentrations with iron and barium provide information concerning the possible relationships between radium levels and drilling activity (excess barium) and natural variations in sediment texture (iron).

Subtractions of accidental coincidences and Compton scattered events by multi-channel time scaling technique in γ-ray spectrometry

The multi-channel time scaling (MCTS) technique is well suited to γ-ray spectrometry, especially for cases where both statistical and timing analysis of event distributions is required. In this paper the technique is applied to the subtraction of Compton scattered events in single γ-ray spectrums obtained using a Ge detector and NaI (Tl) gamma detector. The efficiency curve for a Ge detector was obtained with a standardized gamma emitting nuclide in the energy regions from 80 to 1300 keV by means of the Compton suppression setup. Virtues of the MCTS technique are demonstrated by estimating the photopeak intensities of γ-rays in the decay of 133Ba and 152Eu.

Detectors and detection limits in INAA

Detection limit improvement factors have been calculated to predict the improvement in detection limits when replacing a traditional 20% coaxial detector by either a 100% coaxial detector, a well-type detector or by equipping the detector with a Compton suppression shield. It is show that, when measuring sources on the end-cap of the detector, a system with a Compton suppression shield and a 100% detector have almost equal potentials. But when e.g. geometrical consideration larger source-to-end-cap distances are required with coaxial detectors, the well-type detector offers the best outlook for attaining better detection limits. The calculated predictions have been compared with experimental observations and the results agree well.

Analysis of the Arctic aerosol for a ten year period using various neutron activation analysis methods

During the past decade we have determined the concentrations of a variety of trace elements in the Arctic aerosol by using themal and epithemal neutron activation analysis (NAA). More recently we have employed Compton suppression NAA to lower the detection limits for radionuclides that are characteristic of single or mainly single gamma-ray emission. Using these various methods, we have been able to use elements such as indium and silicon. Furthermore we have achieved extremely low detection limits for iodine, arsenic and antimony. The usefulness of these NAA methods are discussed in a large sampling program that incorporates more than one thousand samples.

Non-destructive analysis of copper in human brain tissue by neutron activation analysis using coincidence and anti-coincidence techniques

Copper (Cu) is an essential element and is incorporated in many biomolecules that are involved in protecting the brain from oxidative damage. Many brain regions strongly affected by neurodegene rative diseases are small. A sensitive nondestructive procedure to determine Cu is desirable to preserve samples for additional studies. Copper is not easily determined by instrumental neutron activation analysis (INAA) due to high activity levels produced by major abundance elements such as sodium (Na) and chlorine (Cl), which produce a high Compton background. An INAA method involving a short epithermal neutron irradiation and counting with a Compton suppression system was developed to determine Cu in brain, via 5.1-min66Cu. These short irradiation results are compared to those based on coincidence spectrometry of annihilation photons from positron emitting 12.7-h64Cu after a long irradiation.

An innovative Compton suppression spectrometer and its capability of evaluation in multi-elemental neutron activation analysis

A new Compton suppression system was established. A low energy HPGe detector (LO-AX) is used as the analyzing detector. It is completely shielded by a 50%n-type HPGe and two NaI(TI) detectors. Experiments show that a substantial Compton suppression effect is also achieved in the low energy region down to 15 keV. The capability of applying this system in INAA was evaluated for 13 elements (As, Cd, Co, Cr, Hg, Mo, Ni, Sb, Se, Sr, Th, U and Zn) in biological and environmental samples. The benefits and problems of using this Compton suppression spectrometer in INAA are examined for each element.

Cold neutron prompt gamma-ray activation analysis at NIST — Recent developments

The cold neutron capture prompt γ-ray activation analysis (CNPGAA) spectrometer located in the Cold Neutron Research Facility (CNRF) at NIST has proven useful for the analysis of hydrogen and other elements in a wide variety of materials. Modifications of the instrument and the CNRF have resulted in improved measurement capabilities for PGAA. The addition of an atmosphere-controlled sample chamber and Compton suppression have reduced γ-ray background and increased signal-to-noise ratio. More recent revisions are expected to yield still further improvement in analytical capabilities. Replacement of the D2O ice cold source with a liquid H2 moderator is expected to yield a 5–10 fold increase in neutron capture rate, and improved neutron and γ-ray shielding will result in further reduction of the background. Other modifications to the instrument allow easier sample mounting and more precise positioning of samples in the neutron beam. Significant improvements in detection limits and analytical accuracy are expected.

Performance of a Ge-BGO Compton-suppression spectrometer and its application to photon activation analysis

A Compton-suppression spectrometer, consisting of a pure Ge detector and BGO shield detectors is described. A cylindrical symmetry type of the shield detector configuration, with the Ge detector inserted on the symmetry axis, has been designed. The Compton-suppression factor is 6.0 for the photopeak of 60Co at 1332.5 keV. The peak-to-Compton ratio for the peak is 42 : 1 when the BGO detectors are used as a passive shield only, and is 250 : 1 in the suppression mode. Photon activation analysis with this detector has been made for the determination of trace elements in a bulk dysprosium sample. The signal-to-noise ratio for detecting trace elements in the sample has been improved by a factor from 1.3 to 3.1 by Compton suppression compared with that obtained in the non-suppression mode.

Improvement in the counting statistics and in the limit of detection with Compton suppression spectrometers—a contribution to instrumental neutron activation analysis

An analytical description of improvements in the counting statistics and in the limit of detection obtained with Compton suppression methods in instrumental neutron activation analysis (INAA) is described. The experimental data, deduced from the INAA of air filters, show the advantage of counting systems where the samples are placed outside the Compton shields.

Application of EGS-4 code for evaluating a Compton suppression system

The response evaluation of the Compton suppression system is usually conducted in controlled experiments with specific radiation sources. However, such sources alone cannot be used to evaluate the advanced system designs to be used in the high background radiation field of a nuclear power plant. A computational code is required to evaluate such a system. We experimentally determined the responses of a Ge detector, a scintillation detector, and a Compton suppression system; and we evaluated the validity of the results using the EGS-4 Monte Carlo Simulation code. Results showed that when the Compton suppression system is simulated, high accuracy cannot be achieved without spending at least 100 hours developing the model for the peripheral structures of the detector and for the /spl gamma/-ray scattering. However, the experimental results agreed closely with the simulation results, and the EGS-4 code was validated to be applicable to the advanced system design.

Development of nuclear spectroscopy systems using high resolution semiconductor detector

Abstract Two kinds of spectrometers have been developed at Dalat Nuclear Research Institute. In this paper an XRF spectrometer and Compton suppression and pair spectrometers have been described. The XRF spectrometer consists of Si(Li) detector and Dalat Electronics. Compton suppression and pair spectrometers consist of a 20% high purity germanium gamma ray detector inserted into two large NaI(TI) shielding detectors, coupled with Compton suppression and pair electronics of Dalat. Improving the resolution of XRF spectrometer and increasing the peak to Compton ratio of gamma ray spectrometers have been done at Dalat Nuclear Research Institute. Block diagram of systems and results of testing spectroscopy systems have been given.