Compton Suppression Research Articles

A Compton suppression spectrometer for neutron activation analysis

Abstract A Compton suppression spectrometer consisting of a system of three Ge detectors surrounded by a BGO shield and a CsI(T1) active collimator has been set up for the use in neutron activation analysis. Tests of the performance of the system concerning Compton suppression, peak reduction and effects of the count rate in an energy range 300–2000 keV are described. A practical test for the determination of iodine in iodized salt is presented.

Enhancement of Compton suppression ratios in anti-Compton techniques: the Garching and Karlsruhe photon spectrometers

Two compact, high-purity germanium and sodium iodide detector systems with very high signal-to-background ratios are described. These systems are designed to identify weak photons emitted from radionuclides with single and multiple photon emissions in the presence of background radiation induced by photons of higher energy. The mounting and housing of the germanium detector are built for minimum gamma-ray absorption and is surrounded by three sodium iodide detectors. The fast and slow timing electronic circuits can be used for a wide range range of photon energies down to 10 keV, for both the pure 4π Compton suppression mode and the mixed energy and time coincidence/anticoincidence mode. The systems are also efficient for low-level spectrometry and high count rates up to 10 5 cps. These spectrometers exhibit Compton reduction ratios (unsuppressed/suppressed) of up to 23 for the single photon emission of 137Cs, up to 60 for the two-photon emission of 60Co, and in excess of 97 for the mixed 3- to 4-photon emission of 82Br.

Compton suppression neutron activation analysis: Past, present and future

In this work a review of the development of compton suppression is presented. It was shown that the application of Compton-suppression counting in instrumental NAA reduces the detection limits and improves the accuracy for a list of elements by substantial reduction of the background of the γ-spectroscopy. Results for certified reference materials obtained through the use of Compton suppression are normally more accurate and in agreement with the published values. Compton suppression is particularly helpful for low level concentrations in environmental samples to those elements which exhibit severe special interferences in the normal NAA counting. A list of the elements with isotopes having single or close to single γ-ray decay schemes and which could benefit from Compton-suppression counting is presented. Also, evaluation is made regarding the reliability of Compton suppression with increase in the overall dead-time of the counting. It was concluded that this method does not provide accurate quantification of the isotopes when the overall dead-time exceeds the 10% range. Investigation of the natural background was performed with Compton suppression for the purpose of neutron activation analysis application. The method presented proves to broaden the application of NAA and helps in its competition for simplicity, accuracy and reliability with the modern methods of elemental analysis. Future application of coincidence spectrometry in activation analysis should include better enclosing of the primary detector, utiliza5tion of x-ray and well type detectors, γ-γ, β-γ, and β-γ-γ coincidence techniques.

Structure of neutron-rich nuclei and neutron multiplicities in spontaneous fission

New insights into the fission process and the structure of neutron-rich nuclei have been gained by measuring γ-γ and γ-γ-γ coincidences of the prompt γ-rays following the spontaneous fission of 252Cf. The data were taken with the Compton Suppression Spectrometer System at HHIRF. New high spin levels were observed in 143,145,147Ba Only in 143Ba are intertwined bands observed with the characteristics of the octupole structure found only in 144Ba. New high spin states observed in the N = 84 136Te, 138Xe, 140Ba nuclei exhibit a symmetry pattern suggestive of a pseudospin f ̃ - shell . For the first time in fission, the relative yields and neutron multiplicities were measured directly. Yields and multiplicities were measured directly for the ZrCe and MoBa correlated fragment pairs by observing the coincidences between the γ-rays in each fragment. Zero-neutron to ten-neutron emission yields were determined; the latter for the first time. The zero-neutron emission can be understood as a new example of cluster radioactivity. These results demonstrate that the use of the high resolution γ-γ-γ coincidence technique employed here for the first time opens up a new approach to the study of the fission process that will provide previously inaccessible details of the process.

The impact of heavy metals from environmental tobacco smoke on indoor air quality as determined by Compton suppression neutron activation analysis

The method of instrumental neutron activation analysis (NAA) has been improved for air filter samples in the determination of low level heavy metals in indoor air. By using the techniques of epithermal neutron irradiation in conjunction with Compton suppression, the detection limits of cadmium, arsenic and antimony measurements have been dramatically reduced to 2 ng for Cd, 0.2 ng for As, and 0.03 ng for Sb. The determination of these heavy metals in particulate material generated from cigarette smoking in indoor environments has been conducted. Other elements, Br, Cl, Na, K, Zn were also found at elevated levels.

Sensitivity improvements in the determination of mercury in biological tissues by neutron activation analysis

The possible association of dental amalgam surface exposure, brain mercury (Hg) levels, and pathological markers of Alzheimer's disease (AD) in the brain is the subject of an on-going study in our laboratory. Two radiochemical neutron activation analysis methods and the use of instrumental neutron activation analysis (INAA) with Compton suppression spectrometry have been evaluated for improving our INAA Hg detection limit (2.8±0.6 ng/g, wet-weight basis) in human tissue. Large numbers of samples dictated the use of a purely instrumental method or rapid, simple radiochemical separations. Human brain tissues and NIST biological standards were analyzed using a precipitation of Hg2Cl2, a solvent extraction utilizing sodium diethyldithiocarbomate, conventional INAA, and INAA with Compton suppression. The radiochemical precipitation of Hg2Cl2 proved to be the most useful method for use in our study because it provided a simultaneous, quantitative determination of silver (Ag) and a Hg detection limit in brain tissue of 1.6±0.1 ng/g (wet-weight basis).

Improved detection limits for trace elements on aerosol filters using Compton suppression counting and epithermal irradiation techniques

Aerosol samples were collected on Whatman 41 filters at two sites near Lake Huron and one site near Lake Ontario. These samples were then analyzed by instrumental neutron activation analysis (NAA) at the University of Illinois. The detection limits for certain trace elements were enhanced by irradiation with both thermal and epithermal neutrons and also by counting with Compton suppression techniques. The sample was divided in half to allow for four irradiations. Short-lived thermal NAA resulted in the determination of Al, Br, Ca, Cl, Cu, K, Mn, Na, Ti, and V. A short epithermal irradiation was used to determine Cu, I, In, Si, Sn and U. A one and one-half hour epithermal irradiation was utilized for the determination of As, Au, Br, Sm, Sb, and W. The elements Cr, Cs, Fe, Hf, Ni, Sc, Se, Th. Zn, and several rare earths were determined with a long thermal irradiation. Utilizing a Compton suppression gamma-ray counting system reduced the background and enhanced the detection of several isotopes which primarily emit only a single gamma-ray upon decay. Counting was simultaneously performed with a normal counting mode so that the detection of isotopes with multiple decay gamma-rays was not impaired.

A comprehensive study for the determination of forty eight elements in the certification of a hazardous standard reference material

The centification of standard reference materials is of fundamental importance for environmental scientists to proceed with their own quality control programs or calibration procedures. As part of the National Institute of Standard and Technology (NIST) program, we participated in the certification of a hazardous waste material. Our efforts concentrated on judiciously using thermal and epithermal neutron activation analysis in conjunction with Compton suppression techniques. We have demonstrated that besides lowering the detection limit for several elements usually not reported by conventional NAA, Compton suppression is ideally suited to substantially reduce certain spectrla interferences. A detailed comprehensive study was undertaken using this method to highlight which isotopes can be most favorably used.

Characterization of household plastics for heavy metals using neutron activation analysis

Highly enriched concentrations of several heavy metals have been found in municipal solid waste incinerator (MSWI) ash. In an effort to identify possible sources of these metals in MSWI ash, a variety of disposable household plastic products was examined for heavy metal content. Using both thermal and epithermal neutron activation analysis (NAA) along with Compton suppression techniques, concentrations of several trace and heavy metals including Ag, As, Au, Ba, Br, Cd, Cr, Cu, Fe, Mn, Ni, Sb, Se, Sn, Sr, V, W and Zn were determined. Results indicate a wide range of concentrations for these elements, with large variations in plastics of similar color and intended use. As limits dealing with heavy metal content of consumer products are lowered, NAA techniques will provide a useful method for verification of product compliance.

A study of an asymmetric Compton suppression spectrometer by Monte Carlo simulation

The geometrical parameters which affect the suppression factor for a HpGe-NaI(Tl) Compton suppression spectrometer are studied by Monte Carlo simulation. The simulation parameters are designed for comparison of the experimental results of the specified Compton suppression spectrometer with a HpGe detector as the central detector. The simulated results are in coincidence with the experimental results. The optimal position of the HpGe detector within the shielding of the NaI(Tl) detector in two kinds of asymmetry Compton suppression spectrometer, which we named L(T)-shape and L(C)-shape is studied. The optimized suppression position is consistent with the previous simulated results, that the front face of the central detector is about at 3 4 – 4 5 length of the shielding along the hole dimension. Further simulation indicates that the suppression factor is quickly saturated by increasing the shielding thickness along the dimension perpendicular to the hole for the central detector.

Performance and promise of the Compton suppression well counter

Determination of gamma-ray activities in small environmental samples poses a challenge to measurement technologies: to achieve the lowest possible detection limits, an instrument must combine high efficiency with superior peak-to-background characteristics. These requirements are simultaneously met by a Compton-suppression well counter (CSWC) of intrinsic germanium. The Brookhaven CSWC has a 5-cm deep, 1-cm diameter well and active volume of ∼ 125 cm 3. It is positioned in the 3.3-in through-hole of 10 × 12 in NaI(Tl) crystal. This CSWC was first assembled in the mid 1980s, and used in several research studies: double β − decay of 48Ca, the radioactivity of 180mTa, and high-sensitivity neutron activation analysis of iridium in rocks and minerals. More recently it has been employed in environmental radioactivity research on the transport of the members of the uranium and thorium series in natural soils. The counter typically measures samples of ∼ 3 g of soil, has a peak-to-Compton ratio of ∼ 300:1, and a sensitivity for 235U of ∼ 50 ng. It can detect gamma rays of less than 30 keV, hence can be used to measure environmentally significant nuclides like 210Pb, 230Th and 241Am in small soil samples or soil fractions. Because of the simple geometry self-absorption corrections are straightforward. The paper will discuss the performance of this device, and the projected performance of an advanced model which will incorporate a number of changes materially increasing the sensitivity.

Determination of silicon in biological and botanical reference materials by epithermal INAA and Compton suppression

Silicon determination in sixteen botanical and biological standard reference materials is described using the 29Si(n, p) 29Al reaction through instrumental epithermal neutron activation analysis and Compton suppression gamma-ray spectroscopy. By simultaneous utilization of both cadmium and boron epithermal filters along with anticoincidence gamma-counting, detection limits as low as 12 ppm were obtained for certain matrices, much lower than previously reported values for this type of analysis. The method is applicable to many botanical and biological matrices and is attractive with its interference free, purely instrumental nature, compared with methods using the 28Si(n, p) 28Al reaction or chemical separation techniques.

Determination of chromium in biological reference materials by instrumental NAA using Compton suppression

Instrumental neutron activation analysis in conjunction with Compton suppression methods has been used to determine chromium concentrations in 13 biological reference materials. Special attention was focussed on the interferences and the ways of lowering their contribution. Separate data was obtained for normal and Compton suppressed modes of counting so that a comparison could be performed. The results for chromium as compared to the certified or compilation values were in excellent agreement.

Comparison of NAA methods to determine medium-lived radionuclides in NIST soil standard reference materials

In order to determine the elemental concentrations of three new soil standard reference materials SRMs 2709, 2710 and 2711 from the National Institute of Standards and Technology (NIST), a comparative study of different medium-lived neutron activation analysis methods was carefully performed. Three irradiation conditions (1-hour thermal, 1-hour epithermal and 5-minute epithermal) and two counting modes (normal and Compton suppression) have been evaluated for following ten elements: As, Au, Cd, Ga, K, La, Mo, Sb, Sm, and W. The results show that the method of 5-minute epithermal and a 1-day decay is the optimum way to analyze Ga, while the addition of the Compton suppression is very beneficial for the determination of K. Using the Compton suppression system in conjunction with the 1-hour epithermal and a 1-week decay, is ideal to determine Au, Cd, Mo and W, while routine 1-hour thermal and a 1-week decay, is adequate to determine As, Sb, Sm and La in concentrations found in soil.

Compton suppression neutron activation methods in environmental analysis

An overview of Compton suppression neutron activation analysis (NAA) is given. The basic theory of the technique, its experimental design, uniqueness, and limitations are discussed. Experimental data showing its usefulness for the low level determination of several key elements in environmental samples that can not be obtained using conventional non-destructive neutron activation is deliberated.

Precision energy determination of gamma-rays from the 66Ga( β++ ϵ) 66Zn decay

The energies of 29 γ-rays from the 66Ga( β + + ϵ) 66Zn decay have been measured with Ge(Li) detectors, of which one in a Compton suppression shield. Energy calibration standards were provided by 46Sc, 56Co, 65Zn, 110Ag m and 124Sb radioactive sources. The resulting excitation energies of 18 66Zn levels, obtained in a least-squares fit, then provide the energies for 26 strong lines ( E γ = 0.83−4.81 MeV) with a final precision of better than 10 ppm. Of particular importance are the five E γ = 3.76−4.81 MeV lines, which now can serve as calibration standards in this hitherto uncovered high-energy region.

Thermal neutron capture?-ray spectroscopy of59Ni and61Ni

Theγ-radiation emitted after thermal neutron capture in isotopically enriched58Ni and60Ni was measured at the ILL high flux reactor by means of Ge/NaI detectors operated in Compton suppression and pair spectrometer mode. The neutron binding energies were determined asBn (59Ni)=8999.15(23) keV and Bn(61Ni)=7820.07(20) keV; some 95% of the totalγ-ray fluxes through59,61Ni were assigned. Theγ-ray strength functions of the primary transitions and the level densities are discussed.

Determination of airborne cadmium in environmental tobacco smoke by instrumental neutron activation analysis with a compton suppression system.

Concentrations of cadmium, a toxic trace element, were measured in the indoor air of several public places where environmental tobacco smoke was present. Particulate-phase cadmium concentrations were determined by analyzing air filter samples using epithermal instrumental neutron activation analysis in conjunction with a Compton suppression gamma-ray detection system, in which the detection limit for cadmium was reduced to a few nanograms per filter. A cascade impactor and a personal filter sampler were used to collect the indoor suspended particulate matter for size-fractionated mass as well as total mass, respectively. Results show that where environmental tobacco smoke is present, cadmium concentrations are significantly higher than background and that about 80% of the cadmium found in indoor airborne particulate matter is associated with particles with aerodynamic diameters less than 1.8 microns. In one instance, airborne cadmium concentrations in a music club were found to be 38 ng/m, which is at least 30 times higher than background.

The characteristics of the prompt gamma-ray analyzing system at the neutron beam guides of JRR-3M

A reactor-neutron-induced prompt γ-ray analyzing system was constructed at JRR-3M. The system can be set at cold and thermal neutron beam guides with a neutron flux at the sample position of 1.1 × 10 8 and 2.4 × 10 7 n cm −2 s −1, respectively. The system was designed to achieve the lowest γ-ray background. This is done by using lithium fluoride tiles as neutron shielding, by placing the samples in a He atmosphere, and by using a Ge-BGO detector system for Compton suppression. The γ-ray spectrometer is equipped to acquire three modes of spectra simultaneously: single mode, Compton suppression mode, and pair mode in an energy range of up to 12 MeV. Due to the cold neutron guide beam and the low background system, sensitivities and detection limits better than those in other PGA systems have been achieved. For typical elements such as H, B, Cd, and Gd, analytical sensitivities with the cold neutrons are 10 to 18 times higher, and detection limits are lowered by factor of 6 to 8, compared to the case with thermal neutrons under the same geometrical condition. Detection limits down to a range of 2 to 7 ng were recorded for B, Cd, Sm, and Gd by the cold neutrons. Hydrogen, one of the most important elements in material studies, could be detected down to 1.0 and 2.8 μg by cold and thermal neutrons, respectively.

A study on T-shape Compton suppression spectrometer by Monte Carlo simulation

The geometrical parameters which affect the suppression factor for a T-shape HPGe-NaI(Tl) Compton suppression spectrometer are studied by a Monte Carlo simulation method. We simulate two combinations of this sort of Compton suppression spectrometer (113 and 146 cm 3 high-purity germanium detector is inserted into 22.9 cm Ø × 25.4 and 14.0 cm Ø × 17.8 cm annular NaI(Tl) shield detector, respectively). The simulation results of suppression factor are consistent with the results from the experiment. The optimal position of the HPGe detector within the shield is about at the half length of the radius of the NaI(Tl) detector for Compton suppression. Further simulation finds that the suppression factor is independent of the length of the shield but dependent on its radius. Other geometrical topologies for the shield are also discussed.