It is a standard procedure in many countries that response to a nuclear or radiological accident or incident would involve mobile aerial- or ground-based survey with highly sensitive gamma-ray ...detectors to map the distribution of radioactivity. There may however arise situations in which ground- or air-based detectors are not able to access an area to survey for radioactive materials, therefore technologies and techniques that can estimate the position and activity of radioactive materials from a distance are under development. Tomographic reconstruction methods, well-known in medical physics, permit the reconstruction of an N-dimensional map or image, from a number of N-1-dimensional cross-sectional images, or back-projections. We are investigating a tomographic reconstruction method to reconstruct the radioactivity distribution within a restricted-access zone using measurements from a Compton gamma imager placed at several locations around the perimeter of the zone. In this work an extended source of La-140 with an activity of 35 GBq was deposited within a 500 m by 500 m zone that was surveyed from the perimeter at six locations using a Silicon photomultiplier-based Compton Telescope for Safety and Security (SCoTSS) gamma imager. The reconstructed Compton images from multiple viewpoints were then projected back into the zone to reconstruct the distribution of La-140 within it. This tomographic method reconstructed high intensity along the known location of the La-140 source, suggesting that the method is able to localize the radioactive material. A simple fit to measured counts using a point-source approximation of the source distribution yielded a strength estimate of (7 ± 2) GBq at time of deposition, a reasonable result given the presence of soil and snow attenuation. Our method provides an expedient estimate of the distribution of radioactivity using tomographic techniques. It may be used to inform decisions made on the scene in urgent situations where the distribution of radioactivity must be reconstructed from a distance.
•Deployed a scintillator and silicon-photomultiplier-based Compton imager, SCoTSS.•Developed a tomographic reconstruction method for Compton gamma-ray imaging.•Applied tomographic technique to large-area La-140 source at large distances.
In response to the Fukushima nuclear reactor accident, on March 20th, 2011, Natural Resources Canada conducted aerial radiation surveys over water just off the west coast of Vancouver Island. ...Dose–rate levels were found to be consistent with background radiation, however a clear signal due to
133Xe was observed. Methods to extract
133Xe count rates from the measured spectra, and to determine the corresponding
133Xe activity concentration, were developed. The measurements indicate that
133Xe concentrations on average lie in the range of 30–70 Bq/m
3.
► Xe-133 from Fukushima was detected off the west cost of Vancouver Island. ► Natural background is subtracted from the signal. ► The signal is modelled using a simulation of an infinite uniform plume. ► The plume concentration varies strongly over a ∼15 km distance. ► Concentrations lie in the range of 30–70 Bq/m
3.
We are designing an all-scintillator Compton gamma imager for use in security investigations and remediation actions involving radioactive threat material. To satisfy requirements for a rugged and ...portable instrument, we have chosen solid scintillator for the active volumes of both the scatter and absorber detectors. Using the BEAMnrc/EGSnrc Monte Carlo simulation package, we have constructed models using four different materials for the scatter detector: LaBr 3 , NaI, CaF 2 , and PVT. We have compared the detector performances using angular resolution, efficiency, and image resolution. We find that while PVT provides worse performance than that of the detectors based entirely on inorganic scintillators, all of the materials investigated for the scatter detector have the potential to provide performance adequate for our purposes.
Development of a Compton imager based on bars of scintillator MacLeod, A.M.L.; Boyle, P.J.; Hanna, D.S. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2014, Letnik:
767
Journal Article
Recenzirano
Odprti dostop
We have developed a compact Compton gamma-ray imager with a large field of view and a low channel-count that is capable of quickly localizing gamma-ray sources in the few hundred keV – several MeV ...range. The two detector planes (scatter and absorber) employ bars of NaI(Tl) read out by photomultiplier tubes (PMTs) located at each end. The long-range imaging performance has been tested from 392keV to 1274keV. An angular resolution measure of 2.72±0.06° and an efficiency of (1.79±0.04)×10−3 at 662keV is obtained. A 137Cs (662keV) source equivalent to a 10mCi source 40m away can be located in 60s with an uncertainty of about a degree. No significant degradation in imaging performance is observed for source angles up to 40° off axis.
We are developing a rugged and person-transportable Compton gamma imager for use in security investigations of radioactive materials, and for radiological incident remediation. The imager is composed ...of layers of scintillator with light collection for the forward layers provided by silicon photomultipliers and for the rear layer by photomultiplier tubes. As a first step, we have developed a 1/5th-scale demonstration unit of the final imager. We present the imaging performance of this demonstration unit for 137Cs at angles of up to 30° off-axis. Results are also presented for 113Sn and 22Na. This represents the first demonstration of the use of silicon photomultipliers as an embedded component for light collection in a Compton gamma imager.
► A Compton gamma imager based on layers of inorganic scintillator is built. ► The scintillation light is collected using silicon photomultipliers. ► Performance characteristics of the imager are measured for different gamma sources. ► The imager performs well across a range of energies and angles.
A novel, water-based, neutron spectroscopy system has been designed, simulated, and tested at the National Research Council of Canada. The active part of the detection system is a small proportional ...counter sensitive to thermal neutrons. This is suspended along the symmetry axis of a large, rectangular water bath, and can be positioned remotely anywhere along this axis down to the bottom face. The energy spectrum of any constant neutron field impinging on this face from below the detector may be reconstructed with this system by mapping the thermal neutron response as a function of distance from the face, and using singular value decomposition to deduce the energy distribution. Agreement between experimental data from americium-beryllium neutron sources and the Monte Carlo software MCNP5 is excellent. Monte Carlo techniques are used to test the energy spectrum reconstruction using singular value decomposition for americium-boron and americium-beryllium neutron spectra.
The BEAMnrc/EGSnrc Monte Carlo code system is employed to develop a model of the National Research Council of Canada primary standard of absorbed dose to tissue in a beta radiation field, comprising ...an extrapolation chamber and Sr90∕Y90 beta source. We benchmark the model against the measured response of the chamber in terms of absorbed dose to air, for three different experimental setups when irradiated by the Sr90∕Y90 source. For the first setup, the chamber cavity depth is fixed at 0.2cm and the source‐to‐chamber distance varied between 11 and 60cm. In the other two cases, the source‐to‐chamber distance is fixed at 30cm. In one case the response for different chamber depths is studied, while in the other case the chamber depth is fixed at 0.2cm as different thicknesses of Mylar™ are added to the front surface of the extrapolation chamber. The agreement as a function of distance between the calculated and measured responses is within 0.37% for a variation in response of a factor of 29. In the case of dose versus chamber depth, the agreement is within 0.4% for the ISO‐recommended nominal depths of 0.025–0.25cm. Agreement between calculated and measured responses is very good (between 0.02% and 0.2%) for added Mylar foils of thicknesses up to 10.8mgcm−2. For larger Mylar thicknesses, deviations of 0.6%–1.2% are observed, which are possibly due to the systematic uncertainties associated with the restricted collisional stopping powers of air or Mylar used in the calculations. We conclude that our simulation model represents the extrapolation chamber and Sr90∕Y90 source with adequate accuracy to calculate correction factors for accurate realization of dose rate to tissue at a depth of 7mgcm−2 in an ICRU tissue phantom, despite the fact that the uncertainties in the physical characteristics of the source leave some uncertainty in certain calculated quantities.
The BEAMnrc/EGSnrc Monte Carlo code system is employed to develop a model of the National Research Council of Canada primary standard of absorbed dose to tissue in a beta radiation field, comprising ...an extrapolation chamber and
Sr
90
∕
Y
90
beta source. We benchmark the model against the measured response of the chamber in terms of absorbed dose to air, for three different experimental setups when irradiated by the
Sr
90
∕
Y
90
source. For the first setup, the chamber cavity depth is fixed at
0.2
cm
and the source-to-chamber distance varied between 11 and
60
cm
. In the other two cases, the source-to-chamber distance is fixed at
30
cm
. In one case the response for different chamber depths is studied, while in the other case the chamber depth is fixed at
0.2
cm
as different thicknesses of Mylar™ are added to the front surface of the extrapolation chamber. The agreement as a function of distance between the calculated and measured responses is within 0.37% for a variation in response of a factor of 29. In the case of dose versus chamber depth, the agreement is within 0.4% for the ISO-recommended nominal depths of
0.025
–
0.25
cm
. Agreement between calculated and measured responses is very good (between 0.02% and 0.2%) for added Mylar foils of thicknesses up to
10.8
mg
cm
−
2
. For larger Mylar thicknesses, deviations of 0.6%–1.2% are observed, which are possibly due to the systematic uncertainties associated with the restricted collisional stopping powers of air or Mylar used in the calculations. We conclude that our simulation model represents the extrapolation chamber and
Sr
90
∕
Y
90
source with adequate accuracy to calculate correction factors for accurate realization of dose rate to tissue at a depth of
7
mg
cm
−
2
in an ICRU tissue phantom, despite the fact that the uncertainties in the physical characteristics of the source leave some uncertainty in certain calculated quantities.
Purpose: To benchmark an EGSnrc Monte Carlo calculated response against the high quality measured response of an extrapolation chamber used as NRC's primary standard of absorbed dose to tissue in a
...90
Sr
/
90
Y
beta field. Method and Materials: The BEAMnrc code was used to model the NRC's beta source and indigenously developed extrapolation chamber. The calculated response was compared to the measured response in 3 different series of measurements. An overall scale factor was determined by a global fit. It was used to scale the calculated values to the measured values and was compared to the known activity of the source. A single measurement configuration (30 cm distance, 0.2015 cm chamber depth) was common to all 3 sets of experimental data. Results: The scale factor led to an estimated source activity of 1.237±0.08% GBq which is consistent with the nominal value of 1.2±0.1 GBq. As the source‐detector distance was varied from 11 cm to 60 cm, values of calculated and measured responses agreed within 0.37% for a variation in response by a factor of 29. As chamber depth was varied from 0.05 cm to 0.25 cm the values agreed within 0.4%. As Mylar thicknesses up to 11 mg/cm2 were added to the face of the chamber, the values agreed within 0.2%, and agreed within 1.2% up to 150 mg/cm2. Conclusion: This project demonstrates EGSnrc's ability to calculate the response of extrapolation chamber with a remarkable degree of accuracy. Such high precision comparisons with experimental data are rare. This benchmarking of the Monte Carlo model will allow it to be used to calculate correction factors needed for the NRC's primary standard.
We are developing a compact, rugged Compton-scatter gamma-ray imager with a large field of view and a low channel count for use in investigations involving radioactive threat material. The two ...detector planes, (scatterer and absorber), employ bars of NaI(Tl) read out by photomultiplier tubes (PMTs) located at each end. The position of interaction along the bar is reconstructed from the relative pulse heights of the PMTs. We present the imaging performance of a reduced-scale prototype using three bars in the scatter layer and three bars in the absorber layer. A 100 μCi 662 keV source located 180 cm away from the detector is well localized to within 2° with 60 s of data over 90% of the time.
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