Diameter NaI Research Articles

Soil analysis using a 2″ NaI gamma detector

We describe an undergraduate physics laboratory experiment that uses a 2-inch diameter NaI gamma detector to measure natural radiation in soils. Students first calibrate the detector for the source-detector geometry, energy-dependent detection efficiency, and sample self-absorption. Then, the activity of the 238U and 232Th decay series, as well as the activity of 40K in the soil, are measured. The results are comparable in accuracy to those obtained using a high-resolution germanium gamma detector.

Approaches for single channel large area silicon photomultiplier array readout

We present the results of experiments applying a large area tiled array of silicon photomultipliers (SiPM) to gamma ray spectroscopy with 50.8 mm diameter NaI(Tl) and CsI(Tl) scintillators and 25.4 mm diameter CeBr3. These scintillators are many times larger than an individual SiPM, the largest of which are 36 mm2 at present. This work compares two methods of summing 64 SiPMs into a single output signal for use with standard charge integration electronics. The methods are a straightforward passive method, which simply combines all anode and cathodes together, and an active method, which employs high bandwidth op-amps in a summing topology. Comparisons are also made to a high quantum efficiency photomultiplier tube (PMT) for both resolution and pulse shape. Resolution and noise floor are satisfactorily comparable to the PMT from 21 keV to 2614 keV. The passive method provides the most straightforward approach, and slightly better energy resolution than the active method. The active method shows a substantially faster pulse rise time and undistorted fall time compared to the passive method.

Thin NaI(Tl) crystals to enhance the detection sensitivity for molten 241Am sources

A thin 5-mm NaI(Tl) scintillator detector was tested with the goal of enhancing the detection efficiency of 241Am gamma and X rays for steelworks operations. The performance of a thin (5 mm) NaI(Tl) detector was compared with a standard 76.2-mm thick NaI(Tl) detector. The 5-mm thick detector crystal results in a 55% smaller background rate at 60 keV compared with the thicker detector, translating into the ability to detect 30% weaker 241Am sources. For a 5 mm thick and 76.2 mm diameter NaI detector in the ladle car tunnel at Outokumpu Tornio Works, the minimum activity of a molten 241Am source that can be detected in 5 s with 95% probability is 9 MBq.

Estimation of counting efficiencies of a portable NaI detector using Monte Carlo simulation for thyroid measurement following nuclear accidents

Radioiodine can be released in nuclear accidents and can cause internal contamination of the thyroid gland in members of the public. For population monitoring in radiation emergencies, measurement of counting efficiency is very important to accurately determine thyroid activity. Here, we estimate the counting efficiencies of a portable NaI detector using Monte Carlo simulations. Two different types of neck phantoms and a 7.62 cm long by 7.62 cm diameter NaI detector were modeled and the counting efficiencies were calculated depending on the thyroid size and distance from the neck phantom to the detector. We found that distance is a more important parameter than thyroid size. The optimal distance of the NaI detector from the neck surface was determined to be greater than 12 cm, beyond which the counting efficiency was not affected by thyroid size.

SU‐C‐209‐05: Monte Carlo Model of a Prototype Backscatter X‐Ray (BSX) Imager for Projective and Selective Object‐Plane Imaging

Purpose:To develop a Monte Carlo N‐Particle (MCNP) model for the validation of a prototype backscatter x‐ray (BSX) imager, and optimization of BSX technology for medical applications, including selective object‐plane imaging.Methods:BSX is an emerging technology that represents an alternative to conventional computed tomography (CT) and projective digital radiography (DR). It employs detectors located on the same side as the incident x‐ray source, making use of backscatter and avoiding ring geometry to enclose the imaging object. Current BSX imagers suffer from low spatial resolution. A MCNP model was designed to replicate a BSX prototype used for flaw detection in industrial materials. This prototype consisted of a 1.5mm diameter 60kVp pencil beam surrounded by a ring of four 5.0cm diameter NaI scintillation detectors. The imaging phantom consisted of a 2.9cm thick aluminum plate with five 0.6cm diameter holes drilled halfway. The experimental image was created using a raster scanning motion (in 1.5mm increments).Results:A qualitative comparison between the physical and simulated images showed very good agreement with 1.5mm spatial resolution in plane perpendicular to incident x‐ray beam. The MCNP model developed the concept of radiography by selective plane detection (RSPD) for BSX, whereby specific object planes can be imaged by varying kVp. 10keV increments in mean x‐ray energy yielded 4mm thick slice resolution in the phantom. Image resolution in the MCNP model can be further increased by increasing the number of detectors, and decreasing raster step size.Conclusion:MCNP modelling was used to validate a prototype BSX imager and introduce the RSPD concept, allowing for selective object‐plane imaging. There was very good visual agreement between the experimental and MCNP imaging. Beyond optimizing system parameters for the existing prototype, new geometries can be investigated for volumetric image acquisition in medical applications.This material is based upon work supported under an Integrated University Program Graduate Fellowship sponsored by the Department of Energy Office of Nuclear Energy.

Background measurements in the Gran Sasso Underground Laboratory

The gamma background flux below 3000 keV in the Laboratori Nazionali del Gran Sasso (LNGS), Italy, has been measured using a 3 in. diameter NaI(Tl) detector at different underground positions: in hall A, hall B, the interferometer tunnel, and inside the Large Volume Detector (LVD). The integrated flux is 0.3–0.4 s −1 cm −2 at the first three locations, and is lower by two orders of magnitude inside LVD. With the help of Monte Carlo simulations for every location, the contribution of the individual primordial isotopes to the background has been determined. Using an 11 in. diameter NaI(Tl) detector, the background neutron flux in the LNGS interferometer tunnel has been estimated. Within the uncertainties, the result agrees with those from other neutron measurements in the main halls.

Validation of a new TDCR system at NPL

A new triple to double coincidence ratio (TDCR) system has been established at NPL. The system incorporates a spherical optical chamber, a manual sample changing facility and an integral light-tight housing. A 6 in diameter NaI(Tl) detector has been incorporated to allow 4pibeta-gamma coincidence measurements to be performed in parallel. Details of the detectors, the electronics and the overall TDCR system are given. Validation measurements using suitable low-energy beta and electron capture nuclides, i.e. (3)H and (55)Fe have been performed. The highest efficiency achieved with a (3)H solution in Ultima Gold AB in a glass vial was 53% and in a high-efficiency LS cocktail 65%. This indicates that the optical chamber is performing well. After varying the efficiency by de-focusing the PM tubes, the activity of the sources was determined. The activity concentration determined with TDCR agreed with certified values within the range of uncertainties. Further results from validation measurements and the corresponding uncertainty budgets are presented.

Design of Floating-Boundary Multi-Zone Scintillation Camera With Pileup Prevention and Misplaced Pileup Suppression (MPS)

Image quality in nuclear medicine is a trade-off between image contrast and resolution, since there are conflicting requirements of crystal thickness and collimator-hole dimensions. The sensitivity, or efficiency, which determines the contrast-to-noise ratio, however, is constrained primarily by pulse-mode operation, in which gamma ray interactions are recorded once every deadtime period. Nuclear images also suffer from image blurring due to gamma rays penetrating the collimator septa and due to misplaced pileup events arising from the coincidence detection of Compton-scattered gamma rays. Multi-zone detector architecture (MZDA) and the use of pileup prevention circuitry (PPC) lead to reduction of the detector's effective deadtime, and hence to significant extension of the count-rate range of the camera. This in turn allows the source activity to be increased to make up for sensitivity that will be lost on changing the detector and collimator-hole dimensions to improve spatial resolution. Misplaced pileup suppression (MPS) circuitry, also described in this paper, enables image blurring from Compton-scattered radiation to be reduced to 1/P of its level in a single zone detector, P being the PMT array size. The paper first develops the theoretical relationships relating to the trade-offs between efficiency and resolution, the distribution of scintillation light from interaction points in the crystal, the share of the light exposure of individual PMTs and of clusters of PMTs, and the cross-talk between clusters of PMTs. The paper then presents an outline of the implementation of an S-zone camera with PPC of order 1, based on hexagonal and rectangular arrays of PMTs. A hexagonal array of 331 1.27-cm PMTs, which will have S ap 35 detector-zones on average, can, for instance, use a 1.7-mm thick and 29-cm diameter NaI(Tl) crystal to design a camera with twice the sensitivity of a single-zone camera of the same array size, which will also provide a spatial resolution of 2.1 mm at the collimator face and 3.4 mm at 10 cm from the collimator face.

Measurement of the quenching factor of Na recoils in NaI(Tl)

Measurements of the quenching factor for sodium recoils in a 5 cm diameter NaI(Tl) crystal at room temperature have been made at a dedicated neutron facility at the University of Sheffield. The crystal has been exposed to 2.45 MeV mono-energetic neutrons generated by a Sodern GENIE 16 neutron generator, yielding nuclear recoils of energies between 10 and 100 keVnr. A cylindrical BC501A detector has been used to tag neutrons that scatter off sodium nuclei in the crystal. Cuts on pulse shape and time of flight have been performed on pulses recorded by an Acqiris DC265 digitiser with a 2 ns sampling time. Measured quenching factors of Na nuclei range from 19% to 26% in good agreement with other experiments, and a value of 25.2±6.4% has been determined for 10 keV sodium recoils. From pulse shape analysis, the mean times of pulses from electron and nuclear recoils have been compared down to 2 keVee. The experimental results are compared to those predicted by Lindhard theory, simulated by the SRIM Monte Carlo code, and a preliminary curve calculated by Prof. Akira Hitachi.

SU-FF-T-235: Fluence Determination in a Polarized Synchrotron-Generated Monochromatic X-Ray Beam

Purpose: K-edge capture radiotherapy using monochromatic x-ray beams necessitates development of corresponding dosimetry techniques, one being utilization of Monte Carlo methods. Since Monte Carlo simulations yield dose per particle, a methodology to measure the beam fluence of a monochromatic synchrotron-generated x-ray beam was developed using Compton scattering techniques. Method and Materials: X-rays were produced at the LSU CAMD synchrotron by passing a 1.3-GeV electron beam (≈200 mA) through a 7-T superconducting wiggler. The resulting polychromatic beam passed through a double multilayer monochromator to generate a ∼0.1×2.8-cm2 beam. Since high fluence rates make direct measurement of beam fluence impossible, photons Compton scattered by thin polyethylene targets into a 1-mm thick × 2.54-cm diameter NaI(Tl) scintillation detector were counted. Incident beam fluence was calculated from net peak counts via application of the Klein-Nishina collisional cross section for polarized radiation at multiple scattering angles. Simultaneously, percent polarization of the beam was determined by separating the Klein-Nishina cross section into its two orthogonally-polarized components. Measurements were taken at 4 scattering angles (15°–60°) with the fraction of photons (f), polarized such that the angle between the direction of incident polarization and the scattering plane is 0°, determined by minimizing the standard deviation of the calculated fluences. Results: Incident beam fluence rates for 100 mA of synchrotron ring current were determined with ∼1% precision and ranged from to . Measurement showed that the 35-keV beam was highly polarized in the plane of the synchrotron, e.g. f =7%. Conclusion: We have demonstrated a feasible method based on Compton scattering to determine incident synchrotron-generated x-ray beam fluence and polarization. Fluences determined via this method can provide input data that will allow currently underway Monte Carlo simulations to predict dose distributions in homogeneous phantoms for comparison with more traditional dosimetry methods (ion chamber and radiochromic film).

TH‐C‐230A‐04: Characterizing a Monochromatic X‐Ray Beam From a 1.3 GeV Synchrotron for Auger Electron Radiotherapy and Dosimetry Studies

Purpose: Auger electron radiotherapy and dosimetry methods are being studied in preparation for future small animal irradiations using monochromatic x‐ray beams and IUdR. Aims of the present study are: (1) establish methods for characterizing the LSU CAMD synchrotron monochromatic x‐ray beam and (2) validate MCNP dose calculations in polymethylmethacrylate (PMMA). Method and Materials: The synchrotron's tunable (6–40 keV), monochromatic beam was set to 15 keV and collimated to ≈2.5‐mm wide. Beam energy was determined from photons Compton scattered by a 56 mg⋅cm−2 aluminum target, whose energy was measured using a thin window 1‐mm thick × 2.54‐cm diameter NaI(Tl) scintillation detector. Beam cross section and divergence were measured using radiochromic film digitized with an Epson 1680 scanner. Depth‐dose measurements within a PMMA phantom were made using a 0.23 cm3 air‐ionization chamber. Ionization was converted to dose in air and PMMA at each depth and a percent depth‐dose curve generated. Results of MCNP5 Monte Carlo dose calculations simulating measured conditions were compared with measured data. Results: Measurements indicated the nominal 15 keV beam had energy of 15.5 keV, horizontal width of 3.1 cm, Gaussian distribution vertically with FWHM = 0.1 cm, and beam divergence <0.004 horizontally and vertically. A dose rate of 69 cGy⋅s−1, measured at 0.58‐cm depth in PMMA with 92 mA beam current, corresponds to 3.4×1011 photons⋅cm−2⋅s−1. Measured percent depth‐dose curve agreed with MCNP5 simulated curve, yielding a PMMA mass attenuation coefficient of 1.1 cm2 g−1, approximately equal to the NIST value. Conclusion: The LSU CAMD 15‐keV monochromatic beam has been characterized demonstrating utility of the measurement methods for future studies at energies suitable for iodine k‐edge capture (>33.2 keV). MCNP5 Monte Carlo calculations have been shown to predict depth dose in PMMA validating its use and showing its potential for future treatment planning dose calculations in small animals.

Comparative study of PP0275C hybrid photodetector and XP2020Q photomultiplier in scintillation detection

The properties of a hybrid photodetector (HPD), type PP0275C, produced by Delft Electronic Products B.V., for scintillation detection and spectrometry were studied and compared to a standard XP2020Q photomultiplier. The study was performed for several scintillators, such as NaI(Tl), CsI(Tl) and LSO(Ce) of different dimensions. The excellent capability of the HPD to resolve single photoelectron events was fully confirmed. However, the study of the HPD with the scintillators showed a dramatically reduced number of photoelectrons and a further deterioration of energy resolution, depending on the size (diameter or length) of the crystals. For a 10 mm diameter NaI(Tl) a number of 5000±250 photoelectrons/MeV was measured, which corresponds to about 56% of that observed with the XP2020Q with comparable quantum efficiency. An energy resolution of 9.2% for 662 keV γ-rays from a 137Cs source as measured with the HPD light readout indicated on a serious degradation, larger than that arising from the statistics of photoelectrons. In conclusion, the study showed that this HPD is optimized for single photon detection but its application to scintillation detection is very limited.

Evaluation of new photonis XP1472 nine-channel photomultiplier

The performance of a new Photonis XP1472 nine-channel photomultiplier (PMT) was studied with the main emphasis to the photoelectron numbers and photoelectron collection efficiency, as measured with a 10 mm diameter NaI(Tl) crystal coupled to the separate channels. Simultaneously, energy resolution and crosstalk were measured for all channels with a /sup 137/Cs source. Time jitter and time resolution for an LSO crystal coupled to the different channels were measured for 511keV annihilation quanta and compared to those measured earlier with XP2020Q PMTs. The studies confirmed high photoelectron collection efficiency in the PMT due to a new design of the input focalization system.

Design and Operation of a Whole-body Monitoring System for the Goiania Radiation Accident

With as many individuals involved in the Goiânia 137Cs accident who had high levels of internal contamination, it was necessary to improvise a whole-body counter installation in loco. The in-vivo counting system was located in a 4.0 X 3.5 X 3.5-m room, where seven layers of 2-mm lead sheets with dimensions of 2.0 m X 1.0 m were overlaid on the floor at loci that were equidistant from the walls. A 20-cm diameter NaI (Tl) detector was installed at a height of 2.05 m above the floor at the center of the room. The detector was shielded and collimated with 5 cm of lead. The enormous amounts of activity in the subjects required the detector to be positioned at a height of 2.05 m. Subjects were required to wear disposable clothing and lie on a reclining, fiberglass chair. Counting time for the subjects was 2 min (live-time). The minimum detectable 137Cs activity for this counting time was 7.3 kBq* (0.05 significance level). Besides the accident victims, all individuals who had direct or indirect contact with contaminated people or areas were also monitored. More than 300 people of both sexes, with ages varying from a few months to 72 y, were measured for whole-body radioactivity. The observed activities ranged from less than the minimum detectable activity (MDA) to 59 MBq.

Angular correlation of double internal bremsstrahlung in the electron capture decay of 55Fe

The angular correlation of photons emitted in double internal bremsstrahlung (DIB) associated with the electron capture (EC) decay of 55Fe has been investigated. Relative emission angles ranged from 50° to 150°. Photons from the 1 mCi source were detected in two 5 cm × 5 cm diameter NaI(Tl) counters; the sum energies of photons was in the range of 68–231 keV. Experimental results were compared with the theoretical predictions for 1s, 2s, 2p and 3p initial electron state contributions.

Comparison of in vivo scintillation probes and gamma cameras for detection of small, deep tumours

An in vivo probe was compared with a gamma camera for the task of detecting radiolabelled tumour models in a water phantom. The probe, which contained a 1 cm diameter NaI(Tl) detector, was designed and used by the authors for surgical staging studies of gynaecological patients. Tumours were spherical sources of different sizes and activities per unit volume of cobalt-57. The phantom was a tank of water (45 l) containing dissolved radioactivity to simulate background activity. Detector-to-source separations and tank depths were also varied. Camera images of 10 min duration were compared with probe counts of 15 s. For each configuration a large number of source and background runs were analysed using an ideal-observer ROC technique. Area under the ROC curve was used as the figure of merit. Results show that for approximately uniform background the probe should perform substantially better than the gamma camera in detecting small, deep tumours provided that the probe can be manoeuvred to within a few centimetres of the tumour. Mathematical modelling of the results indicates that this conclusion is not dependent on radiopharmaceutical dose, tumour uptake or camera type.

The 12C(3He, γ1+2)15O reaction in the giant dipole resonance region

The 90° differential cross section of the 12C(3He, γ1+2)15O reaction has been measured at 3He particle energies from 5.3 to 16.7 MeV in steps varying from 100 to 300 keV. The γ-rays were detected by a 23 cm long × 23 cm diameter NaI(Tl) crystal spectrometer enclosed in an anticoincidence shield. The yield curve shows a resonant behaviour at the energies Ex = 17.56 ± 0.06, 19.02 ± 0.12, and 23.80 ± 0.10 MeV.

Calibration factors for Ge detectors used for field spectrometry.

Calibration factors to convert a measured full-absorption peak count rate to activity in the soil and dose rate in air are given for Ge detectors that are used for field measurements of radionuclides. The appropriate factors for a given detector are derived using three primary parameters: the manufacturer's quoted efficiency at 1332 keV relative to a 7.6 cm (3 in.) long by 7.6 cm (3 in.) diameter NaI(Tl) detector, the detector's orientation in the field (up or down) and the Ge crystal length/diameter ratio. The accuracy of the results obtained by using this simplified calibration technique is estimated to be 10-15%.

Position Sensitive Photomultiplier Tubes for Scintillation Imaging

The Position Sensitive Photomultiplier Tube (PMT) was developed mainly for scintillation imaging in the field of high energy physics and medical applications. This PMT consists of a proximity-type photocathode and 11 staged mesh dynodes. For performing two-dimensional detection, two sets of wire anodes arranged in perpendicular with each other performs a read-out of the secondary electrons emitted from the last dynode. The intrinsic spatial spread of 4.3 mm FWHM and resolution of 0.3 mm FWHM were obtained for 4000 photons/pulse with LED by centroid calculation. Also, this PMT features an amplification gain of 10/sup 6/ and a quantum efficiency of 20% allowing for low energy radiation measurements and providing an energy resolution that is nearly the same as that of the conventional PMT. Finally, the authors show the reconstruction of two dimensional/scintillation image with 3'' diameter NaI(T1) crystal.

Development of the EXITE Detector: A New Imaging Detector for 20 - 300 keV Astronomy

We report on the development and testing of a detector to be used in the Energetic X-ray Imaging Telescope Experiment (EXITE). It consists of a 34 cm diameter NaI(Tl) cyrstal coupled directly to a single large image intensifier tube with associated silicon PIN diode readout. The measured spatial and energy resolutions at 122 keV are 6mm (FWHM) and 9% (FWHM), respectively. This energy resolution is ~50% better than that of any previously flown hard x-ray experiment. These resolutions decrease with the square root of the energy of the incident x-ray, indicating that they are determined by the number of photons emitted in the NaI(Tl) scintillator light flash.