Experimental determination of absolute efficiency and energy resolution for NaI(Tl) and germanium gamma ray detectors at energies from 2.6 to 16.1 MeV

Measurements have been made to characterize the response of NaI(Tl) and Ge(Li) gamma ray detectors to gamma rays in the energy range of 2.6 to 16.1 MeV. Both absolute efficiency and energy resolution are reported. At 16.1 MeV the absolute full energy efficiency of a 10 cm × 10 cm NaI(Tl) detector is about 2% and the energy resolution is also about 2%. For a 65 cm³ Ge(Li) detector, the full energy peak absolute efficiency at 16.1 MeV is 0.2% (the escape peak efficiencies are 5 times larger), and the fwhm energy resolution is about 0.1%.     

A modular NaI(Tl) detector for intermediate energy photons

We describe the properties of a detector array made up of 64 NaI(Tl) 406 × 63 × 63 mm³ modules, used as an intermediate energy photon spectrometer. We obtain an energy resolution of 6% FWHM at 129 MeV, a time resolution of 1 ns FWHM and a resolution of 48 mm FWHM for the location of the impact point on the front face of the detector. The modularity allows to some extent a discrimination between photons and neutrons. We also present the response of the detector to 69 MeV neutrons.     

Development of a NaI(Tl) detector with superior photon energy resolution for use above 100 MeV

We have designed and tested a new high resolution NaI(Tl) total absorption scintillation counter. The detector is a cylinder composed of a 26.7 cm diameter by 55.9 cm long NaI core with a concentric 10.8 cm thick NaI annulus that is divided into quadrants. The NaI detector is surrounded by a 12.7 cm thick plastic scintillator to veto both cosmic rays and events with significant shower leakage from the NaI. High uniformity of light production and collection throughout the detector is required for superior resolution. The detector has a measured resolution of 1.3% and 1.7% FWHM for 130 MeV photons and 330 MeV electrons, respectively. Computer simulations to account for loss of resolution due to pileup and energy spread of the beam indicate that the ultimate experimental resolutions at these energies are 1.2±0.1% and 1.3±0.1%. The resolutions at these two energies are at least a factor of 2 better than that of any other total absorption scintillation counter available today. Based on shower simulations, the detector is expected to have a resolution of approximately 1.3% for collimated 130–2000 MeV photons.     

Energy measurement of 50 MeV proton beam with a NaI(Tl) scintillator

Energy measurement of 50 MeV proton beam produced on the AVF MC-50 Cyclotron was conducted using a detector telescope with a NaI(Tl) scintillator as an E counter. Protons of various energies, elastically and inelastically scattered from the ¹²C target nucleus were measured at four different angles of 35°, 40°, 50° and 55°. We applied the chi-square method to determine the beam energy, which showed a well defined minimum chi-square corresponding to a beam energy of 49.6 ± 2.3 MeV at the 68% confidence level. Also the light output response of NaI(Tl) to proton energies between 31 and 44 MeV is linear within 0.5 MeV and is in good accord with the recent result of Romero et al. [Nucl. Instr. and Meth. A 301 (1991) 241].     

A large high-resolution sodium iodide spectrometer

The new NaI detector system for high-energy gamma-ray detection at The Svedberg Laboratory is presented together with results from test experiments. The system has high efficiency, good energy resolution and rejects cosmic radiation efficiently. For example, the resolution is 1.6% at 22.6 MeV, the best value obtained so far for a NaI detector in this size category.     

Dosimetry of a nearly monoenergetic 6 to 7 MeV photon source by NaI(Tl) scintillation spectrometry

The dosimetry of a nearly-monoenergetic 6–7 MeV photon source developed at the National Bureau of Standards (NBS) for radiation protection instrument calibration has been carried out by NaI(Tl) scintillation spectrometry. This approach uses calculated 3 in. × 3 in. NaI(Tl) detector-response functions that have been shown to be reasonably accurate up to 20 MeV. A least-squares fit of the appropriate response functions to a selected region of the pulse-height distribution determines the primary 6–7 MeV photon fluence. The uncertainty in the fluence determination is based on the χ² of the fit, the statistics of the data, and the uncertainty in the response functions. The air kerma delivery due to the primary photons at a reference point in the photon field was calculated from the primary photon fluence. The uncertainty in the determination of air kerma delivery for primary photons was less than 5% (1 std. dev.). The primary high-energy photon contribution to the detector response was subtracted from the data and the remaining distribution due to lower-energy photons was evaluated by spectrum unfolding analyses. The spectrum-unfolded results indicate that a contribution of approximately 12% of the total air kerma was mostly from a continuous distribution of photons extending up to 4.5 MeV.     

Application of CLTD’s for High Resolution Mass Identification and for Stopping Power Measurements of Heavy Ions

An array of calorimetric low temperature detectors (CLTD’s) for energy sensitive detection of heavy ions was combined with time-of-flight (TOF) detectors to obtain a detector system for high resolution mass identification of low energy heavy ions. In addition the same setup was used to prove the ability of CLTD’s to be used in electronic stopping power measurements for heavy ions in matter. Experiments with 50?MeV ⁶³Cu and ⁶⁵Cu ions at the tandem accelerator at the MPI at Heidelberg, and with 25 to 250?MeV ²³⁸U ions at the UNILAC accelerator at GSI at Darmstadt have been performed. For ^63,65Cu at 50?MeV a mass resolution of Δm(FWHM)=0.9?amu, and for ²³⁸U in an energy range of 65 to 150?MeV a resolution of Δm(FWHM)=1.28?amu, was obtained. The results for stopping powers of ²³⁸U in carbon and gold are presented and compared with theoretical predictions and data from the literature.     

Determination of the number of pulsed laser-Compton scattering photons

We measured pulse-height spectra of 16.7 MeV laser-Compton scattering photons with a 6 in.×5 in. NaI(Tl) detector for blank and three lead materials of 75.8, 50.9, and 25.9% transmissions at the NewSUBARU facility to investigate how the original Poisson distribution of the pulsed photons is modified after passing through thick-target materials. We present a well-prescribed method of determining the number of incident photons within 3.5% accuracy based on the response of the NaI(Tl) detector to the pulsed photon beams.     

The trigger detector for APEX: An array of position-sensitive NaI(Tl) detectors for the imaging of positrons from heavy-ion collisions

We describe a 24-element position-sensitive cylindrical NaI(Tl) detector which allows positron imaging with an on-axis position resolution of 3 cm FWHM and annihilation radiation energy resolution of 13% FWHM. Two such detectors will provide the trigger for the APEX experiment at Argonne National Laboratory, which is exploring anomalous positron and electron production observed in heavy-ion collisions.     

Energy non-linearity effects in the response of ionic crystal scintillators to X-rays with energy in the region of the K-absorptions edges: experimental results

The response of a YAP, NaI(Tl) and BaF₂ scintillators to X-rays with energies around the Y, I, and Ba K-absorption edges, respectively, was investigated. For all the scintillators, the amplitude response follows different linear trends for X-ray energies below and above the respective K-edges, presenting a discontinuity at these energies. An abrupt decrease of about 3%, 5% and 2% were observed in the detector amplitude at the K-edges, for the YAP, the NaI(Tl) and the BaF₂ scintillator, respectively, corresponding to a decrease of 0.5±0.1, 1.7±0.3 and 0.8±0.2 keV in the energy calibration line. These discontinuities result in a region within 0.5±0.1, 1.6±0.3 and 0.9±0.2 keV where the X-ray energy cannot be obtained unambiguously. The scintillation yields for X-rays present abrupt decreases of about 3%, 4% and 2%, respectively, at the K-edges. The measured non-linearity effects are significantly larger than those obtained for gaseous and semiconductor detectors. The higher amplitude non–linearity observed in NaI(Tl) is attributed to the larger light yield non-linearity in the electron response of this crystal.     

Evaluation of two Compton camera models for scintimammography

We study the performance of a Si/LaBr₃:Ce Compton camera model for scintimammography, and compare it with a Si/NaI(Tl) model of similar geometry. The GEANT4 simulation toolkit was used to study the behaviour of the cameras at 511 keV. Certain simulation steps, such as the modelling of radionuclide decay times, scintillation photon transport and interactions with photomultipliers, as well as detector dead time corrections were included to make the modelling of the cameras more realistic than previous studies. The Si/LaBr₃:Ce Compton camera shows superior efficiency of 2.0×10⁻³ and resolution of 5.3 mm over the Si/NaI(Tl) Compton camera model which has the efficiency of 1.6×10⁻³ and resolution of 6.9 mm at a source-to-scatterer distance of interest, 2.5 cm. A similar result sequence is obtained for two breast tumours of 5 mm diameter embedded in the medial region of an average-size breast phantom of thickness 5 cm. Notably, the signal-to-noise ratios (SNR) obtained for the Si/LaBr₃:Ce camera are 9.7 and 3.4 for tumour/background radiation uptakes of 10:1 and 6:1, whereas 6.8 and 2.4 were obtained for the Si/NaI(Tl) camera model for the same tumour/background radiation uptakes respectively. It is therefore envisioned that with lower cost, LaBr₃:Ce could replace NaI(Tl) as the Compton camera absorber.     

A background rejection efficiency calculation for a proposed gaseous xenon TPC in a double beta decay experiment

A detailed calculation of the double beta (ββ) decay distribution and probability are presented, together with the results of a Monte Carlo simulation. The expected background rejection efficiency and achievable neutrinoless ββ decay half-life for a proposed high pressure (5–10 atm) gaseous xenon time projection chamber are calculated. The half-life is comparable to or longer than the present limit (∼ 10²³yr). It is found that ∼ 4π sr active shielding with NaI scintillator is effective in rejecting e^-e⁺ events when the energy resolution is 2% or better at 2.5 MeV. Monte Carlo calculations of electron tracks from neutrinoless ββ decay events show that a magnetic field can be used to distinguish ββ decay events from e^-e⁺ events when the energy resolution is ≥ 2%.     

Performance of a BGO calorimeter with photodiode readout and with photomultiplier readout at energies up to 10 GeV

Bismuth germanate (BGO) calorimeter arrays, consisting of up to 12 elements of 30 × 30 × 200 mm³ have been tested at the CERN PS with pions and electrons of up to 10 GeV/c momentum, and at SIN with pions, electrons and protons up to 450 MeV/c. Both photomultiplier (PM) and photodiode (PD) readouts were used. Accurate calibration in the 100 MeV energy range was achieved with stopping protons, stopping pions and minimum ionizing pions. With 212 MeV electrons and PM readout, a time resolution of the BGO signal of 640 ps fwhm has been measured. The energy resolution for electrons above 1 GeV (PD readout) was found to be roughly constant at σ/E ～ 1%. This is consistent with a negligible intrinsic resolution for BGO at these energies, after taking into account shower leakage and PD noise. For electrons of 92 and 200 MeV, we obtained (PM readout) energy resolutions close to the theoretical limit given by photon statistics and shower leakage. The electron/hadron separation was better than 1:500 over the energy range of 0.5 to 10 GeV, and improved to better than 1:1000 after a simple pattern cut. The energy deposition of the e.m. showers, both laterally and longitudinally (rear leakage), was found to be in agreement at the 0.1% level with Monte Carlo calculations using the SLAC-EGS program.     

Mass resolution of recoil fragment detector telescopes for 0.05–0.5 A MeV heavy recoiling fragments

The factors governing the mass resolution for 0.05–0.5 A MeV recoil nuclei have been investigated for detector telescopes in which carbon-foil time zero detectors and ion-implanted silicon detectors are used to determine the time of flight and energy respectively. Experimentally determined second moments of the mass distribution have been compared with theoretical estimates based on literature data. The experimental mass resolution is in reasonably good absolute agreement with theoretical estimates. For low energy (< 0.3 A MeV) particles the mass resolution is dominated by the contribution from the silicon detector and thus largely independent of timed flight length. In fact for detection of very low energy (0.1 A MeV) recoil nuclei timed flight lengths of less than 0.22 m are sufficient.     

Resolution tests of CsI(Tl) scintillators read out by pin diodes

Cylindrical CsI(Tl) scintillators of 38 mm diameter and 100 mm length read out with PIN diodes of 400 mm² area were tested with respect to their response to medium energy light particles (p, d, t, α). Resolutions of better than 1% were achieved for 50 MeV protons and 90 MeV α-particles. For many crystals the resolution was found to be limited to 2–3% by local crystal nonuniformities which caused variations of the light output efficiency of several percent. A bench test is described which allows the detection of inhomogeneities to better than 0.5% accuracy. The quality of particle identification obtained with ΔE-E and pulse shape discrimination techniques are investigated as a function of count rate.     

High-resolution short range ion detectors based on 4H-SiC films

The energy resolution of SiC detectors has been studied in application to the spectrometry of α particles with 5.1–5.5 MeV energies. The Schottky barrier structure of the detector was based on a CVD-grown epitaxial n-4H-SiC film with a thickness of 26 μm and an uncompensated donor concentration of (1–2)×10¹⁵ cm^?3. An energy resolution of 0.5% achieved for the first time with SiC detectors allows fine structure of the α particle spectrum to be revealed. The average energy of the electron-hole pair formation in 4H-SiC is estimated at 7.71 eV.     

Improvement on the light yield of a high-Z inorganic scintillator GSO(Ce)

Cerium-doped gadolinium silicic dioxide crystal, GSO(Ce), is a high-Z non-hydroscopic scintillator that gives higher light yield than BGO, and can potentially replace NaI(Tl), CsI(Tl) and BGO in many applications. Its production cost, however, has been substantially higher than any of them, while its energy resolution has been worse than that of NaI(Tl) or CsI(Tl). The merit did not overcome these deficiencies except in limited applications.

We developed a low background phoswich counter (the well-type phoswich counter) for the Hard X-ray Detector of the Astro-E project based on GSO scintillator. In the developmental work, we have succeeded in improving the light yield of GSO(Ce) by 40–50%. For energies above 500 keV, a large GSO(Ce) crystal (4.5 cm×4.5φ cm) now gives energy resolution comparable to or better than the best NaI(Tl) when read out with a phototube. With a small GSO(Ce) crystal (5×5×5 mm³) and a photodiode, an energy resolution comparable to or better than the best CsI(Tl) has been obtained. With this improved performance, we find that the much higher photopeak efficiency and the shorter scintillation decay time of GSO(Ce) offsets its higher cost for many applications.

We summarize our past developmental work to decrease radioactive contamination and to increase light yield of GSO(Ce) for astronomical hard X-ray detection. Included also are measurements done after the unsuccessful launch of the Astro-E mission. The work is still continuing for the remake version of Astro-E Hard X-ray Detector.     

A Si(Li)-Pb shower calorimeter for p-p collider experiments

A silicon/lead sandwich calorimeter with 38 cm² in active area and 10 radiation lengths in depth has been constructed. The performance has been investigated for incoming electrons of 250 to 750 MeV. The calorimeter shows a good linearity over the electron energy region and the energy resolution was well expressed by σ(rms)/E = (16.5 ± 0.5)/√E(GeV) %. Also, it is shown that the deposited energy and energy resolution do not change greatly even when the incident beam position is very close to the detector edge. The agreement between these results and a Monte Carlo simulation is quite satisfactory.     

Effects of packaging SrI2(Eu) scintillator crystals

Recent renewed emphasis placed on gamma-ray detectors for national security purposes has motivated researchers to identify and develop new scintillator materials capable of high energy resolution and growable to large sizes. We have discovered that SrI₂(Eu) has many desirable properties for gamma-ray detection and spectroscopy, including high light yield of ∼90,000 photons/MeV and excellent light yield proportionality. We have measured <2.7% FWHM at 662 keV with small detectors (<1 cm³) in direct contact with a photomultiplier tube, and ∼3% resolution at 662 keV is obtained for 1 in.³ crystals. Due to the hygroscopic nature of SrI₂(Eu), similar to NaI(Tl), proper packaging is required for field use. This work describes a systematic study performed to determine the key factors in the packaging process to optimize performance. These factors include proper polishing of the surface, the geometry of the crystal, reflector materials and windows. A technique based on use of a collimated ¹³⁷Cs source was developed to examine light collection uniformity. Employing this technique, we found that when the crystal is packaged properly, the variation in the pulse height at 662 keV from events near the bottom of the crystal compared to those near the top of the crystal could be reduced to <1%. This paper describes the design and engineering of our detector package in order to improve energy resolution of 1 in.³-scale SrI₂(Eu) crystals.