Range verification of particle beams in real time is considered a key for tapping the full potential of radio-oncological particle therapies. The novel technique of prompt gamma-ray timing (PGT), recently proposed and explored in first proof-of-principle experiments, promises range assessment at reasonable expense but challenges detectors, electronics, and data acquisition. Energy-selected time distributions have to be measured at very high throughput rates to obtain the statistics necessary for range verification with single pencil beam spots. Clinically applicable systems should provide a time resolution of about 200 ps, to be obtained with large (about 2” diameter) scintillators, detector loads in the few-Mcps range, and data acquisition rates around 1 Mcps, if possible with compact and inexpensive systems. Such requirements can be met best with CeBr3 scintillators read out with conventional photomultiplier tubes, coupled to commercial but customized electronics featuring high-resolution pulse digitization and fast digital signal processing. The paper deduces design parameters from the constraints given by typical treatment conditions, and presents first results obtained with prototype detectors and electronics developed in accordance with the derived specifications.

A functional prototype two-plane Compton camera array for localization and identification of remote radiation sources, consisting of four PVT and four NaI(Tl) scintillation detectors with PMT readout, is presented. The large-volume, 76 × 76 ×76 mm scintillators provide a broad field of view for scattered photons and facilitate maximum efficiency at moderate cost. Each detector is equipped with a voxelSPEC, a compact electronic module that provides high voltage for the PMT, signal processing, detector stabilization, and an Ethernet communication interface. The voxelSPEC delivers list-mode event data with nanosecond precision timing over non-proprietary Ethernet and makes a system extension very easy. A software package has been developed for real-time data processing and image reconstruction. Advantages in the hard- and software allow stable, unattended operation of the camera array for many days, and provide easy-to-read information on the radiation source in real time. Measurements with the prototype array have been performed for a few standard scenarios and geometries to verify the model predications obtained by Monte-Carlo simulations. Simulations have been further performed to explore larger camera arrays with 2 × 4 × 4, 2 × 8 × 8, 2 × 10 × 10, 2 × 13 × 13 and 2 × 16 × 16 detectors.

Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF 2 (Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of −20°C to +55°C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.

The performance of a silicon drift detector (SDD) with an integrated FET, delivered by the company PNSensor, Munich, Germany, was studied in gamma spectrometry at room temperature (23-25 degrees C) with a LaBr3:Ce crystal of 6 mm diameter and 6 mm height. The SDD characteristics were compared with those measured with a Photonis XP5212 photomultiplier, a Large Area Avalanche Photodiode (LAAPD) of Advanced Photonix, Inc., and a Hamamatsu S3590-18 Photodiode (PD). Energy resolution versus gamma ray energies and its components related to the photoelectron/electron-hole pair statistics and dark noise were measured and compared. At low energies, below 100 keV, the light readout by the photomultiplier gives the best results, while for high energies, above 300 keV, the light readout by the SDD delivers superior energy resolution. In particular, the best energy resolution of 2.7% was determined for 662 keV gamma rays from a Cs-137 source.

Liquid scintillators loaded with boron-10 or lithium-6 are capable to detect gamma rays, fast neutrons and also thermal neutrons. One of the popular methods applied in order to distinguish events originating from different particles is the pulse shape discrimination (PSD). The previously presented study of boron-10 loaded liquid scintillators using the PSD method showed different discrimination performance in scintillators such as BC523A, BC523A2, EJ339A2 and EJ309B5. It triggered a further study of the light pulse shapes in these scintillators originating from events related to gamma rays, fast and thermal neutrons. The light pulse shapes, measured using the single photon method, were recorded together with the 2-dimensional n/gamma discrimination data. Next, the recorded light pulses were gated using energy and the PSD information to extract pulses characteristic of the only one kind of particles. Finally, the analysis of the light pulse shapes with multi-exponential fits and calculation of decay time constants and intensities of components were performed. The results were compared with the data obtained for liquid scintillators not sensitive to thermal neutrons BC501A, EJ301 and EJ309.

Performance of a He-3 counter and a B-10 loaded liquid scintillator EJ309B5 has been studied in terms of neutron detection efficiency. The measurements were carried out in a mixed field of neutron and gamma radiation from an intense (~106 neutrons/s/47t) 252Cf source. The response of both detectors to background and high intensity gamma radiation (~100 μSv/h at a detector) from a 60Co source has been measured to establish background count rate and gamma rays cut-off point, respectively. A method to compare the performance of various neutron detectors for border monitoring was proposed. The analysis showed that the properties of a He-3 counter are significantly better than that of EJ309B5. However, it has been pointed out how to improve the performance of a liquid scintillator in order to fulfill standards for neutron detection at borders.

The performance of several BrilLanCetrade LaBr3 crystals with the size of O6 times 6 mm2 up to O38 times 38 mm2 were studied coupled to the XP5212 and R6231MOD photomultipliers and in case of the small crystals, coupled also to large area avalanche photodiodes of Advanced Photonix, Inc. First, several photomultipliers of Photonis and Hamamatsu were compared in the work with LaBr3 to select the best one, not affecting energy resolution, besides the photoelectron statistics. The light output and energy resolution for 662 keV gamma rays from 137Cs source were measured for all crystals. Moreover, for some of them, the non-proportionality of the light yield and energy resolution versus gamma rays energy were measured and the intrinsic resolution of the crystals was calculated. For the smallest crystals of O6 times 6 mm2 further comparative tests with LAAPD were carried out.

A BC523A liquid scintillator loaded with boron-10 was tested as a detector for both fast and thermal neutrons. Pulse shape discrimination (PSD) method based on a zero-crossing principle was applied to distinguish between neutron and gamma radiation. High quantum efficiency Photonis XP5500B photomultiplier was used to enhance light detection from the scintillator. This allowed a good registration of the energy spectrum of neutron capture events on boron-10, corresponding to about 60 keVee. The applied PSD method proved to be useful for n/y discrimination. A good resolving power of the method was achieved even without gating on neutron capture events. A comparison with a standard BC501A liquid scintillator was done to evaluate thermal neutrons detection efficiency in BC523A.

Boron-10 loaded liquid scintillators were studied in order to improve n/γ separation. Pulse shape discrimination (PSD) was done by means of a zero-crossing (ZC) method to distinguish between γ-rays and fast/slow neutrons. A significant progress was achieved for BC523A2 and EJ339A2 loaded with reduced amount of 10B (2% and 2.5%, respectively), as compared to the results obtained earlier with BC523A loaded with 5% of 10B. The improvement was probably caused by the reduction of 10B content, as indicated in the latter study. A 10B loaded scintillator EJ309B5 based on a non-flammable liquid was also studied, showing the best separation of thermal neutron capture events from fast neutrons, γ-rays and noise.

The temperature dependence of light output, energy resolution and decay time constants of the light pulses of NaI(Tl), LaCl3(Ce) (LaCl3) and LaBr3(Ce) (LaBr3) crystals were measured over the temperature range of −30 to 60°C. In the study of the light output, the number of photoelectrons produced by the scintillators in the XP2020 photomultiplier was measured and corrected for by the temperature dependence of the quantum efficiency determined for 360 and 420nm, respectively. It showed a high stability of the light output of LaBr3 of about 0.01%/°C and a comparable uniformity of LaCl3 at a long peaking time of 12μs. The well-known thermal instability of NaI(Tl) was confirmed at a short peaking time of 2μs. However, a much better stability of NaI(Tl) at low temperatures was observed for a long peaking time. The study of the decay of light pulses from LaCl3 and LaBr3 crystals confirmed earlier measurements, while NaI(Tl) showed a complex behavior at different temperatures. At low temperatures a strong contribution of a slow component of up to 60% of the total light was observed, while at elevated temperatures a well-known initial slow decay was replaced by a delayed maximum and the slow component became insignificant. The results of the study of energy resolution seem to be correlated with the variation of both the light output and a dependence of the decay time constants of the light pulses at changing temperature. This is particularly interesting in the case of NaI(Tl), where different dependencies of the energy resolution as a function of temperature for different peaking times in the spectroscopy amplifier were found. Tests of the XP2020 PMT itself showed that the thermal instability of the gain of the dynode structure of about−0.4%/°C is a dominating effect. The opposite effect on an increasing quantum efficiency, partly compensating for the gain instability, was observed above 10°C for the longer wavelength of 420nm.

Scintillation detectors must tolerate a wide range of ambient temperatures and strong temperature slopes when used in outdoor applications. Such demanding conditions are typical for all homeland security applications. An effective and efficient detector stabilization compensating for temperature dependent gain shifts is essential to maintain energy calibration and resolution. Reliable, well established solutions are based on radioactive reference sources; however, alternatives are much asked for. The gain shift correction for the temperature dependence of the scintillation light output requires elaborate hard and software means without a reference source. Strong and rapid temperature changes further complicate the situation as there is no thermal equilibrium in the detector but rather a temperature field. Our paper demonstrates the measurement of an effective scintillator temperature by analyzing the pulse shape of detector signals. The pulse shape is correlated with the scintillation light decay time which can be extracted online from the digitized signals. The decay time data are used to eliminate all the temperature determined system gain shifts without radioactive reference source. This new stabilization procedure has been verified in extensive climate chamber measurements. The results are discussed.

Photomultiplier tubes are often stabilized with pulsed LEDs used as reference light sources. However, if detectors must be operated in a wide temperature range, the temperature dependence of the LED light emission must be considered as well. Such demanding conditions are typical for outdoor applications, e.g. in the field of homeland security. This problem is solved by means of a new method. The actual LED temperature is derived from a measured pulse height spectrum while the LED is operated in two alternating modes distinguished by different pulse voltages. Since the temperature dependence of the light output varies with the pulse voltage, the ratio R of the peak positions corresponding to different voltages is a function of the LED temperature as well. This ratio can be determined with an unstable PMT. Measuring L and determining R at different temperatures yields a calibration function L(R). With this knowledge, commercial off-the-shelf LED components can be used as precision light sources in a wide range of ambient temperatures.

The properties of CdWO/sub 4/ (CWO) crystals in gamma spectrometry were studied. Several small samples of 10/spl times/10/spl times/3 mm size, typically used in CT X-ray detectors, were tested and then compared to the performance of a larger crystal of 20 mm in diameter and 20 mm in height. The light output, energy resolution, and nonproportionality of the CWO response versus gamma-ray energy, were measured and compared with those of a small BGO to discuss further the origin of the intrinsic resolution of pure undoped scintillating crystals. A high light output of 6500/spl plusmn/200 phe/MeV and a good energy resolution of 6.6/spl plusmn/0.2% for 662 keV gamma rays from a /sup 137/Cs source were measured for the small samples coupled to an XP3212 photomultiplier. Common nonproportionality curves and consequently common intrinsic resolutions of small CWO and BGO suggest that they represent fundamental characteristics of the heavy oxide scintillating material themselves.