Jürgen Stein

Stabilizing scintillation detector systems by exploiting the temperature dependence of the light pulse decay time

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 hardware 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. This paper presents a new technique of gain stabilization which considers the effective scintillator temperature by analyzing the average pulse shape of detector signals. The pulse shape is correlated with the scintillation light decay time. This parameter can be extracted online from digitized detector signals. The decay time data are used to eliminate all the temperature determined system gain shifts without radioactive reference source. This technique has been verified in extensive climate chamber measurements. The results are discussed.

Stabilizing scintillation detector systems with pulsed LEDs: a method to derive the LED temperature from pulse height spectra

Photomultiplier tubes (PMTs) can be stabilized with light emitting diodes (LEDs) used as reference light sources. The LED is supplied with short voltage pulses that produce well-defined light portions if the LED temperature is kept constant. However, if the PMT must be operated in a wide temperature range, the LED light output is no longer a constant but becomes a function of the junction temperature. This problem can be solved at low expense by means of a new method. The LED is operated in two alternating pulse modes distinguished by the pulse voltage. The LED light output depends on the pulse voltage and, therefore, the PMT pulse height spectrum shows two distinct peaks. However, not only is the total amount of light L emitted per pulse but also the shape of the temperature dependence L(T) varies with the pulse voltage. The numerical ratio R of the peak positions corresponding to the two different LED modes is therefore a function of the LED temperature as well. Since it is a ratio, R is independent of the actual PMT gain but characterizes the LED junction temperature. Thus, the pulse height ratio of the different LED signals can serve as an LED thermometer. Moreover, measuring L and R at several temperature points covering the full operational range yields a calibration function L(R). With this knowledge, commercial off-the-shelf LED components can be used as precise reference light sources in a wide range of ambient temperatures

Radiation detector signal processing using sampling kernels without bandlimiting constraints

For the development of digital signal processing systems for fast scintillation detectors we comprehensively study the modeling of nuclear signals, deconvolution of detector pulses and signal sampling. Applications for new scintillators with light decay times of a few nanoseconds demand suitable low power digital systems running at lowest possible sampling rates. We are interested in accurate sub-nanosecond timing and optimal energy resolution. The generalized non-bandlimiting sampling theorems allow filter structures with lower than Nyquist sampling rates for certain signals, where the classical sampling theorem fails. Recently it was shown that, by using a sampling rate greater or equal to the rate of innovation, it is possible to reconstruct certain restricted signals uniquely. The class of sampling kernels that can be used contains transfer functions with rational Fourier transforms. We introduce a physically realizable sampling scheme combined with a deconvolution timing filter algorithm for radiation detector signals. The novel architecture achieves sub-sampling rate timing accuracy together with optimal energy resolution and a high throughput for LaBr3:Ce scintillation detector system.

Stabilizing scintillation detector systems: determination of the scintillator temperature exploiting the temperature dependence of the light pulse decay time

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.

Comparison of different Cs 2 LiYCl 6 :Ce crystals: Energy resolution and pulse shape dependences on temperature

Portable radiation detection systems will greatly benefit from the use of the same detector for gamma and neutrons, on the roadmap towards miniaturization, provided such devices are capable of robust discrimination of gammas against neutrons. One of the most promising and yet commercially available inorganic scintillator, delivering different pulse shapes for thermal neutrons and gammas is the Cs2LiYCl6:Ce (CLYC) [1,2]. The detection for thermal neutrons follows through the 6Li(n,alpha)3H capture reaction, and with the appropriate levels of 6Li enrichment, which have currently reached 95%, the CL YC crystals could become very efficient neutron detectors. In order to successfully integrate this new scintillator in our digital devices, the variability of the energy resolution across various crystals, the dependence of the energy resolution on energy, shaping time and temperature, the pulse shape parameters variability across the crystals and the thermal neutron sensitivity were investigated.

Anomalous gain drop effects in Hamamatsu 3998-01 photomultiplier

Currently, in our IdentiFINDER LGH instruments, commercially available 30S30_B380 (LaBr3) detectors, purchased from Saint-Gobain Crystals, are deployed. They are equipped with R3998-01 photomultipliers from Hamamatsu Photonics. With this type of photomultiplier, we recently encountered a new issue in our devices related to a unprecedented gain drop/recovery, occurring at a certain temperature and count rate. The experiments will be outlined and the results presented.

CaF 2 (Eu): An “Old” scintillator revisited

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.