ia om M. M Scintillator Dose-rate diat e de ve m fine the device be sensitive and simultaneously small, pocket-sized, of robust mechanical design and carriable on the user’s body. To serve these specialized purposes and requirements, we developed the alarm sitive, lly car The sensitivity requirements and standards required by HLS from the crystal is performed by a photomultiplier tube (PMT). uired f the nted Contents lists available at ScienceDirect .e Nuclear Instrument Physics Re Nuclear Instruments and Methods in Physics Research A 652 (2011) 41–44 production and low power requirements. Based on previousE-mail address:
[email protected] (D. Ginzburg). However, in the past few years a new photoelectric sensor, the 2.1. Radiation sensor The advantages of SiPM enable the production of a small and robust PRD yet with highly efficient light collection, low noise 0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.01.027 n Corresponding author. applications necessitate the use of a scintillation crystal for radiation detection. Traditionally, the readout of scintillation light 2. Methods training session. The inherent portability of this instrument provides a great advantage that allows the FLO to approach a suspected radiation source while maintaining safety and assuring that the radiation level preset is below the permitted value. refinishing of the crystal volume to comply with the req radiological performance, optional directional localization o source and specifications relating to this detector are prese herein. front-line officer (FLO). PRD is designed to alert the FLO regarding the presence of radioactive or nuclear materials and also to provide an alert when increasing background radiation levels are detected. PRD should feature a clear and simple indication of radiation presence and its intensity. This type of instrument can be used for automatic detection and for the assessment and localization of radioactive materials by nonexperts after a short scintillation crystal, has been intensively investigated for applica- tions of radiation detection by our research group [3–5]. Previous works were focused on optimizing the SiPM configuration to reduce the noise level and minimize electrical power consump- tion. In this article, we present a novel alarming device based on the successful optical coupling of a CsI(Tl) scintillation crystal with SiPM, resulting in the SENTIRAD PRD. Design considerations, 1. Introduction The main design goals of the detector (PRD) [1] are to be a sen lightweight radiation monitor usua SENTIRAD, a new radiation detector designed to meet the performance criteria established for counterterrorist applications. SENTIRAD is the first commercially available PRD based on a CsI(Tl) scintillation crystal that is optically coupled with a silicon photomultiplier (SiPM) serving as a light sensor. The rapidly developing technology of SiPM, a multipixel semiconductor photodiode that operates in Geiger mode, has been thoroughly investigated in previous studies. This paper presents the design considerations, constraints and radiological performance relating to the SENTIRAD radiation sensor. & 2011 Elsevier B.V. All rights reserved. ing personal radiation small, automatic and ried on the body by a silicon photomultiplier (SiPM) [2], was introduced in the field of radiation detection mainly for medical imaging applications due to its properties (such as size, time of flight and cost). Our innovative research focused on the implementation of SiPM into a portable radiation detector. SiPM, optically combined with a SENTIRAD—An innovative personal rad scintillation detector and a silicon phot A. Osovizky a, D. Ginzburg a,n, A. Manor a, R. Seif b, V. Bronfenmakher a, V. Pushkarsky a, E. Gonen b, T. a Radiation Detection Department, Rotem Industries Ltd., Israel b Electronics and Control Laboratories, Nuclear Research Center—Negev, Israel a r t i c l e i n f o Available online 12 January 2011 Keywords: Silicon photomultiplier Personal radiation detector a b s t r a c t The alarming personal ra applications. This portabl photon-emitting radioacti scope of specifications de journal homepage: www tion detector based on a ultiplier Ghelman b, I. Cohen-Zada b, M. Ellenbogen a, azor b, Y. Cohen b ion detector (PRD) is a device intended for Homeland Security (HLS) vice is designed to be worn or carried by security personnel to detect aterials for the purpose of crime prevention. PRD is required to meet the d by various HLS standards for radiation detection. It is mandatory that lsevier.com/locate/nima s and Methods in search A A. Osovizky et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 41–4442 studies [3], the CsI scintillation crystal doped with Tl was selected for this application due to its mechanical properties [6], relatively high density, light yield and scintillation-decay period. The max- imum emission wavelength of 550 nm perfectly matches the quantum efficiency of SiPM. The long decay constant (1 ms) enables collection of all of the scintillation light produced per g interaction during several generated pulses by a fast light sensor (integration mode). In HLS applications, the main difficulty to overcome is the low rate of radiation to be detected due to the high sensitivity requirements. Therefore, the limitation of high rates at which pulses can be accepted without pileup resulting from the long decay time is alleviated by these sensitivity requirements. SiPM was chosen for this application due to its advantages over PMT and the PIN diode based on the results of a comparative study, which are summarized in Table 1. SiPM was coupled to a CsI(Tl) crystal covered with a white diffusive reflector (Teflons type) to achieve optimized scintillation light collection. The best optical coupling was achieved using optical grease as the coupling medium. Bias voltage Low High Medium Low/none Bias power consumption Low High High Low Amplifier power consumption Low Medium High High Sensitivity to microphonics No No Medium Yes size Small Big Small Small Direct interaction No No No Yes Magnetic field No Yes No No Fig. 1. Radiation sensor design. Table 1 Summary of survey of alternative light sensors. Device SiPM PMT APD PIN diode The coupled sensor was then covered and sealed using opaque black tape to prevent external light from reaching the sensor. Readout from the radiation sensor is performed by an electronic unit, which includes a bias-voltage-supply circuit for SiPM and a fast pre-amplifier to integrate the output signal from SiPM. The pre- amplifier was specially designed and its parameters were optimized for radiation detection applications. Optimization was achieved by setting the amplifier impedance according to the SiPM electrical module characteristic, thereby reducing noise level and increasing the output pulse at the expense of response time. A further reduction of the noise level was achieved by enclosing the entire radiation sensor in an aluminum housing (see Fig. 1). Among the aspects of mechanical design of the radiation sensor and the instrument (see Fig. 2), it should be noted that the electronic components for the instrument were selected using the criteria of miniaturization and low power consumption, especially regarding the pre-amplifier components, as they were located inside the radiation sensor housing. 2.2. Sensitivity A cylindrical crystal 14 mm in diameter and 20 mm high was chosen on the basis of calculation of the intrinsic sensitivity volume complying with ANSI N42.32 Performance Criteria for Alarming Personal Radiation Detectors for HLS [5]. According to these requirements, the detector is required to respond within 2 s to an ambient radiation level of 50 mR/h above background level. The maximum count rates of a sensor remaining in compliance with the requirements cannot exceed 10,000 cps. The effective dose-rate indication should be a minimum of 2 mR/h. The required sensitivity of the detector is determined by the following parameters: signal (S), background (B), measuring time (t) and the detection reliability for false alarm (s1) and for missed alarm (s2). All PRD parameter values are defined by the ANSI 42.32 standard. Although the primary HLS application for these devices is radiation detection, they are still required to be capable of converting the detector count rate measurement (cps) into dose-rate units (e.g., mR/h) to provide a certain level of personal radiation safety to the FLO. To perform the cps-to-dose-rate conversion, a weighting function based on the interacting photon energy was employed. The radiation field caused by one photon with energy of 1 MeV is 10 times higher than the one caused by one photon with energy of 100 keV; however, the scintillator detection efficiency for the 100 keV photon is higher. Therefore, to define the required detector sensitivity, the detector count rate for radiation field caused by high-energy photons must be utilized. The scintillation crystal can provide this information from the signal pulse height, which is proportional to the inter- acting photon energy. The pulse height can be evaluated by digital signal processing (DSP); however, using a sampler for multi- channel analysis is a power-consuming method. An alternative method can be based on setting several energy windows with a Fig. 2. SENTIRAD PRD and its accessories. On the right: screen display unit with seven segments. different weighting factor for counts in each window. For an accuracy of 50% in the conversion of a reading into a radiation field, approximately five windows are needed. These five win- dows should cover the full energy spectrum from 50 keV up to 1.6 MeV (e.g., 50–100, 100–200, 200–400, 400–800 and 800– 1600 keV). This spectrum division demands a detector with fine pulse height resolution. The following calculations were proposed to calculate and verify the compatibility of detector efficiency (e). Using the values S¼50 mR/h, B¼25 mR/h and t¼2 s, as dictated by the standard, the decision level of the instrument, which is determined accord- ing to the deviation in false alarms, can be assumed to be much lower than the signal: ka ffiffiffiffiffiffiffi Bet p {Set ð1Þ Considering the worst case (when the left- and right-hand terms approach equality), the value of e can be calculated as e¼0.075 [cps/mR/h]. When using CsI(Tl) as the scintillation crystal, the required volume is 0.243 cm3, as calculated from 3:7eþ4 G� 4pr2 að1�e �mxÞ ¼ e ð2Þ where G(137Cs)¼0.32 [mR/h per mCi] at 1 m, a is the detector face area (cm2) and r is the distance from source to the detector face (cm). This volume is enough to fulfill the standard demand of ten alarms out of ten tests for the specified S, B and t values given above. The volume required for 60Co, calculated in the same way and using the appropriate G factor, is 1.82 cm3. 3. Results In our case, the crystal was a cylinder with r¼7 mm, h¼20 mm and a volume of 3 cm3. The sensitivity was sufficient to meet the standard requirements as validated using the linear- ity curve of the detector (Fig. 3). The slope of this curve is the sensor efficiency (the ratio of the photon flux through the detector to the number of interacting photons). This curve also defines the detector measuring range as the range where the curve maintains linearity. The value of sensor efficiency obtained from the slope of linearity curve was compared to the required sensitivity derived from the calculations above. The value of e calculated for the cylindrical crystal described above is 0.56 cps/mR/h; this value matches the empirically determined slope of the linearity curve. Fig. 4 presents the spectrum obtained from measurements of a cocktail of three isotopes: 241Am, with a photopeak at the energy of 60 keV, 137Cs, with a photopeak at the energy of 662 keV and 60Co, with a photopeak at the energies of 1173 and 1332 keV. All different count rates received at each photopeak demonstrate the need for a weighting function to convert the detector reading from cps into dose-rate units. The spectrum shows that although the resolution is poor (about 15% for 137Cs), energy windows can be implemented by dividing the spectrum into several regions, each having a multiplication factor for converting the readings into dose-rate units. Poor resolution can be observed from the 60Co photopeaks, which are almost combined into one broad peak. 3.1. Directionality Another advantage that SiPM provides is the ability to deter- mine the direction of the radiation source relative to the detector. This advantage can be implemented based on the miniature size of the light sensor and its pixilated nature. This is an important feature for the FLO once an alarm is triggered. A feasibility experiment was conducted and the results are presented in Table 2. Here the crystal volume was divided into four parts, and each part was covered with a reflector and coupled to an SiPM (see Fig. 5, left panel); therefore, the total sensitivity of the sensor was unchanged. All four sensors were read and compared against the geometrical location of the source (see Fig. 5, right panel). The results indicated a significant difference between detector readings. For each direction, there was a crystal (or two) with a higher count rate. The crystal closer to the source A. Osovizky et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 41–44 43 Fig. 4. Obtained spectrum of 241Am, 137Cs and 60Co; all three isotopes are placed in a way such that they produce the same radiation field. On the inset: 60Co three isotopes produce the same radiation field of 50 mR/h. The y = 0.5645x R2 = 0.9982 0 20000 40000 60000 80000 100000 120000 140000 160000 180000 0 350000 To ta l c ou nt s [c ps ] Field [uR/h] Total counts vs Field [mR/h] 50000 100000 150000 200000 250000 300000 Fig. 3. Measured linear response up to 300 mR/h. region-of-interest. attenuated the photon flux for the other crystals and the direction of the source could thus be established by comparing the count rates of the different sensors. To maintain low power consumption, all four sensors were wired to a single amplifier, which is used as long as the reading is below the alarm threshold level. Once the gross reading exceeds the threshold level, the output of each of the four SiPMs is routed to a separate amplification channel. Another advantage of this configuration is the ability to improve the detector noise threshold, which increases with temperature. Additionally, a higher ratio of the crystal’s coupling- surface area to the light-sensor surface area results in poorer light collection. Therefore, by dividing the volume into four smaller units, Table 2 Measurement results following the irradiation diagram. Source location Left (%) Left forward 451 (%) Right forward 451 (%) Forward (%) Right (%) Left back 451 (%) Right back 451 (%) Back (%) Crys 1 100 92 83 96 68 77 43 57 Crys 2 72 82 99 100 86 48 68 54 Crys 3 100 61 56 58 68 86 69 84 Crys 4 66 51 54 48 100 56 70 71 Forward Back Le ft R ig ht 45° 45° 45° 45° Crys 1 Crys 3 Crys 2 Crys 4 Fig. 5. Left: irradiation scheme (top view). Right: crystal divided into four parts and coupled to SiPM. the multiple amplifiers can be activated when the operating condi- tions require a lower noise level. The improved light collection also provides a better resolution, although a better resolution does not yield a major advantage in the current applications of PRD. The experimental setup included four identical scintillation crys- tals (CsI; 8�8�30mm3) optically coupled to corresponding SiPMs. The setup was irradiated with a 137Cs source and counts at the photopeak were evaluated as the criterion for detector exposure. The results presented in Table 2 demonstrate the correlation of counts received from each of the four detectors and the source position relative to the detector. The correlation shows the possibility of developing an algorithm based on intensity attenua- tion for 2D and partial 3D estimation of gamma source location (i.e., positioning of the gamma source). 4. Conclusion A novel configuration for an alarming radiation detector was presented. The advantages of the device were demonstrated with respect to both properties and application levels. The instrument can display significant information for the operator about the radiation level and thus eliminates the need for expert assistance. The draw- backs of the device, such as limited dynamic range, temperature dependence and relatively poor resolution compared to PMT-based detectors, do not prevent the use of SENTIRAD as an alarming device; conversely, its groundbreaking advantages enable the minimization of overall detector dimensions and, being a silicon based-device, afford the opportunity for cost-effective product fabrication. The size of the light sensor enables the division of the crystal volume into smaller units, thus achieving a directional property. Further research will be focused on coupling advanced scintilla- tion crystals with high-light-output emission and appropriate wavelengths for SiPM (such as SrI2). This configuration may reveal a route for a miniature spectroscopic personal radiation detector. However, for this type of device, stabilization of the spectrum will be required. In summary, a new personal radiation detector based on a scintillation crystal and silicon photomultiplier was presented. The thorough consideration given to the process of mechanical design of SENTIRAD resulted in the provision of a user-friendly and fully featured instrument for effective use by first-line officers in compliance with ANSI N42.32. References [1] American National Standards Institute, Performance Criteria for Alarming Personal Radiation Detectors for Homeland Security, ANSI N42.32 2006. [2] P. Buzhan, B. Dolgoshein, et al., Nucl. Instr. and Meth. A 504 (2003) 48. [3] A. Osovizky, D. Ginzburg, et al., in: Proceedings of the IEEE Nuclear Science Symposium, Orlando, FL, 2009. [4] A. Osovizky, U. Wengrowicz, et al., in: Proceedings of the IEEE Nuclear Science Symposium, Dresden, 2008. [5] A. Osovizky, D. Ginzburg, et al., IEEE Trans. Nucl. Sci. 57 (2010) 2758. [6] J.D. Valentine, W.W. Moses, S.E. Derenzo, D.K. Wehe, G.F. Knoll, Nucl. Instr. and Meth. A 325 (1993) 147. A. Osovizky et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 41–4444 SENTIRAD--An innovative personal radiation detector based on a scintillation detector and a silicon photomultiplier Introduction Methods Radiation sensor Sensitivity Results Directionality Conclusion References