Development of a position-sensitive fast scintillator (LaBr3(Ce)) detector setup for gamma-ray imaging application

We have characterized a Cerium doped Lanthanum Bromide (LaBr3(Ce) ) crystal coupled with the position-sensitive photo-multiplier system for the gammaray imaging application. One can use this detector set-up for the scanning of high purity germanium detectors for pulse shape analysis in gamma-ray spectroscopy experiments and the image formation of an object by Compton backscattering . The sensor has been tested for energy, timing and position information of the gamma-rays interacting within the detector crystal. The GEANT4 simulation results are consistent with the experimental results. We have reconstructed the image of irradiation spots in different positions throughout the detector crystal. Position resolution is found to be around 3.5 mm with the 2 mm collimated gamma-rays. The 2-d image of hexagonal Bismuth Germanate (BGO) crystal and a cylindrical LaBr3(Ce) crystal have been reconstructed in coincidence technique. The performance of the detector for imaging application has been investigated by coincidence technique in GEANT4 simulation and compared with the experimental data. We have reconstructed the 2-d images of objects with various geometrical shapes by Compton back-scattered events of the gamma-rays. This position-sensitive detector can be used as an absorber of a Compton camera for the image reconstruction of an extended radioactive source. One can also use this kind of set-up as in radiation imaging and many other applications where the energy and source position of the gamma-ray is the main interest.


I. INTRODUCTION
Position Sensitive Photo Multiplier Tube (PSPMT) coupled with scintillator crystal converts the associated scintillation light into a current signal in the localized photo-cathode [1,2]. The current signal distribution contains important information regarding energy, timing and the interaction position of the gamma-ray in the detector crystal [3,4,5]. Such position measurements are also often carried out using other detector arrays by combining signal channels in a multichannel analog readout, or by using complex segmented detectors, but potentially, at the expense of other performance parameters, and also not viable in compact arrangements. In the present work, a Ce doped LaBr3 crystal coupled with a position sensitive photo-multiplier assembly has been characterized, where the signal processing unit could be made highly compact by coupling a resistive chain with the PMT. In this way, the electronics for signal processing can be reduced without impacting the image quality [4]. There are some work on position characterization of the similar kind of detector where individual readout has been read and digitized [6,7]. This setup has a versatile impact, either as a stand-alone assembly or in concurrence with an ancillary detector in various important fields. It is expected to find use in i) scanning of segmented high purity Ge (HPGe) detectors for pulse shape analysis [8][9][10][11] in gamma-ray spectroscopy experiments, in order to reconstruct the interaction path within the detector volume, ii) testing the electrical response of granular Si charged-particle detectors/sensors as a function of the interaction position of incident particle [12], iii) medical imaging of large organs/sections of the body, for situations where conventional cameras may be difficult to optimize, iv) small animal imaging for pre-clinical research in the pharmaceutical sector, v) the defense and nuclear wastedisposal sectors for identification of radiation sources as well as for environmental radiation surveys by means of a scattererabsorber configuration.
In this work, a LaBr3(Ce) scintillator crystal, with high light output, low self-absorption, and high sensitivity [13,14] for low energy gamma-rays, has been coupled with a compact readout PSPMT and employed for gamma-ray imaging. This detector has an energy resolution value of about 6% for 511 keV gamma-ray photo peak. The signal produced in the photomultiplier tube by scintillation light output of the crystal has been distributed through a resistive chain circuit. The front, side, and back view of the detector assembly are shown in Fig. 1. The scintillator crystal is chosen on account of its good timing resolution, in order to reject the scattered and background events by coincidence technique for image reconstruction [15]. Spots at different positions on the PSPMT-coupled-crystal have been irradiated, and the position resolution has been determined therefrom. Subsequently, the 2-d images of auxiliary detectors, such as a hexagonal-shaped BGO crystal as well as a cylindrical LaBr3(Ce) crystal, have been reconstructed in the PSPMT detector using the coincidence technique. The experimental results are found to be in good agreement with simulations carried out using the GEANT4 architecture (version 4.10.05). Subsequently, the Development of a position-sensitive fast scintillator (LaBr 3 (Ce)) detector setup for gamma-ray imaging application applicability of the present system for imaging via the Compton back-scattering technique has been illustrated by simulating the 2-d image of a ring-shaped scatterer. Further more, this kind of setup can be use as a Compton's camera by adding a thin position sensitive detector as scatterer with this LaBr3(Ce)-PSPMT detector.

II. MATERIALS AND DETECTOR DETAILS
The position sensitive detector crystal used in this work has been manufactured using pure LaBr3 with 5% doping of Ce. The crystal was grown by Saint Gobain Crystals and Detectors [16]. It has a high density (5.08 gm/cm 3 ), high light output of ≈ 63 photons/keV, and a fast timing characteristics with time resolution less than 500 ps. 138 La (natural abundance 0.089%) is known to decay by two parallel paths, namely (i) βdecay to 138 Ce (34.4%) and (ii) electron capture decay to 138 Ba (65.6%) [17]. These processes are associated with the emission of 789 keV and 1436 keV γ-transitions [18], respectively, which also contribute to the total internal activity of the detector material, measured to be 3 counts sec ∼ −1 cm −3 in the present work. An appropriate thickness of the crystal is chosen that allows seamless detection for a few hundred keV gamma-rays, without much contamination owing to internal activity. The crystal has dimensions measuring 5.08 cm × 5.08 cm × 0.6 cm , and has been coupled to a compact readout PSPMT (Hamamatsu-8500C) [19] through a 5 mm long light guide. The entire assembly is housed inside a 0.5 mm thick aluminium casing. The PSPMT of demountable design is equipped with a mesh of 8 horizontal (X) and 8 vertical (Y) anode-wires, as well as a bi-alkali photo-cathode. The output signal from the photo-cathode is distributed among the anode mesh-wires through an external network of resistors, from which four position signals are derived by means of chargedividing detection. This allows a compact setup with nominal electronic circuitry.

III. EXPERIMENTAL DETAILS
The cathode signal of the LaBr3(Ce)-PSPMT gives the gamma-ray energy and timing information, and the four anode signals contain X-Y position information of the gamma-ray interaction point in the detector crystal. An operating voltage of -1050 V leads to low internal noise and high signal gain. For characterization of the position sensitive detector, such as estimation of spatial resolution of irradiation spots, as well as for position calibration of the crystal, a 57 Co (0.5 μCi) radioactive source (122 keV gamma-ray) [5] has been used. In the course of data acquisition, the performance of the crystal as an imaging detector has been investigated, wherein the events due to internal radioactivity and scattering within the crystal have been suppressed as a result of acquiring data in coincidence mode. In this regard, a 22 Na (14 μCi) radioactive source has been used where positron-electron annihilation leads to simultaneous emission of two 511 keV gamma-rays in the diametrically opposite directions [20]. The schematic diagram of the set-up for the coincidence technique used in this work is shown in Fig. 2. To detect these two 511 keV gamma-rays, the LaBr3(Ce)-PSPMT is used along with an auxiliary detector kept on the opposite side of the radioactive source. The distance of the source from both the detectors, placed face-to-face, has been optimized for count rate handling capacity of the data acquisition system and also to obtain an optimum size of the image of the complementary detector reconstructed by the LaBr3(Ce)-PSPMT. For this purpose, a hexagonal BGO detector and a cylindrical LaBr3(Ce) detector have been used as auxiliary detectors, with dimensions of 5.59 cm × 7.62 cm (height × length) and 5.08 cm × 5.08 cm (diameter × length) respectively. Only those yields recorded by the LaBr3(Ce)-PSPMT assembly have been registered as true events which were in coincidence with the BGO/cylindrical LaBr3(Ce) detector.
All current approaches to the characterization of the position sensitive detector are based on the standard CAMAC acquisition system. To extract energy and position information, the cathode and four anode signals of the LaBr3 (Ce)-PSPMT, respectively, have been processed using spectroscopic amplifiers. The coincidence interval is defined using a TAC module, wherein the signal of the complementary BGO/cylindrical LaBr3(Ce) detector, after being processed through a Constant Fraction Discriminator (CFD) module, triggers/starts the coincidence event while the corresponding CFD output of LaBr3(Ce)-PSPMT cathode terminates the window. The common timing reference window for the registered events, i.e. the master gate, has been generated by the AND output of the two aforementioned CFD signals, in order to control the trigger rate of the data acquisition system. A typical time profile of the raw energy signal and position signal are shown in Fig. 3 . Only those events have been accepted during acquisition wherein the four anode signals as well as the TAC output remain well within this window. The analog pulses have been digitized using 8channel 12-bit ADC. The Linux Advanced Multi-Parameter System (LAMPS) [21,22] package has been used for the data acquisition system and further analysis has been performed by employing the ROOT package [23].

A. Energy and time resolution
The gamma-ray energy spectrum of the LaBr 3 (Ce)-PSPMT detector has been calibrated using standardized 22 Na, 54 Mn, 57 Co, 133 Ba, 137 Cs, and 152 Eu radioactive sources. Energy resolutions of 9.5% for 122 keV and 6% for 511 keV (Fig. ∼ ∼ 4(a)) have been measured, in good agreement with GEANT4 simulations. In coincidence with the auxiliary cylindrical LaBr 3 (Ce) crystal, timing resolution of 761(2) ps has been obtained from the TAC spectrum generated between the auxiliary detector and the position sensitive detector, as shown in Fig. 4(b).

B. Position resolution
Nine different spots were selectively irradiated on the LaBr 3 (Ce)-PSPMT detector with 122 keV gamma-ray source, collimated using a perforated (2 mm) lead block of 20 mm thickness.

4(a)
4(b) Fig. 4. a) Simulated and experimental gamma-ray energy spectra of 22 Na source for the LaBr3Ce)-PSPMT detector obtained in coincidence with 511 keV γ-energy in the BGO spectrum. The calculated energy resolution of the 511 keV photo-peak is 6%. b) TAC spectrum, with the timing from the ∼ auxiliary cylindrical LaBr3(Ce) taken as the START signal, and that from the position sensitive detector as the STOP signal.
The arrangement is represented in Fig. 5 . The resultant amplitudes of the four anode signals corresponding to each irradiated spot depend on the voltage drop through the resistive network, and the interaction points (X-position and Y-position) of the incident gamma-rays on the detector crystal have subsequently been determined as follows [3]: where, A, B, C and D are the amplitudes of the four anode signals, a 1 , a 2 , a 3 and a 4 are the gain factors of the amplifiers (adjusted to be identical in the present measurement), with b as a manually-defined offset factor for the position spectra. The spectrum of the nine irradiated spots has been generated with an energy gate on 122 keV gamma-rays recorded from LaBr 3 (Ce)-PSPMT cathode, and the X-and Y-positions of the interaction points have been obtained therefrom. The observed spectrum has also been validated by means of GEANT4 simulation, taking all possible physics processes into account. gamma-ray photons, electrons and positrons are expected to be produced during the interaction of the incident gamma-rays. The considered interaction mechanisms for the γ-photons are photoelectric effect, Compton scattering, and pair production. The electrons and positrons so produced can undergo multiple scatterings, ionizations and emit Bremsstrahlung radiation, while in addition, the positron can also undergo annihilation. The image has been reconstructed using those points at which maximum energy is deposited by the electrons generated along the interaction path of the incident gamma-rays in the crystal volume. The FWHM for each spot is ≈ 3.5 mm in the simulated spectrum ( Fig. 6(a)), while the measured FWHM for the different spots lies in the range of about 3.5-4.0 mm ( Fig. 6(b)). Since the measured spectrum is susceptible to electronic noise, a slight degradation is seen compared to the simulated outcome.

C. Performance as an imaging detector
The images of a hexagonal BGO detector and a cylindrical LaBr 3 (Ce) crystal have been generated using the position sensitive detector in a coincidence setup, as illustrated in Fig.  7 for the BGO detector, with energy gate on 511 keV gammaray photo-peak of both the detectors. The results compared with simulations are found to be in reasonable agreement in either case. The typical imaging performance is illustrated in Fig. 8. The position resolution, measured from the fit of the 1-d Y-projection of a slice of Xaxis (22.5-27.5 mm) of the BGO image ( Fig. 9), is 3.77(08) mm.  An optimum geometry for image reconstruction by backscattering technique has been investigated in the GEANT4 framework using the position sensitive LaBr3(Ce) detector in coincidence with a BGO detector as shown in Fig. 10. A steel ring having inner and outer diameter of 3.6 cm and 5.6 cm respectively with 1.1 cm thickness has been used as a scatterer for the incident photons. The position of the lead brick, scatterer, radioactive source, and detectors are chosen in such, so that the ancillary detector can't see the source directly. The lead brick having appropriate thickness to block the gammaray directly coming from the source. The 150 keV -230 keV of the gamma-rays recorded in the BGO detector correspond to the back-scattering events from the ring-shaped scatterer. We have reconstructed the image of the ring as shown in Fig. 11, in the simulation with the energy gating condition: 490 keV < EPSPMT <530 keV in the position sensitive LaBr3(Ce) detector and 150 keV < EBGO < 250 keV in the BGO detector. In future, we are planing to do this backscattering experiment to verify the simulated result as well as to investigate some applications of back-scattering technique.

V. SUMMARY AND CONCLUSIONS
A LaBr3(Ce)-PSPMT detector setup is developed for gamma-ray tracking and imaging applications. The energy and time characterization of the detector has been studied. A spatial resolution of about 3.5 mm has been obtained with a 2 mm collimated source. GEANT4 simulation for this detector reproduces the measured energy response of the detector and the spatial resolution. The reflection of the scintillation light from the periphery of the detector slightly hampers the position resolution. Sharp 2-d images of a hexagonal shaped BGO and a cylindrical LaBr3(Ce) crystals have been reconstructed using this position-sensitive detector setup, by employing data acquisition in coincidence mode. The 2-d image of a ring shaped scatterer has been reconstructed in GEANT4 by Compton back-scattering technique of the gamma-rays. This setup is expected to find applications in the fields of nuclear and high energy physics for scanning of detectors, as well as for the purpose of imaging in the medical and defense sectors.