Fast Timing with BGO (and other Scintillators) on Digital Silicon Photomultipliers for Prompt Gamma Imaging J. Petzoldt, K. R¨omer, T. Kormoll, M. Berthel, A. Dreyer, W. Enghardt, F. Fiedler, F. Hueso-Gonz´alez, C. Golnik, T. Kirschke, A. Wagner, and G. Pausch
Abstract—Particle therapy is supposed to be an advanced treatment modality compared to conventional radiotherapy because of the well-defined range of the ions. Prompt gamma rays, produced in nuclear reactions between ion and nuclei, can be utilized for real-time range verification to exploit the full potential of particle therapy. Several devices have been investigated in the field of Prompt Gamma Imaging (PGI), like Slit and Compton Cameras. The latter need very high detection efficiency as well as good time and energy resolution, requiring a versatile scintillation detector. In Positron Emission Tomography (PET), LSO and LYSO are known for their good time resolution, while the lower cost alternative BGO shows worse performance. In PGI however, where gamma rays have energies up to 10 MeV, the light output of a scintillator is up to 20 times larger compared to PET. This reduces the statistical contribution of the time resolution, which is the dominant part in case of BGO. Thus, BGO could be a reasonable alternative to LSO/LYSO for applications in PGI. Hence, experiments at the ELBE accelerator at HZDR (Germany) were performed using digital silicon photomultiplier (dSiPM) from Philips with monolithic BGO and LYSO crystals, and for completeness with GAGG, CeBr3 , CsI, CaF2 , and GSO. The time resolution of BGO compared to the other scintillators will be presented for a wide range of trigger- and validation levels as well as validation lengths of the dSiPM. Timing resolutions below 220 ps are obtained for BGO, while LYSO and CeBr3 achieve about 170 ps. Index Terms—timing resolution, scintillation detectors, digital silicon photomultipliers, prompt gamma imaging.
I. I NTRODUCTION Particle therapy is supposed to be an advanced treatment modality compared to conventional photon therapy. Protons and heavier ions deposit the maximum dose directly in the target volume while sparing healthy tissue due to their welldefined range. In practice, range uncertainties due to organ motion, patient mispositioning, anatomical changes between J. Petzoldt, M. Berthel, A. Dreyer, W. Enghardt, F. Hueso-Gonz´alez, C. Golnik, T. Kormoll, and G. Pausch are with OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universit¨at Dresden, HelmholtzZentrum Dresden - Rossendorf, Fetscherstr. 74, PF 41, 01307 Dresden, Germany. W. Enghardt, F. Fiedler, T. Kirschke, K. R¨omer and A. Wagner are with Helmholtz-Zentrum Dresden - Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany. Contact:
[email protected]. This work was supported by the German Federal Ministry of Education and Research (BMBF-03Z1NN12) and the European Commission (FP7 Grant Agreement N. 241851 and N. 264552). The authors would like to thank the ELBE accelerator crew for stable operation as well as the Philips DPC team for the support.
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treatment fractions, and other factors, can cause critical deviations between planned and actual dose with the possible result of damaging organs at risk and only partially irradiating the tumor. Therefore, verifying the particle range and monitoring the dose in real time are key factors to exploit the major advantages of particle therapy. Since protons are fully stopped inside the patient, secondary radiation emerging from the patient has to be utilized for range control. Prompt Gamma Imaging (PGI) deploys the emission of high energetic gamma rays from nuclear reactions betweeen ions and target nuclei because of the nearly instantaneous emission time and the close correlation to the dose deposition [1]. However, high photon energies from 2 − 10 MeV as well as high background rates pose major problems for currently investigated PGI systems, like Slit [2] and Compton Cameras [3]. Time-of-flight (TOF) techniques can be deployed to suppress the background contribution from neutrons and scattered photons [1], thus demanding good timing resolution of the detector. Systems using the energy information of photons like Compton Cameras need additionally high absorption efficiency, i.e. high density and effective atomic number Z. Well-known in medical application (especially in positron emission tomography (PET)), Lu2 SiO5 (LSO) and Lux Y(1−x) SiO5 (LYSO) crystals combine those two properties. Bi4 Ge3 O12 (BGO) is a lower cost alternative to LSO/LYSO, which is also very dense but with less light yield than the lutetium containing scintillators (about 8,500 versus 33,000 photons per MeV [4]). This leaves BGO with worse timeand energy resolution in the energy region of PET. However, in PGI, prompt gamma rays have energies up to 10 MeV. Thus, the total light yield of the scintillator is up to 20 times higher compared to PET. Theoretically, the energy-resolution ∆E is dominated by a statistical and an √ intrinsic contribution, where the statistical one scales with N where N is the number of scintillation photons. BGO is known to have a lower intrinsic contribution than LSO/LYSO at energies below 1.5 MeV [4]. This suggests that the energy resolution of BGO could improve beyond LSO/LYSO in case of energies of prompt gamma rays. BGO could thus be a reasonable alternative to LSO/LYSO. Analogously, the timing resolution has a statistical contribution as well as effects from the readout electronics. Therefore, the timing of a scintillator is expected to improve with increasing number of scintillation photons.
Motivated by those expectations, we performed measurements to compare the time resolution of various scintillators with the light readout we intend to use for our absorber detectors of a medical Compton Camera [3], namely the pixelated digital silicon photomultipliers (dSiPM) DPC-3200 from Philips Digital Photon Counting (DPC) [5]. The dSiPM features excellent timing resolution due to its trigger scheme [6], where possibly only one fired microcell is needed to create the timestamp, while noise reduction can be obtained using the internal validation scheme. However, due to the limited number of microcells (3200 per pixel), monolithic scintillation crystals have to be used in the energy region of PGI. Otherwise, the pixels would rapidly go into saturation, preventing spectroscopic measurements. Further information about the Philips dSiPM can be found in [7]. The paper presents the techniques used and the results obtained in such experiments, performed at an accelerator providing excellent timing capabilities. II. M ETHODS Monolithic BGO, LYSO, Gd3 Al2 Ge3 O12 (GAGG), CsI:Tl, CaF2 , and CeBr3 crystals, matching the shape of the dSiPM (32×32 mm2 for BGO, CaF2 , CsI:Tl, and LYSO, 33×33 mm2 for GAGG and CeBr3 ), were coupled to the sensor. BGO, CsI:Tl, CaF2 , and GAGG had a thickness of 20 mm, while the LYSO and CeBr3 crystals were thinner: 10 mm and 15 mm respectively. Additionally, a cylindrical Gd2 SiO5 (GSO) crystal with a diameter of 40 mm and a thickness of 25 mm was investigated. To achieve complete light collection, all crystals but the CeBr3 were wrapped in several layers of white teflon tape with another layer of black tape to prevent light from outside. The hygroscopic crystal was capsulated by the manufacturer also providing complete light collection. The dSiPM DPC-3200 from Philips was used in the experiments. The detector, also called tile, is divided into 16 subunits, the so called dies. Two TDCs are implemented on each die, working with a phase-shift of 180 ◦ to cover the full clock cycle of the electronics. Every die is again divided into four pixels with 3200 microcells each. In total, one tile consists out of 64 pixels with about 200,000 microcells, which is sufficient to avoid saturation effects of the dSiPM in the energy region of PGI. The linear electron accelerator ELBE at Helmholtz Zentrum Dresden Rossendorf (HZDR) in Dresden, Germany, was used as source for high energetic bremsstrahlung with time-spread of about 50 ps. The electron beam with a radio frequency (RF) of 13 MHz and an energy of 13 MeV is guided on a Nb foil, where the electrons produce bremsstrahlung. To achieve low background rates in the experimental cave, the electrons are deflected by a dipole magnet behind the Nb irradiator and the bremsstrahlung beam is guided through a collimator system. To obtain a stable time reference in the measurements, the accelerators’ RF signal had to be coupled to the dSiPM controller board, which accepts only frequencies between 110 MHz and about 210 MHz as internal clock. Therefore, the RF signal was fed through a phase-locked loop (PLL) to increase the frequency up to 182 MHz.
The dark count map (DCM) of the DPC-3200 was measured using the 182 MHz signal as internal clock, which is also used to calibrate the 32 time-to-digital converters (TDCs) per tile. To ensure low dark count rates (DCRs), the temperature of the sensor was kept constant at about 2.0 ◦ C with ∆T < 1.0 ◦ C. Additionally, 10 % of the cells, the most active ones, were disabled using the internal inhibit map function of the dSiPM [5]. Measurements were performed for a set of various trigger and validation levels, as well as for all existing validation lengths of the sensor. The integration time was set to at least three times the decay time of the scintillation material to ensure complete light integration. Energy calibration of all scintillators coupled to the dSiPM was performed using 60 Co and 22 Na sources as well as the end point energy of the bremsstrahlung spectrum with an energy Ex−ray = 12.5 MeV. In case of BGO and GSO, this is only a rough estimate because the gamma lines of the calibration sources could not be separated. Thus, an additional source for high energetic photons was found with the tandem accelerator Tandetron at HZDR. Here, 4.4 MeV gamma rays are produced in the nuclear reaction 15 N(p,α)12 C, thus closing the gap between calibration measurements in the laboratory and high energies relevant in the experiment and PGI. III. DATA A NALYSIS When using monolithic crystals on the pixelated DPC-3200, one has to make sure that events are only valid if the light was readout completely. In some cases, dies are in recharge mode or not all dies receive enough light to get validated, thus scintillation light gets lost. This results in a dependancy between number of readout dies and light yield (see Fig. 1), invalidating the energy calibration of the scintillator. When used in raw mode, the listmode files taken by the dSiPM system contain the number of fired microcells per pixel and two TDC fine counter (FC) values per die. Thus, an event with all dies triggered and validated has up to 32
Fig. 1. Light yield of CeBr3 in units of fired microcells over number of validated dies. Complete collection of the scintillation light is only guaranteed in case of CeBr3 when 16 dies are readout.
timestamps. Several techniques for data analysis were tested, e.g. using the FC of the event’s first die or using the FC of the brightest die, as well as averaging over all FC values per event. For simplicity, all data presented use the timestamp of the event’s brightest die. Averaging the timestamps created slewing artifacts, while using the first timestamp of each event performed worse in most cases than using the brightest die. Additionally, skew correction of the dies’ timestamps was performed to account for wire lengths. IV. R ESULTS To obtain the best performance by means of timing resolution, the optimal setup of the DPC-3200 has to be found. CeBr3 will be used in the following as probe, because we expect the best performance due to the high light yield (68,000 photons per MeV) and the low decay time (τ = 17 ns). The trigger level (TL) plays a crucial role in timestamp generation due to the statistical character of light collection and is therefore the most important parameter. When applying the lowest trigger threshold TL1 in a measurement with CeBr3 ((a), Fig. 2), the peak in the energy over time distribution becomes more narrow compared to a measurement with TL2 and TL4 ((c) and (d) resp., Fig. 2). This shows the statistical uncertainty in detecting enough photons for the timestamp generation. The validation level (VL) and the validation length (VT) are more important for the full light readout. When increasing the validation level from VL2 to VL16 ((a) and (b) resp., Fig. 2), the minimum number of fired cells required for full light readout is increased, thus the minimum energy of the incident photon needs to be higher. The validation length can be changed from 5 ns to 40 ns, where a shorter length reduces the background coming from dark counts. On the other hand, a higher VT improves the light readout which is crucial for most scintillators. Therefore, the longest available validation length (VT40) is chosen, while a slight increase in dark counts is accepted. To obtain a quantitative dependency of timing resolution over energy, the 2-dimensional spectra are divided into slices and each projection on the time axis is fitted with a Gaussian function, where the full width at half maximum (FWHM) is taken as the time resolution. The results of the exemplary four data sets of CeBr3 (see Fig. 2) are shown in Fig. 3. It is visible, that a higher trigger level results in a worse timing resolution, while increasing the validation level requires more scintillation light for a complete readout of the tile. Summarizing, CeBr3 shows the best performance when applying trigger level 1, validation 2 and the highest validation length of 40 ns. BGO has a lower light yield and a higher decay time compared to the standard probe CeBr3 , both decreasing the expected performance by means of timing resolution and increasing the energy threshold needed to achieve full light readout of the tile. Especially the latter criteria reduces possible setups of the dSiPM to more or less only one working configuration in case of BGO (TL1 VL2 VT40). Exemplary data are shown in Fig. 4. When using BGO, photons with about 3 MeV are required to obtain enough scintillation light in order to validate all dies.
(a) TL1 VL2 VT40
(b) TL1 VL16 VT40
(c) TL2 VL4 VT40
(d) TL4 VL8 VT40
Fig. 2. Energy over time measurements measured with CeBr3 coupled to Philips dSiPM using a bremsstrahlung continuum with an endpoint energy of 12.5 MeV. (a): The trigger level was set to TL1 resulting in a narrow peak in the energy over time distribution. (b): The same setup as in (b) but with a higher validation level (VL16) increasing the minimum energy for full light readout. (c): The increase from TL1 to TL2 broadens the peak reflecting a worse timing resolution compared to (a). (d): An even broader peak when increasing to TL4.
Fig. 3. Timing resolution over deposited energy of CeBr3 obtained with different setups of the Philips dSiPM DPC-3200. The measurements with TL1 (blue and green line) provide the best performance, while higher trigger levels (red and black line) increase the statistical uncertainty of the timestamp generation and worsen the timing resolution. The minimum energy needed for complete light readout of the tile is higher when increasing the validation level from VL2 (blue) to VL16 (green).
For each scintillator, the best setup was found using the described data analysis procedure. The timing resolution over energy for each material is summarized in Fig. 5. V. C ONCLUSIONS CeBr3 shows by far the best performance with about 180 ps at 4 MeV, which is expected due to the high light yield of about 68,000 photons per MeV and the very short decay time of about 17 ns.
(a) Energy over Time
(b) Timing Resolution over Energy
Fig. 4. (a) Energy over time measured with BGO coupled to Philips dSiPM using a bremsstrahlung continuum with an endpoint energy of 12.5 MeV. (b): The timing resolution over energy of BGO using the setup from (a). The high energy threshold of about 3 MeV, required for full light readout, is visible.
It could be shown that the timing resolution of BGO, GAGG, LYSO, and CeBr3 is below 400 ps for energies over 3 MeV, which is sufficient for PGI system due to a beam spread at clinical accelerators of about 2 ns [1]. When regarding timing resolution as main objective, the advantage of fast scintillators becomes negligible for high photon energies when using dSiPM as light readout. Therefore, BGO would be a suitable alternative to the more expensive LSO/LYSO detectors in PGI. VI. O UTLOOK Several experimental improvements can be deployed to overcome the energy threshold limitations, especially in the case of BGO and GSO. One possibility would be the reduction of the crystal size, like a matrix of 2 × 2 crystals per tile. Here, saturation effects are still avoided for the scintillators with low light yield, but less scintillation photons are required to validate the full matrix. Additionally, the internal neighbor logic of the DPC-3200 can be used in future experiments with monolithic crystals, triggering a full readout of the tile after only one die was successfully validated. Furthermore, optimized electronics for feeding the accelerator RF into the dSiPM controller board will be installed in the near future, reducing the time jitter due to electronic noise. R EFERENCES
Fig. 5. Timing resolution (FWHM) over deposited energy of all investigated scintillators obtained with the best setup of the Philips dSiPM DPC-3200. CeBr3 (pink) shows the best performance with about 180 ps at 4 MeV, followed by LYSO (green), BGO (red) and GAGG (blue) with about 220 ps to 250 ps. CaF2 (dark green) and GSO (brown) achieve about 420 ps, while CsI (black) has the worst performance with about 800 ps timing resolution at 4 MeV.
LYSO performs worse for lower energies which could probably be explained due to the internal activity coming from 176 Lu as well as the 22 Na source (only present during the LYSO measurement). The gamma rays could pre-trigger the dSiPM, while the bremsstrahlung photons validate the signal. However, the resolution improves up to about 170 ps with increasing energy, which could confirm the hypotheses. Despite its long decay time and low light yield, BGO shows very good timing resolution of about 230 ps at energies above 3 MeV. This might be explainable with the timestamp generation of the dSiPM with the first detected photon and the absence of background or slower decay components. In contrast, BGO has even a minor faster decay component with τ = 60 ns, which could improve the timestamp generation. The relatively bright and fast material GAGG as well as GSO have also worse timing resolution compared to CeBr3 , which could be related to afterpulsing due to a second slower decay component with τ = 260 ns and τ = 600 ns respectively. CaF2 and CsI have very long decay times of about 1 µs and perform reasonably.
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