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Multiple photon counting coincidence (MPCC) technique for scintillator characterisation and its application to studies of CaWO4 and ZnWO4 scintillators
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Nuclear Instruments and Methods in Physics Research A 553 (2005) 522–534 www.elsevier.com/locate/nima
Multiple photon counting coincidence (MPCC) technique for scintillator characterisation and its application to studies of CaWO4 and ZnWO4 scintillators H. Kraus, V.B. Mikhailik, D. Wahl Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK Received 13 June 2005; received in revised form 9 July 2005; accepted 10 July 2005 Available online 2 August 2005
Abstract We describe a new method for measurements of the scintillation characteristics of crystals. The multiple photon counting coincidence (MPCC) technique involves recording the sequence of individual photon pulses resulting from a scintillation event. The timing of the individual photons allows determination of the scintillation decay time constants. The number of photons recorded per scintillation event is proportional to the scintillation light yield. The decay time constants and the relative light yield of CaWO4 and ZnWO4 scintillators have been investigated in the temperature range 9–350 K. An important advantage of the MPCC method is the possibility to reject spurious events through offline analysis, taking into account the entire data set of scintillation events. This procedure allows cleaning of the data set from multiple scintillation events (pile-up). The MPCC technique is an excellent complement to conventional characterisation techniques and is particularly suited for investigation of slow scintillation processes. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Mc Keywords: Photon counting; Scintillator characterisation; Decay time; Light yield; CaWO4; ZnWO4
1. Introduction During recent years there has been a steady increase in research activity regarding the development of new instrumentation for low-backCorresponding author. Tel.: +44 1865273361;
fax: +44 1865273418. E-mail address: [email protected] (H. Kraus).
ground particle physics experiments. Recent progress in the field of low-temperature detection techniques has made available highly sensitive energy resolving cryogenic detectors [1–3]. A new generation of these detectors, which offer the capability of background discrimination by simultaneous measurement of scintillation and phonon signals [4], is now in use. These detectors play an important role in experiments searching for rare
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.07.011
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events, such as interaction of Weakly Interacting Massive Particles (WIMPs) likely to comprise galactic Dark Matter [5,6], double beta decay [7] and radioactive decay of very long-lived isotopes [8,9]. Scintillating crystals are a key element in these experiments and the identification and characterisation of promising scintillator materials for cryogenic applications is an important objective for future experiments in this field. A number of experimental techniques commonly used for luminescence characterisation of materials can be implemented readily in these studies, specifically UV, VUV and X-ray excited spectroscopy [10–12]. In addition there is strong motivation for an experimental technique capable of measuring the temperature dependences of the light yield and the decay time constants under a-, b- and g-excitation. In this paper we describe an original method for measuring these scintillator characteristics using the multiple photon counting coincidence (MPCC) technique. The development of this technique is part of a research initiative aimed at the identification and optimisation of scintillation materials for cryogenic phonon-scintillation detectors (CPSD). The structure of this paper is as follows: in the first part we discuss the conventional approach to measurements of the light yield and decay time constants of scintillators. We analyse inherent limitations of these methods and in particular those arising in low-temperature scintillation studies. In the second part we present the MPCC method, explaining technical detail and the data analysis. Finally, to illustrate the performance of the technique, we discuss our results for light yield and decay time constants of CaWO4 and ZnWO4 scintillators in the temperature range 9–350 K.
2. Conventional techniques of scintillator characterisation 2.1. Light yield measurements in a low-temperature experiment A large number of papers on scintillators have focused on measurements close to or at room temperature. There are quite a few challenges to be
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overcome when expanding characterisation to a wider temperature range. Firstly, we consider the measurement of the light yield. The characterisation of scintillation detectors has so far relied almost exclusively on the use of high-gain photomultipliers (PMT) or avalanche photodiodes (APD) [13,14]. Such experiments imply the detection of integrated voltage pulses produced by a photodetector following the scintillation event. The pulse height spectra obtained from these data give a measure for the scintillator’s light yield. In order to achieve optimum light collection and hence the best possible resolution on the measurement of the light yield, the scintillator and photodetector are arranged in direct optical contact. However when considering studies of the light yield as function of temperature one has to bear in mind that the quantum efficiency of PMTs strongly depends on temperature. The numerous technical and methodological difficulties encountered by experimentalists who have endeavoured to implement a PMT inside a low-temperature setup can be appreciated from [15]. Similar problems, though with the reverse effect, hold for APDs that exhibit a dramatic increase in gain with cooling [16]. In addition, most scintillator studies have been performed only down to the temperature of liquid nitrogen and as far as we are aware none of the photodetectors operate in stable conditions at lower temperatures. In summary we conclude that for carrying out such measurements routinely it is strongly advisable to keep the photodetector at constant (room) temperature while cooling (heating) the sample. This simple concept can be implemented readily by installing the sample into an optical cryostat; the downside, however, is that in measuring the amplitude spectrum not much light is collected by the photodetector. 2.2. Techniques for measurements of the decay time constants In studies of decay time constants as function of temperature additional problems associated with the necessity to cover a large range of decay time constant values arises. For example, the materials
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used in this study, i.e. calcium and zinc tungstates, exhibit a composite decay spectrum with decay time constants varying with temperature from micro-seconds to milli-seconds. Accurate determination of these decay time constants requires the sampling time interval to be at least ten times shorter than the shortest decay time constant while the number of samples taken in one recording process (record length) has to be large enough to record for at least three times the characteristic time constant of the longest component of the scintillation [17]. The delayed coincidence single-photon counting (DC-SPC) technique has excellent timing resolution (ca. 0.5 ns) and it is commonly used for measuring decay characteristics of traditional fast scintillators with decay time constants in the range of nano-seconds to micro-seconds. The method is based on measuring the distribution of the time difference between the excitation of the scintillator and an individual scintillation photon [18] or recording of a start and a stop photon from the same scintillation event [19]. Two PMTs are used to produce a prompt trigger (start) signal associated with the beginning of the scintillation pulse and a stop signal that defines the arrival time of one of the scintillation photons. The time difference is digitized with a time to digital converter (TDC) and processed by down-stream electronics. The accumulated data are plotted in the form of a histogram of the arrival times of scintillation photons from many excitation events and fitted to a mathematical function suitable for describing the decay mechanism. Measurements of decay time constants by the DC-SPC technique require the probability of detection of more than one photon during the recording time to be less than 0.01 [17]; otherwise there will be bias towards a faster decay time constant [20]. To reduce this bias the number of photons is artificially limited using a diaphragm or a filter; but this reduces sampling rates to typically 0.1–1 s1. Accurate measurements of the scintillator decay time constants usually require 105 data points which may result in measurement times of up to a few days [17]. Therefore such a technique is not very suitable for studies involving temperature variation.
If the decay time constant of the scintillator is in the region of several micro-seconds, pulse shape analysis (PSA) can be implemented [13]. The conventional PSA technique works by converting the optical signal following the scintillation event into an analog PMT electrical signal and recording this electrical signal with an oscilloscope, transient recorder or box-car integrator. It should be noted that the PSA technique has limited time resolution that is defined by the width of the PMT response (typically a few nanoseconds) and the minimum sampling interval of the digitizer (typically 5 ns). Given this, one can estimate that this approach allows measuring the decay time constant only if it is X50 ns. Therefore this approach is used for the analysis of ‘‘slow’’ scintillators which exhibit a decay time constant of 4100 ns [21]. 2.3. Limitations of decay time constant techniques There are two sources of error inherent to both decay time measurement techniques. It is not practically possible to fully control the rate at which ionising particles or photons interact with the scintillator. Hence there are always events recorded in which a second (or more) scintillation event has occurred during the measurement period of the first, a so-called multiple excitation event. To reduce this effect the time between events needs to be much longer than the recording period. This can be achieved by decreasing the source activity, but this results in a prolonged measurement time. Although this is not a severe problem when fast scintillation events are studied it does become a serious one in the case of ‘‘slow’’ scintillators. First of all, the source activity cannot be arbitrarily low; for meaningful measurements the rate of scintillation events produced by a radioactive source must exceed that caused by natural background by some 2–3 orders of magnitude. A rough estimate for a decay time constant of 10 ms, a recording time of 50 ms and an event rate of 1000 s1 shows that approximately 2% of the recorded events will include scintillation resulting from more than just one particle interaction, i.e. multiple excitations. This seemingly negligible contribution to the total signal results in a roughly even time difference distribution throughout the decay time spectrum,
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3. MPCC technique
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hence creating a noticeable tailing effect. Thus when analysing such data one might easily misinterpret this background as an additional decay component with a long time constant. Other common sources of error are spurious prompt events caused by a direct hit of the PMT’s photocathode, by scattered ionising radiation from the source and by natural background. In both techniques this effect can cause a prompt response of the PMT, which could be misinterpreted as a fast scintillation component. It is possible to reduce this effect in the DC-SPC technique by shielding the ‘‘stop’’ PMT and by moving it away from the radioactive source but it cannot be eliminated completely and it remains an intrinsic source of error in the PSA technique. In summary there are several potential problems when measuring decay time constants of scintillators that bias the final result in one or the other way. A reduction of this bias is possible via reducing the source activity, and hence event rate, but this inevitably results in a conflict when a series of measurements is required, e.g. in studies of temperature dependences. These problems are in addition to the necessity of finding a way of measuring the amplitude spectrum of scintillators when having very low light collection efficiency. In an attempt to avoid these problems we turned our attention to the idea of detection of multiple photons from individual scintillation events.
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Fig. 1. (a) A typical record displaying the detection of individual photons from a scintillation event in CaWO4 excited by an 241Am a-source at T ¼ 295 K. (b) Zoom of the above record near the beginning of the scintillation event showing the shape of single photon pulses produced by the PMT.
3.1. Method The MPCC technique involves the recording of a sequence of pulses produced by a PMT when detecting photons from a scintillation event (see Fig. 1). Each pulse in the sequence corresponds to an individual photon impinging on the photocathode of the PMT. The distribution of arrival times of these pulses provides information on the decay characteristics of the scintillation process. The number of pulses recorded per event is proportional to the light yield of the scintillator. By recording a large number of scintillation events (103–104) one can obtain the decay time character-
istics and the light yield in a single measurement. Due to the large number of photons detected in each event, typically 25–30 here (see next section), sufficient accuracy can be achieved already with far fewer scintillation events than needed for the DCSPC technique, which only uses one time difference between arriving photons per scintillation event. With merely about 10 min total data acquisition time for one measurement, MPCC allows carrying out a whole series of measurements within a reasonable period of time (a few hours only), hugely simplifying the examination of scintillation parameters and their variation with temperature.
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3.2. Estimation of expected light collection efficiency The light collection efficiency Z of a PMT with photocathode diameter d, positioned at a distance L from the sample, assuming a point-like excitation in the scintillator sample and isotropic propagation of photons, can be expressed as1 2 d Zffi . (1) 4L If the sample is placed inside a cryogenic setup with N optical windows for light to pass through, the reflection from the 2N glass surfaces, each with reflection coefficient r, has to be taken into account, yielding a correction factor ð1 rÞ2N . The light collection efficiency in that case is given by 2 d . (2) Z ffi ð1 rÞ2N 4L Substituting parameters typical of our setup, d ¼ 25 mm, L ¼ 50 mm, r ¼ 0:05 and N ¼ 4 (three cryostat windows and the window of the PMT) one finds that on average only 1% of the photons leaving the scintillator can reach the photocathode. The typical quantum efficiency of a PMT is 25–30%, and assuming further a light yield in the region of 10 000 photons/MeV, which is typical for tungstate crystals [22], we estimate that on average in such an experimental setup the PMT should detect 25–30 photons from scintillation following an energy deposition of 1 MeV. This number of photons should allow already determination of the scintillation time constant, assuming a single time constant, with an error of o20% from just one single event. The samples used in our experiments have a thickness of a few mm. This is too thin for using high-energy g-sources, such as 137Cs (662 keV) or 60Co (1173 and 1333 keV) as most scintillation photons would result from events due to Compton scattering with a continuum of energies, thereby making the determination of the scintillation yield difficult. On the other hand, low-energy g-sources i.e. 57Co (122 and 136 keV), would not provide a 1 We only consider the photons that have escaped the crystal as is standard for light yield measurements.
sufficient number of photons given the low light collection efficiency in a cryogenic setup (see Eq. (2)). This difficulty can be overcome by using aexcitation since a-particles are absorbed within 10 mm below the scintillator’s surface. In our experiments we used 241Am, emitting a-particles of energies 5.44 and 5.48 MeV. One needs to take into account that a-particles produce less scintillation than g-quanta. For tungstates the correction factor, which is known as the a/b-ratio (g-quanta interact with matter via b-particles), is 0.2 [9,21]. Thus, with 11 000 photons produced by aparticles about 30 photons should be detected in the setup discussed here. 3.3. Experimental setup The experimental setup used for our MPCC measurements is shown in Fig. 2. The scintillator sample and the 241Am a-source are placed inside a helium flow cryostat equipped with optical windows to allow the detection of light. Two PMTs see the sample at right angle to each other. The signals produced by each PMT with their associated pre-amplifiers are passed to integrating
Sample 241AM
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Fig. 2. Schematic of the experimental setup used to measure the decay time spectra and relative light yield of scintillating crystals. PMT—photomultiplier, PA—preamplifier, TR—transient recorder, Amp—linear amplifier, SCA—single channel analyser, Delay—gate and delay generator, Coinc.—coincidence unit.
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amplifiers. Only one PMT is connected to the input of the transient recorder while the trigger is based on coincidence in both PMTs in order to minimise the recording of spurious events such as the detection of a direct hit of the PMT photocathode [13]. Event selection for triggering is carried out by processing the slow integrated and amplified signals. The integrating amplifiers produce signals which are a measure of the total energy detected, and are fed into single channel analysers (SCA). The discrimination thresholds of the SCAs are set such as to reduce the number of pulses with low and very high amplitudes that are associated with electronic noise and spurious events caused by cosmic muons, respectively. The logic output pulses of both SCAs are fed into a coincidence unit that provides the trigger for the transient recorder. A gate and delay generator is operating in one channel to tune the arrival times of the logic pulses to the coincidence unit. The transient recorder has sampling intervals ranging from 5 to 80 ns and is able to record up to 16 384 samples per record. With the number of photons expected from one scintillation event estimated to be 30, we continue with an assessment of the instrumental requirements on the PMT, pre-amplifier and transient recorder chain as described above. The aim is to resolve the time structure of scintillation light emission at the single photon level. A data acquisition chain capable of nano-second time resolution seems appropriate. In our experiment we used a PMT (9125B) from Electron Tubes exhibiting a single electron pulse width of 7.4 ns (FWHM). The charge signal from the PMT was converted into a voltage pulse using an integrating amplifier with time constant 2.2 ns (R ¼ 470 O and C ¼ 4:7 pF) with an OPA633 buffer amplifier. In practice there is additional parasitic capacitance associated with the design of a pre-amplifier, e.g. size and position of components, wiring, etc, that increases the integration time constant calculated from the nominal component values. A measurement of the pre-amplifier transfer function allowed determining the bandwidth, which, with a value of 200 MHz, was not too dissimilar to the design value and, in fact, is a perfect match to the shortest possible sampling interval of 5 ns of the transient
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recorder (LeCroy TR8828D). Individual pulses (photons) from scintillation events in CaWO4 were detected, with each pulse spanning 7–8 data points, corresponding to a characteristic individual pulse FWHM of 15 ns (see Fig. 1). The average number of single photon pulses for the singleexcitation events excited by 241Am a-particles at room temperature (for the procedure of multiple event recognition see Section 3.4) is 3275, in good agreement with our earlier estimate. 3.4. Data analysis All scintillation events recorded by the transient recorder are stored in a binary data file by custommade DAQ software. The data analysis software is based on the particle physics analysis framework ROOT [23], with a tailor-made set of C++ classes to represent the individual record, scintillation events, single photon pulses, etc. Each event is essentially an array of floating point numbers of the same size as the record length (usually 16 384) representing the PMT signal as function of time. For each event (record) a set of parameters is calculated, such as the number of photons, the baseline, timing of the photons, etc. The first parameter to find is the baseline of the record. This is achieved by histograming the data points of a record into 256 bins, representing all possible signal voltages the 8-bit transient recorder can produce. Given the nature of the data (see Fig. 1) most of the data points in the record are the baseline and an initial estimate for the baseline level is arrived at by finding the most likely signal voltage in the histogram. This initial baseline estimate then allows one to identify the individual photon pulses by data points departing from the baseline. The initial photon pulse recognition algorithm merely requires the signal voltage (data point) being greater (negative pulses) for two consecutive points before and after the point under consideration in order for it to be labelled a photon pulse. The range occupied by a potential photon pulse is defined as beginning 3 data points before the peak position and ending 8 data points after. Eliminating all photon pulses leaves one with just the baseline including some variation due to electronic
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noise. By averaging the remaining data one obtains a value for the baseline. At the same time the RMS value of the baseline is calculated as an estimate of the contribution of electric noise. With the baseline sufficiently well established, and also knowing how noisy the baseline is, a new peak search algorithm is run based on the following criterion: for a photon pulse to be recognised, its amplitude must deviate from the baseline by at least 1 unit in amplitude, defined as the difference of two consecutive voltage amplitudes measured by the transient recorder (1 bit), 5 sigma (from RMS of the baseline) from the baseline or 5% of the average amplitude of all photon pulses in the record; whichever is greatest. The end of a photon pulse is defined as when the amplitude falls below the threshold for a peak to begin or 5% of its own amplitude, whichever is farthest from the baseline. The latter correlation with the peak height is to avoid problems with electronic ringing seen in some large signals. The photon pulse recognition explained above actually recognises a signal range containing at least one photon, but possibly also more than one. A second routine is added to identify whether there are multiple photons in a range so far identified as one pulse. Once all photon pulses in a record have been identified, a histogram of the arrival times of the first photon in each record is constructed. A typical example of such a distribution is shown in Fig. 3a. It can be seen that the vast majority of ‘first photons’ arrive in a narrow time interval defined by the hardware trigger, identified as the sharp peak in the histogram. However there is a noticeable number of scintillation events having photon pulses prior to the trigger. An example of such an event with an early ‘first photon’ is shown in Fig. 4a together with a normal event (Fig. 4b) and a multiple (double) event (Fig. 4c). The presence of events with an early ‘first photon’ has a simple explanation: these contain photon pulses from a previous scintillation event recorded with the event that produced the actual trigger. In fact this is an effect complementary to multiple events and, if not identified, will introduce a noticeable error in the results. This is because the distribution of arrival times of photons representing the temporal char-
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Fig. 3. Analysis of scintillation events in CaWO4 at T ¼ 144 K: (a) distribution of arrival times of the first photon in a scintillation event; (b) decay time constants as a function of position of the cut on the arrival time of the first photon; and (c) test statistic, Eq. (5), proportional to the likelihood ratio of the decay time constants.
acteristic of the scintillation process (decay time spectrum) is calculated with respect to the time of the first photon pulse in a record. Therefore records with early photons must be eliminated from the analysis and to do this a procedure of cuts has been implemented. This implies the elimination of events exhibiting a first photon pulse that arrives earlier than a specified time (cut-on-time). Fig. 3b shows the dependence of the two dominating decay time constants of CaWO4 on this cut-on-time. One can identify three characteristic regions in the plot. In the first region
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time of the first photon not within the dominant peak in the distribution are rejected. Once the data set has been cleaned of events with early photons it is desirable to eliminate multiple scintillation events. Initially we assume the decay process can be represented by a single exponential decay with just one decay time constant. This simplification allows immediate calculation of the decay time constant t from the arrival times of the photons in an event with respect to the arrival time of the first photon. This simple method of calculating t is based on the probability density function for the detection of a photon at time t:
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1 Pðt; tÞ ¼ expðt=tÞ. (3) t When N photons are detected following a scintillation event, giving N1 time differences, the decay time constant t can be calculated as
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Fig. 4. Records illustrating different types of scintillation events in CaWO4 (T ¼ 144 K): (a) an event with a very early first photon (most likely from a previous scintillation event or a direct photocatode hit), (b) a normal event and (c) a double event (two scintillation events in one record).
the closer to the trigger time the cut-on-time point is located the shorter the decay time constants are. This can be understood as events with an early first photon contribute randomly to the initial part of the decay time spectrum, introducing an overestimate of the calculated decay time constants. Then there is a ‘plateau region’ where the decay time constant does not change much. This is the region where the cut-ontime is implemented; placing the cut at a later time causes the decay time constants to decrease significantly. Obviously a cut-on-time above this point already eliminates good events of the type being consistent with what Fig. 3a shows: this is the region where the peak in the histogram for good events begins. Based on this analysis a criterion is derived such that events with an arrival
1 X 1 N 1 Dti þ Dtm N 1 i¼1 expðDtm =tÞ 1
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where Dti is the arrival time of the ith photon with respect to the arrival time of the first photon and Dtm is the sampling period remaining after detecting the first photon. The second term in Eq. (4) corrects for the bias due to the finite measurement time (record length) available for each record. Assuming DtmX5t, for example, its contribution to t is less than 3%. With just one exponential decay with a single time constant being an approximation anyhow, the second term has been ignored in our calculations, allowing us to estimate the single decay time constant as simply the mean of the photon arrival times since the first photon. Neglecting the bias, which could nevertheless become significant for long decay time constants, does not really affect the final result as the time constant calculated via Eq. (4) is only used to calculate a test statistic being a measure for how consistent the time structure found in the record is with that given by Eq. (3). The algorithm for the elimination of double events is based on the ratio of likelihoods of the decay time constant t calculated as above and the most likely time constant tm taken from the distribution of t-values for all records. The test
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statistic Pðt; tÞ W ¼ 2 ln Pðt; tm Þ t t ttm Dt 2 ¼ 2 ln þ1 ! tm tm tm ð5Þ
represents a measure of the discrepancy between the most likely decay time constant of the process under study and that obtained for an individual record. When t ffi tm , W can be approximated by the square of the relative deviation of the time constant from the most likely time constant. Fig. 3c shows the histogram of this parameter plotted for a large number of scintillation events detected in CaWO4. It is obvious that W is close to zero for the vast majority of scintillation events, i.e. most events having t ffi tm . These represent single scintillation events. Conversely this parameter differs significantly from zero when a double event is detected. Visual inspection of the events exhibiting W X1 shows that they are double events (see Fig. 4c). Therefore by rejecting events with large likelihood ratio one can eliminate multiple events. The cut-onstatistic was tested for a range of cut-off values for W between 0.1 and 1.0. Within that range, the values obtained for the decay time constants of the scintillation process were hardly affected. An illustration of the impact on the decay time spectrum of CaWO4 introduced by the cuts is given in Fig. 5. Curve 1 shows the histogram for no cuts (as measured) and curves 2 and 3 show the results after subjecting the data to the cut-on-time and cut-on-statistic routines, respectively. It is obvious that the cut-on-time eliminates a spurious bump in the middle of the decay spectrum (at around 35 ms). This bump is caused by events having the first photon near the beginning of the sampling period. The cut-on-statistic has a major effect on the shape of the decay curve for long times. In this region the emission caused by single scintillation events is negligible while multiple events contribute to the background. When multiple events are rejected, the decay spectrum shows virtually no background. The example presented here demonstrates that the procedures implemented give a much clearer
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Fig. 5. Decay time spectra for scintillation in CaWO4 (T ¼ 295 K): curve 1, without cuts, as measured; curve 2, after elimination of events with early photons; and curve 3, after elimination of multiple events. The spectra are shifted along the vertical axis for clarity.
decay time spectrum of the scintillation process, which in turn permits the decay time parameters to be determined with better accuracy. Of particular advantage is the possibility to identify inherent background that might mimic a long lasting emission component. Finally it is worthwhile noting that the background caused by multiple events or pile-up effects is inevitably present in measurements carried out with the conventional DC-SPC technique. It has not much effect on the characterisation of traditional ‘‘fast’’ scintillators since the relation between the event rate (103–105 s1) and decay time constant (108–107 s) is such that the probability of detecting multiple events is negligible. However, as is shown in this section, severe complications arise when scintillators with a ‘‘slow’’ decay time constant have to be analysed. The MPCC technique with its associated data analysis is most beneficial for characterising ‘‘slow’’ scintillators and it is definitely needed for characterisation of slow scintillators at low temperatures where their scintillation becomes even slower.
4. Experimental demonstration of the MPCC technique In this section we shall analyse a number of experimental results obtained using the MPCC
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technique. As was highlighted in the introduction, our particular interest lies with crystals from the tungstate family, specifically CaWO4 and ZnWO4. Whereas the former one is already being used in cryogenic searches for Dark Matter [5,6] zinc tungstate is being developed as an additional target material for such experiments [24]. These crystals are quoted as ‘‘slow’’ scintillators because of their relatively long decay time constants, i.e. 10 ms at room temperature with a tendency to increase by at least an order of magnitude when cooled to liquid helium temperatures. Therefore our technique is thought to be well tailored to measurements of scintillation characteristics as a function of temperature. In this study we examined samples of commercially available scintillators: CaWO4 produced by SRC Carat, Ukraine and ZnWO4 produced by Hilger Crystals, UK, using the Czochralski technique. Samples of 5 5 1 mm3 where placed in a cryostat and approximately 104 scintillation events excited by a-particles from an 241Am source were recorded at different temperatures using the MPCC technique. The data where analysed using the routines described above. Fig. 6a shows the scintillation decay time spectra of CaWO4 for different temperatures. A fit to the experimental curves was done using a sum of two exponentials plus a constant offset: f ðtÞ ¼ y0 þ A1 et=t1 þ A2 et=t2 , although in the temperature range 20–50 K a third component was necessary to achieve a good fit. Fig. 6b shows the temperature dependence of the decay time constants. At room temperature, the decay time constants are t1 ¼ 1.370.1 ms and t2 ¼ 8.97 0.2 ms, in good agreement with recent measurements [21] (see also Table 1). It is clear from Figs. 6a and 6b that the decay time constants change relatively little with temperature in the range 50–300 K. At To50 K the character of the scintillation changes drastically: a very slow component appears which becomes pronounced below 20 K. The decay time constant is measured to be 390720 ms at 9 K. This behaviour is characteristic of the decay time spectra of CaWO4 and it can be attributed to the existence of a metastable level a few meV below the emitting level [25]. At low temperature the thermal depopu-
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100 Temperature, K
Fig. 6. (a) Decay time spectra of scintillation in CaWO4 measured at different temperatures; 1–345 K, 2–295 K, 3–170 K, 4–45 K, 5–30 K, 6–17 K and 7–9 K; (b) The decay time constants for various temperatures.
lation of this level is suppressed causing the probability of radiative recombination to decrease significantly, slowing down the entire emission process. It should be noted that the scintillation decay time constant of CaWO4 at milli-kelvin temperatures was found to be ca. 4007100 ms [26], which is in good agreement with our findings. The decay time spectra of ZnWO4 measured at different temperatures are shown in Fig. 7a. The best fit of the decay curves below room temperature is achieved by using a function with three exponentials and a constant as decay function f ðtÞ ¼ y0 þ A1 et=t1 þ A2 et=t2 þ A3 et=t3 . Three components are clearly seen throughout the temperature range 9–300 K (Fig. 7b). The values of decay time constants at two reference temperatures are given in the Table 1. At low temperature
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Table 1 Decay time constants of CaWO4 and ZnWO4 crystals at 295 and 9 K obtained using the MPCC technique Crystal
T, K
A1, (%)
t1, ms
A3, (%)
t2, ms
A3, (%)
t3, ms
CaWO4
295
5075 (6a) 7075
1.470.1 (0.3a) 3.270.4
5075 (18a) 3075
8.970.2 (3.2a) 390720
— (76a) —
— (8.8a) —
4575 (4b) 7575
1.370.1 (0.7b) 1.770.2
2075 (16b) 1075
5.670.3 (5.6b) 17.970.8
3575 (80b) 1575
25.770.3 (24.8b) 110710
9 295
ZnWO4
9
In brackets: the values of decay time constants obtained from the three exponential fit from references. a [21]. b [30].
intensity, a.u.
103
102
101
1
7
4
3
5
6
2
100 0
100
(a)
200 300 Time, µs
400
500
Decay time, µs
100
10
1
10 (b)
100 Temperature, K
Fig. 7. (a) Decay time spectra of scintillation in ZnWO4 measured at different temperatures; 1–350 K, 2–325 K, 3–294 K, 4–150 K, 5–20 K, 6–14 K, and 7–9 K; (b) The decay time constants for various temperatures.
one can notice another difference in the behaviour of the decay time constants of ZnWO4 when comparing with CaWO4. The decay time spectrum
becomes longer but changes rather more gradually and does not exhibit a very drastic change. Therefore, the longest decay time constant of ZnWO4 at 9 K is shorter than that of CaWO4, being 110715 ms; even at milli-kelvin temperatures the decay time is reported as being only 200 ms [27]. This finding indicates that the timing characteristics of zinc tungstate at low temperature are much more favourable than those of CaWO4, providing additional support to our recent choice of this scintillator for cryogenic application [24]. Fig. 8a shows a set of typical amplitude spectra for CaWO4, excited by 241Am a-particles, measured at different temperatures. Each spectrum can be fitted by a single Gaussian and the peak position, which gives the most probable number of photons detected by the PMT, is proportional to the scintillator light yield. The variation of the light yield with temperature of the calcium tungstate scintillator is shown in Fig. 8b. As the temperature decreases the light yield increases in the range from 350 to 270 K and remains fairly constant in the 270–50 K range. According to the simple configuration model, the temperature dependence of the light yield and decay time constant are controlled by the same process of thermal deactivation of the emitting levels. These dependences usually run in parallel unless other decay channels exist [28,29]. Given the fact that the transient recorder has a finite record length, a certain fraction of photons will escape detection if the decay time constant of the emission is comparable with or longer than the recording time. This would result in a decrease of
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350
Number
for the light yield of ZnWO4 which is consistent with the emission intensity being negligible by the end of the record for any temperature (see Fig. 7a).
1
300
3
2
250 4
200
5. Conclusion
150 100 50 0 0
20
(a)
40 60 Number of photons
80
100
80 Number of photons
70 60 50 40 30 20 10 0 (b)
533
10
100 Temperature, K
Fig. 8. (a) Amplitude spectrum of scintillation in CaWO4 excited by an 241Am a-source and measured at temperatures 295 K (1), 150 K (2), 50 K (3) and 27 K (4). The dotted curve shows a single-Gaussian fit to the amplitude spectrum measured at 295 K. (b) Average number of photons detected from CaWO4 (triangles) and ZnWO4 (circles) when excited with an 241 Am a-source as a function of temperature. Open circles show the measured number of photons from CaWO4 before correcting for the bias introduced by the limited measurement time window.
the observed light yield if one only counted the photons recorded. The effect of this is seen in CaWO4 at temperatures below 50 K (open circles in Fig. 8b) and a correction must be applied to remove this bias. Therefore these data points were corrected for the finite recording time assuming the same decay constant for the whole duration of the scintillation event. For example, it can be seen from Fig. 6a that for the decay time spectrum taken at 9 K the correction accounts for 20% of the amplitude before the correction. It is worthwhile noting that no such correction is necessary
In this study we developed and tested the multiple photon counting coincidence (MPCC) technique for measuring the temperature dependence of decay time constants and absolute light yield of scintillating crystals. The method works by recording the entire sequence of photon pulses produced by a PMT during a scintillation event. By statistical analysis of the recorded scintillation events one can obtain the decay time constant and the number of detected photons, which is proportional to the light yield. Technically the method is realised using two PMTs operating in coincidence mode, a set of standard electronic modules for signal processing and a transient recorder. The use of two PMTs is motivated by the necessity to reduce the number of spurious events. The ability to analyse records of scintillation events composed of individual photons offers a unique opportunity to improve the accuracy of the measurement by filtering out spurious events, such as multiple excitations. These events can be the origin of bias in measurements of the scintillation decay time constants of ‘‘slow’’ scintillators, especially at low temperature and it is shown that the MPCC technique can deal with the problem successfully. Using this technique we studied CaWO4 and ZnWO4 and demonstrated that it offers an excellent way for the characterisation of scintillators over a wide temperature range (9–350 K). Both the high and low temperature decay time constants and the light yield of the crystals investigated were found to be consistent with published data. The MPCC technique is particularly useful for research on slow cryogenic scintillators for rare event searches. Finally we would like to emphasise one more important aspect of the MPCC technique, i.e. the ease with which it can merge with the conventional DC-SPC technique that is used routinely. The only capital investment required is towards a fast
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transient recorder or analog-to-digital converter while available detectors and standard electronic modules can be used readily. The new detection technique can be applied to any excitation mode with the same benefits intrinsic to the DC-SPC technique. Because of this, the MPCC technique is an excellent complement to the conventional DCSPC technique and its use can significantly broaden the range of investigations of processes that require the measurement of the time evolution of a light signal in physics, chemistry, biology and medicine.
[10] [11] [12] [13] [14] [15] [16]
Acknowledgments The work was supported by PPARC Grant I/S/ 2001/646. We are grateful to Prof. Ian Bailiff of the University of Durham for lending us a helium cryostat for the low-temperature experiments. We would like to thank our referee, Prof. Marek Moszynski, for his valuable comments.
[17] [18] [19] [20] [21]
[22]
References [1] H. Kraus, Phil. Trans. R. Soc. London A 361 (2003) 2581. [2] G. Angloher, M. Bruckmayer, C. Bucci, M. Bu¨hler, S. Cooper, C. Cozzini, P. Di Stefano, F. von Feilitzsch, T. Frank, D. Hauff, et al., Astropart. Phys. 18 (2002) 43. [3] P. Christ, S. Rutzinger, C. Koy, W. Seidel, S. Uchaikin, M.O. Glocker, F. Pro¨bst, Eur. J. Mass Spectrom. 10 (2004) 469. [4] P. Meunier, M. Bravin, B. Bruckmayer, S. Giordano, M. Loidl, O. Meier, F. Pro¨bst, W. Seidel, M. Sisti, et al., Appl. Phys. Lett. 75 (1999) 1335. [5] G. Angloher, C. Bucci, P. Christ, C. Cozzini, F. von Feilitzsch, D. Hauff, S. Henry S, Th. Jagemann, J. Jochum, H. Kraus, B. Majorovits, et al., Astropart. Phys. 23 (2005) 325. [6] S. Cebrian, N. Coron, G. Dambier, E. Garcı´ a, I.G. Irastorza, J. Leblanc, P. de Marcillac, A. Morales, J. Morales, A. Ortiz de Solo´rzano, et al., Astropart. Phys. 21 (2004) 23. [7] A. Alessandrello, V. Bashkirov, C. Brofferio, C. Bucci, D.V. Camin, O. Cremonesi, E. Fiorini, G. Gervasio, A. Giuliani, A. Nucciotti, et al., Phys. Lett. B 420 (1998) 109. [8] P. de Marcillac, N. Coron, G. Dambier, J. Leblanc, J.P. Moalic, Nature 422 (2003) 876. [9] C. Cozzini, G. Angloher, C. Bucci, F. von Feilitzsch, D. Hauff, S. Henry, Th. Jagemann, J. Jochum, H. Kraus, B.
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Majorovits, V. Mikhailik, et al., Phys. Rev. C 70 (2004) 064606. V.B. Mikhailik, H. Kraus, D. Wahl, M. Itoh, M. Koike, I. Bailiff, Phys. Rev. B 69 (2004) 205110. V.B. Mikhailik, H. Kraus, D. Wahl, M.S. Mykhaylyk, Phys. Status Solidi B 242 (2005) R17. V.B. Mikhailik, H. Kraus, G. Miller, M.S. Mykhaylyk, D. Wahl, J. Appl. Phys. 97 (2005) 083523. G.F. Knoll, Radiation Detection and Measurements, Wiley, New York, 1999. M. Moszynski, M. Szawlowski, M. Kapusta, M. Balcerzyk, Nucl. Instr. and Meth. A 485 (2002) 504. P. Antonini, S. Belogurov, G. Bressi, G. Carugno, P. Santilli, Nucl. Instr. and Meth. A 488 (2002) 591. M. Moszynski, W. Czarnacki, W. Klamra, M. Szawlowski, P. Schotanus, M. Kapusta, Nucl. Instr. and Meth. A 504 (2003) 307. W.W. Moses, Nucl. Instr. and Meth. A 336 (1993) 253. M. Moszynski, B. Bengtson, Nucl. Instr. and Meth. A 142 (1977) 417. L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. W. Zhaomin, X. Tong, X. Zizong, W. Chong, Nucl. Instr. and Meth. A 419 (1998) 154. Yu.G. Zdesenko, F.T. Avignone III, V.B. Brudanin, F.A. Danevich, S.S. Nagorny, I.M. Solsky, V.I. Tretyak, Nucl. Instr. and Meth. A 538 (2005) 657. S.E. Derenzo, W.W. Moses, Experimental efforts and results in finding new heavy scintillators. in: De F. Notaristefani, P. Lecoq, M. Schneegans (Eds.), Heavy Scintillators for Scientific and Industrial Applications, Frontieres, 1993, pp. 125. R. Brun, F. Rademakers, Root—an object oriented data analysis framework, in: Proceedings AIHENP’96 Workshop, Lausanne, September 1996; Nucl. Inst. and Meth. A 389(1997)81. H. Kraus, V.B. Mikhailik, Y. Ramachers, D. Day, K.B. Hutton, J. Telfer, Phys. Lett. B 610 (2005) 37. G. Blasse, G. Bokkers, J. Solid State Chem. 49 (1983) 126. P.C.F. Di Stefano, T. Frank, G. Angloher, M. Bruckmayer, C. Cozzini, D. Hauff, F. Pro¨bst, S. Rutzinger, W. Seidel, L. Stodolsky, J. Schmidt, J. Appl. Phys. 94 (2003) 6887. V. Babin, P. Bohacek, E. Bender, A. Krasnikov, E. Mihokova, M. Nikl, N. Senguttuvan, A. Stolovits, Y. Usuki, S. Zazubovich, Radiat. Meas. 38 (2004) 533. V. Mu¨rk, M. Nikl, E. Mihokova, K. Nitsch, J. Phys. Cond. Mater. 9 (1997) 249. V. Nagirnyi, L. Jo¨nsson, M. Kirm, A. Kotlov, A. Lushchik, I. Martinson, A. Watterich, B.I. Zadneprovski, Radiat. Meas. 38 (2004) 519. F.A. Danevich, V.V. Kobychev, S.S. Nagorny, D.V. Poda, V.I. Tretyak, S.S. Yurchenko, Yu.G. Zdesenko, Nucl. Instr. and Meth. A (2005) in press.
Nuclear Instruments and Methods in Physics Research A 553 (2005) 522–534 www.elsevier.com/locate/nima
Multiple photon counting coincidence (MPCC) technique for scintillator characterisation and its application to studies of CaWO4 and ZnWO4 scintillators H. Kraus, V.B. Mikhailik, D. Wahl Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK Received 13 June 2005; received in revised form 9 July 2005; accepted 10 July 2005 Available online 2 August 2005
Abstract We describe a new method for measurements of the scintillation characteristics of crystals. The multiple photon counting coincidence (MPCC) technique involves recording the sequence of individual photon pulses resulting from a scintillation event. The timing of the individual photons allows determination of the scintillation decay time constants. The number of photons recorded per scintillation event is proportional to the scintillation light yield. The decay time constants and the relative light yield of CaWO4 and ZnWO4 scintillators have been investigated in the temperature range 9–350 K. An important advantage of the MPCC method is the possibility to reject spurious events through offline analysis, taking into account the entire data set of scintillation events. This procedure allows cleaning of the data set from multiple scintillation events (pile-up). The MPCC technique is an excellent complement to conventional characterisation techniques and is particularly suited for investigation of slow scintillation processes. r 2005 Elsevier B.V. All rights reserved. PACS: 29.40.Mc Keywords: Photon counting; Scintillator characterisation; Decay time; Light yield; CaWO4; ZnWO4
1. Introduction During recent years there has been a steady increase in research activity regarding the development of new instrumentation for low-backCorresponding author. Tel.: +44 1865273361;
fax: +44 1865273418. E-mail address: [email protected] (H. Kraus).
ground particle physics experiments. Recent progress in the field of low-temperature detection techniques has made available highly sensitive energy resolving cryogenic detectors [1–3]. A new generation of these detectors, which offer the capability of background discrimination by simultaneous measurement of scintillation and phonon signals [4], is now in use. These detectors play an important role in experiments searching for rare
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.07.011
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events, such as interaction of Weakly Interacting Massive Particles (WIMPs) likely to comprise galactic Dark Matter [5,6], double beta decay [7] and radioactive decay of very long-lived isotopes [8,9]. Scintillating crystals are a key element in these experiments and the identification and characterisation of promising scintillator materials for cryogenic applications is an important objective for future experiments in this field. A number of experimental techniques commonly used for luminescence characterisation of materials can be implemented readily in these studies, specifically UV, VUV and X-ray excited spectroscopy [10–12]. In addition there is strong motivation for an experimental technique capable of measuring the temperature dependences of the light yield and the decay time constants under a-, b- and g-excitation. In this paper we describe an original method for measuring these scintillator characteristics using the multiple photon counting coincidence (MPCC) technique. The development of this technique is part of a research initiative aimed at the identification and optimisation of scintillation materials for cryogenic phonon-scintillation detectors (CPSD). The structure of this paper is as follows: in the first part we discuss the conventional approach to measurements of the light yield and decay time constants of scintillators. We analyse inherent limitations of these methods and in particular those arising in low-temperature scintillation studies. In the second part we present the MPCC method, explaining technical detail and the data analysis. Finally, to illustrate the performance of the technique, we discuss our results for light yield and decay time constants of CaWO4 and ZnWO4 scintillators in the temperature range 9–350 K.
2. Conventional techniques of scintillator characterisation 2.1. Light yield measurements in a low-temperature experiment A large number of papers on scintillators have focused on measurements close to or at room temperature. There are quite a few challenges to be
523
overcome when expanding characterisation to a wider temperature range. Firstly, we consider the measurement of the light yield. The characterisation of scintillation detectors has so far relied almost exclusively on the use of high-gain photomultipliers (PMT) or avalanche photodiodes (APD) [13,14]. Such experiments imply the detection of integrated voltage pulses produced by a photodetector following the scintillation event. The pulse height spectra obtained from these data give a measure for the scintillator’s light yield. In order to achieve optimum light collection and hence the best possible resolution on the measurement of the light yield, the scintillator and photodetector are arranged in direct optical contact. However when considering studies of the light yield as function of temperature one has to bear in mind that the quantum efficiency of PMTs strongly depends on temperature. The numerous technical and methodological difficulties encountered by experimentalists who have endeavoured to implement a PMT inside a low-temperature setup can be appreciated from [15]. Similar problems, though with the reverse effect, hold for APDs that exhibit a dramatic increase in gain with cooling [16]. In addition, most scintillator studies have been performed only down to the temperature of liquid nitrogen and as far as we are aware none of the photodetectors operate in stable conditions at lower temperatures. In summary we conclude that for carrying out such measurements routinely it is strongly advisable to keep the photodetector at constant (room) temperature while cooling (heating) the sample. This simple concept can be implemented readily by installing the sample into an optical cryostat; the downside, however, is that in measuring the amplitude spectrum not much light is collected by the photodetector. 2.2. Techniques for measurements of the decay time constants In studies of decay time constants as function of temperature additional problems associated with the necessity to cover a large range of decay time constant values arises. For example, the materials
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used in this study, i.e. calcium and zinc tungstates, exhibit a composite decay spectrum with decay time constants varying with temperature from micro-seconds to milli-seconds. Accurate determination of these decay time constants requires the sampling time interval to be at least ten times shorter than the shortest decay time constant while the number of samples taken in one recording process (record length) has to be large enough to record for at least three times the characteristic time constant of the longest component of the scintillation [17]. The delayed coincidence single-photon counting (DC-SPC) technique has excellent timing resolution (ca. 0.5 ns) and it is commonly used for measuring decay characteristics of traditional fast scintillators with decay time constants in the range of nano-seconds to micro-seconds. The method is based on measuring the distribution of the time difference between the excitation of the scintillator and an individual scintillation photon [18] or recording of a start and a stop photon from the same scintillation event [19]. Two PMTs are used to produce a prompt trigger (start) signal associated with the beginning of the scintillation pulse and a stop signal that defines the arrival time of one of the scintillation photons. The time difference is digitized with a time to digital converter (TDC) and processed by down-stream electronics. The accumulated data are plotted in the form of a histogram of the arrival times of scintillation photons from many excitation events and fitted to a mathematical function suitable for describing the decay mechanism. Measurements of decay time constants by the DC-SPC technique require the probability of detection of more than one photon during the recording time to be less than 0.01 [17]; otherwise there will be bias towards a faster decay time constant [20]. To reduce this bias the number of photons is artificially limited using a diaphragm or a filter; but this reduces sampling rates to typically 0.1–1 s1. Accurate measurements of the scintillator decay time constants usually require 105 data points which may result in measurement times of up to a few days [17]. Therefore such a technique is not very suitable for studies involving temperature variation.
If the decay time constant of the scintillator is in the region of several micro-seconds, pulse shape analysis (PSA) can be implemented [13]. The conventional PSA technique works by converting the optical signal following the scintillation event into an analog PMT electrical signal and recording this electrical signal with an oscilloscope, transient recorder or box-car integrator. It should be noted that the PSA technique has limited time resolution that is defined by the width of the PMT response (typically a few nanoseconds) and the minimum sampling interval of the digitizer (typically 5 ns). Given this, one can estimate that this approach allows measuring the decay time constant only if it is X50 ns. Therefore this approach is used for the analysis of ‘‘slow’’ scintillators which exhibit a decay time constant of 4100 ns [21]. 2.3. Limitations of decay time constant techniques There are two sources of error inherent to both decay time measurement techniques. It is not practically possible to fully control the rate at which ionising particles or photons interact with the scintillator. Hence there are always events recorded in which a second (or more) scintillation event has occurred during the measurement period of the first, a so-called multiple excitation event. To reduce this effect the time between events needs to be much longer than the recording period. This can be achieved by decreasing the source activity, but this results in a prolonged measurement time. Although this is not a severe problem when fast scintillation events are studied it does become a serious one in the case of ‘‘slow’’ scintillators. First of all, the source activity cannot be arbitrarily low; for meaningful measurements the rate of scintillation events produced by a radioactive source must exceed that caused by natural background by some 2–3 orders of magnitude. A rough estimate for a decay time constant of 10 ms, a recording time of 50 ms and an event rate of 1000 s1 shows that approximately 2% of the recorded events will include scintillation resulting from more than just one particle interaction, i.e. multiple excitations. This seemingly negligible contribution to the total signal results in a roughly even time difference distribution throughout the decay time spectrum,
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3. MPCC technique
0.25 0.24
Intensity, a.u.
0.23 0.22 0.21 0.20 0.19 (a)
0.18 0
10
20
30
40
0.24
Intensity, a.u.
hence creating a noticeable tailing effect. Thus when analysing such data one might easily misinterpret this background as an additional decay component with a long time constant. Other common sources of error are spurious prompt events caused by a direct hit of the PMT’s photocathode, by scattered ionising radiation from the source and by natural background. In both techniques this effect can cause a prompt response of the PMT, which could be misinterpreted as a fast scintillation component. It is possible to reduce this effect in the DC-SPC technique by shielding the ‘‘stop’’ PMT and by moving it away from the radioactive source but it cannot be eliminated completely and it remains an intrinsic source of error in the PSA technique. In summary there are several potential problems when measuring decay time constants of scintillators that bias the final result in one or the other way. A reduction of this bias is possible via reducing the source activity, and hence event rate, but this inevitably results in a conflict when a series of measurements is required, e.g. in studies of temperature dependences. These problems are in addition to the necessity of finding a way of measuring the amplitude spectrum of scintillators when having very low light collection efficiency. In an attempt to avoid these problems we turned our attention to the idea of detection of multiple photons from individual scintillation events.
525
0.22
0.20
5.1 (b)
5.2
5.3 Time, µs
5.4
5.5
Fig. 1. (a) A typical record displaying the detection of individual photons from a scintillation event in CaWO4 excited by an 241Am a-source at T ¼ 295 K. (b) Zoom of the above record near the beginning of the scintillation event showing the shape of single photon pulses produced by the PMT.
3.1. Method The MPCC technique involves the recording of a sequence of pulses produced by a PMT when detecting photons from a scintillation event (see Fig. 1). Each pulse in the sequence corresponds to an individual photon impinging on the photocathode of the PMT. The distribution of arrival times of these pulses provides information on the decay characteristics of the scintillation process. The number of pulses recorded per event is proportional to the light yield of the scintillator. By recording a large number of scintillation events (103–104) one can obtain the decay time character-
istics and the light yield in a single measurement. Due to the large number of photons detected in each event, typically 25–30 here (see next section), sufficient accuracy can be achieved already with far fewer scintillation events than needed for the DCSPC technique, which only uses one time difference between arriving photons per scintillation event. With merely about 10 min total data acquisition time for one measurement, MPCC allows carrying out a whole series of measurements within a reasonable period of time (a few hours only), hugely simplifying the examination of scintillation parameters and their variation with temperature.
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3.2. Estimation of expected light collection efficiency The light collection efficiency Z of a PMT with photocathode diameter d, positioned at a distance L from the sample, assuming a point-like excitation in the scintillator sample and isotropic propagation of photons, can be expressed as1 2 d Zffi . (1) 4L If the sample is placed inside a cryogenic setup with N optical windows for light to pass through, the reflection from the 2N glass surfaces, each with reflection coefficient r, has to be taken into account, yielding a correction factor ð1 rÞ2N . The light collection efficiency in that case is given by 2 d . (2) Z ffi ð1 rÞ2N 4L Substituting parameters typical of our setup, d ¼ 25 mm, L ¼ 50 mm, r ¼ 0:05 and N ¼ 4 (three cryostat windows and the window of the PMT) one finds that on average only 1% of the photons leaving the scintillator can reach the photocathode. The typical quantum efficiency of a PMT is 25–30%, and assuming further a light yield in the region of 10 000 photons/MeV, which is typical for tungstate crystals [22], we estimate that on average in such an experimental setup the PMT should detect 25–30 photons from scintillation following an energy deposition of 1 MeV. This number of photons should allow already determination of the scintillation time constant, assuming a single time constant, with an error of o20% from just one single event. The samples used in our experiments have a thickness of a few mm. This is too thin for using high-energy g-sources, such as 137Cs (662 keV) or 60Co (1173 and 1333 keV) as most scintillation photons would result from events due to Compton scattering with a continuum of energies, thereby making the determination of the scintillation yield difficult. On the other hand, low-energy g-sources i.e. 57Co (122 and 136 keV), would not provide a 1 We only consider the photons that have escaped the crystal as is standard for light yield measurements.
sufficient number of photons given the low light collection efficiency in a cryogenic setup (see Eq. (2)). This difficulty can be overcome by using aexcitation since a-particles are absorbed within 10 mm below the scintillator’s surface. In our experiments we used 241Am, emitting a-particles of energies 5.44 and 5.48 MeV. One needs to take into account that a-particles produce less scintillation than g-quanta. For tungstates the correction factor, which is known as the a/b-ratio (g-quanta interact with matter via b-particles), is 0.2 [9,21]. Thus, with 11 000 photons produced by aparticles about 30 photons should be detected in the setup discussed here. 3.3. Experimental setup The experimental setup used for our MPCC measurements is shown in Fig. 2. The scintillator sample and the 241Am a-source are placed inside a helium flow cryostat equipped with optical windows to allow the detection of light. Two PMTs see the sample at right angle to each other. The signals produced by each PMT with their associated pre-amplifiers are passed to integrating
Sample 241AM
Signal
PMT 1
To PC
PA TR
Cryostat PMT 2
Trigger. PA Coinc. AMP
SCA
AMP
SCA
Delay
Fig. 2. Schematic of the experimental setup used to measure the decay time spectra and relative light yield of scintillating crystals. PMT—photomultiplier, PA—preamplifier, TR—transient recorder, Amp—linear amplifier, SCA—single channel analyser, Delay—gate and delay generator, Coinc.—coincidence unit.
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amplifiers. Only one PMT is connected to the input of the transient recorder while the trigger is based on coincidence in both PMTs in order to minimise the recording of spurious events such as the detection of a direct hit of the PMT photocathode [13]. Event selection for triggering is carried out by processing the slow integrated and amplified signals. The integrating amplifiers produce signals which are a measure of the total energy detected, and are fed into single channel analysers (SCA). The discrimination thresholds of the SCAs are set such as to reduce the number of pulses with low and very high amplitudes that are associated with electronic noise and spurious events caused by cosmic muons, respectively. The logic output pulses of both SCAs are fed into a coincidence unit that provides the trigger for the transient recorder. A gate and delay generator is operating in one channel to tune the arrival times of the logic pulses to the coincidence unit. The transient recorder has sampling intervals ranging from 5 to 80 ns and is able to record up to 16 384 samples per record. With the number of photons expected from one scintillation event estimated to be 30, we continue with an assessment of the instrumental requirements on the PMT, pre-amplifier and transient recorder chain as described above. The aim is to resolve the time structure of scintillation light emission at the single photon level. A data acquisition chain capable of nano-second time resolution seems appropriate. In our experiment we used a PMT (9125B) from Electron Tubes exhibiting a single electron pulse width of 7.4 ns (FWHM). The charge signal from the PMT was converted into a voltage pulse using an integrating amplifier with time constant 2.2 ns (R ¼ 470 O and C ¼ 4:7 pF) with an OPA633 buffer amplifier. In practice there is additional parasitic capacitance associated with the design of a pre-amplifier, e.g. size and position of components, wiring, etc, that increases the integration time constant calculated from the nominal component values. A measurement of the pre-amplifier transfer function allowed determining the bandwidth, which, with a value of 200 MHz, was not too dissimilar to the design value and, in fact, is a perfect match to the shortest possible sampling interval of 5 ns of the transient
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recorder (LeCroy TR8828D). Individual pulses (photons) from scintillation events in CaWO4 were detected, with each pulse spanning 7–8 data points, corresponding to a characteristic individual pulse FWHM of 15 ns (see Fig. 1). The average number of single photon pulses for the singleexcitation events excited by 241Am a-particles at room temperature (for the procedure of multiple event recognition see Section 3.4) is 3275, in good agreement with our earlier estimate. 3.4. Data analysis All scintillation events recorded by the transient recorder are stored in a binary data file by custommade DAQ software. The data analysis software is based on the particle physics analysis framework ROOT [23], with a tailor-made set of C++ classes to represent the individual record, scintillation events, single photon pulses, etc. Each event is essentially an array of floating point numbers of the same size as the record length (usually 16 384) representing the PMT signal as function of time. For each event (record) a set of parameters is calculated, such as the number of photons, the baseline, timing of the photons, etc. The first parameter to find is the baseline of the record. This is achieved by histograming the data points of a record into 256 bins, representing all possible signal voltages the 8-bit transient recorder can produce. Given the nature of the data (see Fig. 1) most of the data points in the record are the baseline and an initial estimate for the baseline level is arrived at by finding the most likely signal voltage in the histogram. This initial baseline estimate then allows one to identify the individual photon pulses by data points departing from the baseline. The initial photon pulse recognition algorithm merely requires the signal voltage (data point) being greater (negative pulses) for two consecutive points before and after the point under consideration in order for it to be labelled a photon pulse. The range occupied by a potential photon pulse is defined as beginning 3 data points before the peak position and ending 8 data points after. Eliminating all photon pulses leaves one with just the baseline including some variation due to electronic
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noise. By averaging the remaining data one obtains a value for the baseline. At the same time the RMS value of the baseline is calculated as an estimate of the contribution of electric noise. With the baseline sufficiently well established, and also knowing how noisy the baseline is, a new peak search algorithm is run based on the following criterion: for a photon pulse to be recognised, its amplitude must deviate from the baseline by at least 1 unit in amplitude, defined as the difference of two consecutive voltage amplitudes measured by the transient recorder (1 bit), 5 sigma (from RMS of the baseline) from the baseline or 5% of the average amplitude of all photon pulses in the record; whichever is greatest. The end of a photon pulse is defined as when the amplitude falls below the threshold for a peak to begin or 5% of its own amplitude, whichever is farthest from the baseline. The latter correlation with the peak height is to avoid problems with electronic ringing seen in some large signals. The photon pulse recognition explained above actually recognises a signal range containing at least one photon, but possibly also more than one. A second routine is added to identify whether there are multiple photons in a range so far identified as one pulse. Once all photon pulses in a record have been identified, a histogram of the arrival times of the first photon in each record is constructed. A typical example of such a distribution is shown in Fig. 3a. It can be seen that the vast majority of ‘first photons’ arrive in a narrow time interval defined by the hardware trigger, identified as the sharp peak in the histogram. However there is a noticeable number of scintillation events having photon pulses prior to the trigger. An example of such an event with an early ‘first photon’ is shown in Fig. 4a together with a normal event (Fig. 4b) and a multiple (double) event (Fig. 4c). The presence of events with an early ‘first photon’ has a simple explanation: these contain photon pulses from a previous scintillation event recorded with the event that produced the actual trigger. In fact this is an effect complementary to multiple events and, if not identified, will introduce a noticeable error in the results. This is because the distribution of arrival times of photons representing the temporal char-
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Fig. 3. Analysis of scintillation events in CaWO4 at T ¼ 144 K: (a) distribution of arrival times of the first photon in a scintillation event; (b) decay time constants as a function of position of the cut on the arrival time of the first photon; and (c) test statistic, Eq. (5), proportional to the likelihood ratio of the decay time constants.
acteristic of the scintillation process (decay time spectrum) is calculated with respect to the time of the first photon pulse in a record. Therefore records with early photons must be eliminated from the analysis and to do this a procedure of cuts has been implemented. This implies the elimination of events exhibiting a first photon pulse that arrives earlier than a specified time (cut-on-time). Fig. 3b shows the dependence of the two dominating decay time constants of CaWO4 on this cut-on-time. One can identify three characteristic regions in the plot. In the first region
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time of the first photon not within the dominant peak in the distribution are rejected. Once the data set has been cleaned of events with early photons it is desirable to eliminate multiple scintillation events. Initially we assume the decay process can be represented by a single exponential decay with just one decay time constant. This simplification allows immediate calculation of the decay time constant t from the arrival times of the photons in an event with respect to the arrival time of the first photon. This simple method of calculating t is based on the probability density function for the detection of a photon at time t:
0.25
0.20
0.15 (a)
Intensity, a.u.
0.25
(b)
0.20
0.15
1 Pðt; tÞ ¼ expðt=tÞ. (3) t When N photons are detected following a scintillation event, giving N1 time differences, the decay time constant t can be calculated as
0.25 0.20 0.15
t¼ 0.10 (c)
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0
100
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300 400 Time, µs
500
600
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Fig. 4. Records illustrating different types of scintillation events in CaWO4 (T ¼ 144 K): (a) an event with a very early first photon (most likely from a previous scintillation event or a direct photocatode hit), (b) a normal event and (c) a double event (two scintillation events in one record).
the closer to the trigger time the cut-on-time point is located the shorter the decay time constants are. This can be understood as events with an early first photon contribute randomly to the initial part of the decay time spectrum, introducing an overestimate of the calculated decay time constants. Then there is a ‘plateau region’ where the decay time constant does not change much. This is the region where the cut-ontime is implemented; placing the cut at a later time causes the decay time constants to decrease significantly. Obviously a cut-on-time above this point already eliminates good events of the type being consistent with what Fig. 3a shows: this is the region where the peak in the histogram for good events begins. Based on this analysis a criterion is derived such that events with an arrival
1 X 1 N 1 Dti þ Dtm N 1 i¼1 expðDtm =tÞ 1
(4)
where Dti is the arrival time of the ith photon with respect to the arrival time of the first photon and Dtm is the sampling period remaining after detecting the first photon. The second term in Eq. (4) corrects for the bias due to the finite measurement time (record length) available for each record. Assuming DtmX5t, for example, its contribution to t is less than 3%. With just one exponential decay with a single time constant being an approximation anyhow, the second term has been ignored in our calculations, allowing us to estimate the single decay time constant as simply the mean of the photon arrival times since the first photon. Neglecting the bias, which could nevertheless become significant for long decay time constants, does not really affect the final result as the time constant calculated via Eq. (4) is only used to calculate a test statistic being a measure for how consistent the time structure found in the record is with that given by Eq. (3). The algorithm for the elimination of double events is based on the ratio of likelihoods of the decay time constant t calculated as above and the most likely time constant tm taken from the distribution of t-values for all records. The test
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statistic Pðt; tÞ W ¼ 2 ln Pðt; tm Þ t t ttm Dt 2 ¼ 2 ln þ1 ! tm tm tm ð5Þ
represents a measure of the discrepancy between the most likely decay time constant of the process under study and that obtained for an individual record. When t ffi tm , W can be approximated by the square of the relative deviation of the time constant from the most likely time constant. Fig. 3c shows the histogram of this parameter plotted for a large number of scintillation events detected in CaWO4. It is obvious that W is close to zero for the vast majority of scintillation events, i.e. most events having t ffi tm . These represent single scintillation events. Conversely this parameter differs significantly from zero when a double event is detected. Visual inspection of the events exhibiting W X1 shows that they are double events (see Fig. 4c). Therefore by rejecting events with large likelihood ratio one can eliminate multiple events. The cut-onstatistic was tested for a range of cut-off values for W between 0.1 and 1.0. Within that range, the values obtained for the decay time constants of the scintillation process were hardly affected. An illustration of the impact on the decay time spectrum of CaWO4 introduced by the cuts is given in Fig. 5. Curve 1 shows the histogram for no cuts (as measured) and curves 2 and 3 show the results after subjecting the data to the cut-on-time and cut-on-statistic routines, respectively. It is obvious that the cut-on-time eliminates a spurious bump in the middle of the decay spectrum (at around 35 ms). This bump is caused by events having the first photon near the beginning of the sampling period. The cut-on-statistic has a major effect on the shape of the decay curve for long times. In this region the emission caused by single scintillation events is negligible while multiple events contribute to the background. When multiple events are rejected, the decay spectrum shows virtually no background. The example presented here demonstrates that the procedures implemented give a much clearer
Intensity, a.u.
with t ¼ tm Dt
104 103
1
102 3 10
2
1
100 0
20
40
60 Time, µs
80
100
120
Fig. 5. Decay time spectra for scintillation in CaWO4 (T ¼ 295 K): curve 1, without cuts, as measured; curve 2, after elimination of events with early photons; and curve 3, after elimination of multiple events. The spectra are shifted along the vertical axis for clarity.
decay time spectrum of the scintillation process, which in turn permits the decay time parameters to be determined with better accuracy. Of particular advantage is the possibility to identify inherent background that might mimic a long lasting emission component. Finally it is worthwhile noting that the background caused by multiple events or pile-up effects is inevitably present in measurements carried out with the conventional DC-SPC technique. It has not much effect on the characterisation of traditional ‘‘fast’’ scintillators since the relation between the event rate (103–105 s1) and decay time constant (108–107 s) is such that the probability of detecting multiple events is negligible. However, as is shown in this section, severe complications arise when scintillators with a ‘‘slow’’ decay time constant have to be analysed. The MPCC technique with its associated data analysis is most beneficial for characterising ‘‘slow’’ scintillators and it is definitely needed for characterisation of slow scintillators at low temperatures where their scintillation becomes even slower.
4. Experimental demonstration of the MPCC technique In this section we shall analyse a number of experimental results obtained using the MPCC
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technique. As was highlighted in the introduction, our particular interest lies with crystals from the tungstate family, specifically CaWO4 and ZnWO4. Whereas the former one is already being used in cryogenic searches for Dark Matter [5,6] zinc tungstate is being developed as an additional target material for such experiments [24]. These crystals are quoted as ‘‘slow’’ scintillators because of their relatively long decay time constants, i.e. 10 ms at room temperature with a tendency to increase by at least an order of magnitude when cooled to liquid helium temperatures. Therefore our technique is thought to be well tailored to measurements of scintillation characteristics as a function of temperature. In this study we examined samples of commercially available scintillators: CaWO4 produced by SRC Carat, Ukraine and ZnWO4 produced by Hilger Crystals, UK, using the Czochralski technique. Samples of 5 5 1 mm3 where placed in a cryostat and approximately 104 scintillation events excited by a-particles from an 241Am source were recorded at different temperatures using the MPCC technique. The data where analysed using the routines described above. Fig. 6a shows the scintillation decay time spectra of CaWO4 for different temperatures. A fit to the experimental curves was done using a sum of two exponentials plus a constant offset: f ðtÞ ¼ y0 þ A1 et=t1 þ A2 et=t2 , although in the temperature range 20–50 K a third component was necessary to achieve a good fit. Fig. 6b shows the temperature dependence of the decay time constants. At room temperature, the decay time constants are t1 ¼ 1.370.1 ms and t2 ¼ 8.97 0.2 ms, in good agreement with recent measurements [21] (see also Table 1). It is clear from Figs. 6a and 6b that the decay time constants change relatively little with temperature in the range 50–300 K. At To50 K the character of the scintillation changes drastically: a very slow component appears which becomes pronounced below 20 K. The decay time constant is measured to be 390720 ms at 9 K. This behaviour is characteristic of the decay time spectra of CaWO4 and it can be attributed to the existence of a metastable level a few meV below the emitting level [25]. At low temperature the thermal depopu-
531
10
1 10 (b)
100 Temperature, K
Fig. 6. (a) Decay time spectra of scintillation in CaWO4 measured at different temperatures; 1–345 K, 2–295 K, 3–170 K, 4–45 K, 5–30 K, 6–17 K and 7–9 K; (b) The decay time constants for various temperatures.
lation of this level is suppressed causing the probability of radiative recombination to decrease significantly, slowing down the entire emission process. It should be noted that the scintillation decay time constant of CaWO4 at milli-kelvin temperatures was found to be ca. 4007100 ms [26], which is in good agreement with our findings. The decay time spectra of ZnWO4 measured at different temperatures are shown in Fig. 7a. The best fit of the decay curves below room temperature is achieved by using a function with three exponentials and a constant as decay function f ðtÞ ¼ y0 þ A1 et=t1 þ A2 et=t2 þ A3 et=t3 . Three components are clearly seen throughout the temperature range 9–300 K (Fig. 7b). The values of decay time constants at two reference temperatures are given in the Table 1. At low temperature
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Table 1 Decay time constants of CaWO4 and ZnWO4 crystals at 295 and 9 K obtained using the MPCC technique Crystal
T, K
A1, (%)
t1, ms
A3, (%)
t2, ms
A3, (%)
t3, ms
CaWO4
295
5075 (6a) 7075
1.470.1 (0.3a) 3.270.4
5075 (18a) 3075
8.970.2 (3.2a) 390720
— (76a) —
— (8.8a) —
4575 (4b) 7575
1.370.1 (0.7b) 1.770.2
2075 (16b) 1075
5.670.3 (5.6b) 17.970.8
3575 (80b) 1575
25.770.3 (24.8b) 110710
9 295
ZnWO4
9
In brackets: the values of decay time constants obtained from the three exponential fit from references. a [21]. b [30].
intensity, a.u.
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102
101
1
7
4
3
5
6
2
100 0
100
(a)
200 300 Time, µs
400
500
Decay time, µs
100
10
1
10 (b)
100 Temperature, K
Fig. 7. (a) Decay time spectra of scintillation in ZnWO4 measured at different temperatures; 1–350 K, 2–325 K, 3–294 K, 4–150 K, 5–20 K, 6–14 K, and 7–9 K; (b) The decay time constants for various temperatures.
one can notice another difference in the behaviour of the decay time constants of ZnWO4 when comparing with CaWO4. The decay time spectrum
becomes longer but changes rather more gradually and does not exhibit a very drastic change. Therefore, the longest decay time constant of ZnWO4 at 9 K is shorter than that of CaWO4, being 110715 ms; even at milli-kelvin temperatures the decay time is reported as being only 200 ms [27]. This finding indicates that the timing characteristics of zinc tungstate at low temperature are much more favourable than those of CaWO4, providing additional support to our recent choice of this scintillator for cryogenic application [24]. Fig. 8a shows a set of typical amplitude spectra for CaWO4, excited by 241Am a-particles, measured at different temperatures. Each spectrum can be fitted by a single Gaussian and the peak position, which gives the most probable number of photons detected by the PMT, is proportional to the scintillator light yield. The variation of the light yield with temperature of the calcium tungstate scintillator is shown in Fig. 8b. As the temperature decreases the light yield increases in the range from 350 to 270 K and remains fairly constant in the 270–50 K range. According to the simple configuration model, the temperature dependence of the light yield and decay time constant are controlled by the same process of thermal deactivation of the emitting levels. These dependences usually run in parallel unless other decay channels exist [28,29]. Given the fact that the transient recorder has a finite record length, a certain fraction of photons will escape detection if the decay time constant of the emission is comparable with or longer than the recording time. This would result in a decrease of
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Number
for the light yield of ZnWO4 which is consistent with the emission intensity being negligible by the end of the record for any temperature (see Fig. 7a).
1
300
3
2
250 4
200
5. Conclusion
150 100 50 0 0
20
(a)
40 60 Number of photons
80
100
80 Number of photons
70 60 50 40 30 20 10 0 (b)
533
10
100 Temperature, K
Fig. 8. (a) Amplitude spectrum of scintillation in CaWO4 excited by an 241Am a-source and measured at temperatures 295 K (1), 150 K (2), 50 K (3) and 27 K (4). The dotted curve shows a single-Gaussian fit to the amplitude spectrum measured at 295 K. (b) Average number of photons detected from CaWO4 (triangles) and ZnWO4 (circles) when excited with an 241 Am a-source as a function of temperature. Open circles show the measured number of photons from CaWO4 before correcting for the bias introduced by the limited measurement time window.
the observed light yield if one only counted the photons recorded. The effect of this is seen in CaWO4 at temperatures below 50 K (open circles in Fig. 8b) and a correction must be applied to remove this bias. Therefore these data points were corrected for the finite recording time assuming the same decay constant for the whole duration of the scintillation event. For example, it can be seen from Fig. 6a that for the decay time spectrum taken at 9 K the correction accounts for 20% of the amplitude before the correction. It is worthwhile noting that no such correction is necessary
In this study we developed and tested the multiple photon counting coincidence (MPCC) technique for measuring the temperature dependence of decay time constants and absolute light yield of scintillating crystals. The method works by recording the entire sequence of photon pulses produced by a PMT during a scintillation event. By statistical analysis of the recorded scintillation events one can obtain the decay time constant and the number of detected photons, which is proportional to the light yield. Technically the method is realised using two PMTs operating in coincidence mode, a set of standard electronic modules for signal processing and a transient recorder. The use of two PMTs is motivated by the necessity to reduce the number of spurious events. The ability to analyse records of scintillation events composed of individual photons offers a unique opportunity to improve the accuracy of the measurement by filtering out spurious events, such as multiple excitations. These events can be the origin of bias in measurements of the scintillation decay time constants of ‘‘slow’’ scintillators, especially at low temperature and it is shown that the MPCC technique can deal with the problem successfully. Using this technique we studied CaWO4 and ZnWO4 and demonstrated that it offers an excellent way for the characterisation of scintillators over a wide temperature range (9–350 K). Both the high and low temperature decay time constants and the light yield of the crystals investigated were found to be consistent with published data. The MPCC technique is particularly useful for research on slow cryogenic scintillators for rare event searches. Finally we would like to emphasise one more important aspect of the MPCC technique, i.e. the ease with which it can merge with the conventional DC-SPC technique that is used routinely. The only capital investment required is towards a fast
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transient recorder or analog-to-digital converter while available detectors and standard electronic modules can be used readily. The new detection technique can be applied to any excitation mode with the same benefits intrinsic to the DC-SPC technique. Because of this, the MPCC technique is an excellent complement to the conventional DCSPC technique and its use can significantly broaden the range of investigations of processes that require the measurement of the time evolution of a light signal in physics, chemistry, biology and medicine.
[10] [11] [12] [13] [14] [15] [16]
Acknowledgments The work was supported by PPARC Grant I/S/ 2001/646. We are grateful to Prof. Ian Bailiff of the University of Durham for lending us a helium cryostat for the low-temperature experiments. We would like to thank our referee, Prof. Marek Moszynski, for his valuable comments.
[17] [18] [19] [20] [21]
[22]
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