1
Comparative Study of PP0275C Hybrid Photodetector and XP2020Q Photomultiplier in Scintillation Detection M. Moszyński, Member, IEEE, W. Klamra, D. Wolski, W. Czarnacki, M. Kapusta, M. Balcerzyk, Abstract-- Properties of a hybrid photodetector (HPD), type PP0275C, produced by Delft Electronic Products B.V., for scintillation detection and spectrometry were studied and compared to a standard XP2020Q photomultiplier. The study was performed for several scintillators, as NaI(Tl), CsI(Tl), and LSO of different dimensions. The excellent capability of the HPD to resolve single photoelectron events was fully confirmed. However, the study of the HPD with the scintillators showed a dramatically reduced number of photoelectrons and a further deterioration of energy resolution, depending on the size (diameter or length) of the crystals. For a 10 mm diameter NaI(Tl) the number of photoelectrons of 5000±250 phe/MeV was measured, which correspond to about 56% of that observed with the XP2020Q with comparable quantum efficiency. Energy resolution of 9.2% for 662 keV γ-rays from a 137Cs source measured with the HPD light readout showed a serious degradation, larger than that arising from the statistics of photoelectrons. In conclusion, the study showed that this HPD is optimized for single photon detection and its application to scintillation detection is very limited.
I. INTRODUCTION recent works [1-6] showed an attractive Numerous capability of hybrid photodetectors (HPD) in detection of weak light signals. The very high pulse height resolution for single photoelectron events allows resolving peaks up to several photoelectrons, in this aspect much superior compare to other photodetectors. A use of the HPD in a scintillation detection is, however, less clear. In ref. [2] a rather low number of photoelectrons were measured for different scintillators and a poor energy resolution was observed. In ref. [5] a HPD equipped with a YAP window showed much better performances. Most of the measurements were done with a DEP-PP0270 series of HPD with a cross-focused focalization system, with a typical gain of 3500 at 15 kV high voltage. The high resolution of the single photoelectron peak and, in consequence, a negligible excess noise factor of the HPD together with the expected better stability of its gain suggested the HPD to be the best photodetector for precise Manuscript received October 15, 2003. This work was supported partially by the Swedish Institute. M. Moszyński, (telephone: +48-22 718-0586, e-mail:
[email protected]), M. Balcerzyk, W. Czarnacki, M. Kapusta, D. Wolski are with the Soltan Institute for Nuclear Studies, PL 05-400 Świerk-Otwock, Poland. W. Klamra is with Royal Institute of Technology, SCFAB, S-106 91 Stockholm, Sweden, (
[email protected]).
studies of an energy resolution and a non-proportionality of scintillators. Thus, an accurate evaluation of its performance for the scintillation detection and spectrometry was performed using several different crystals, as well as a standard XP2020Q photomultiplier for a comparison purpose. Since the energy spectrometry with the PMT light readout is well known, the most of data on the figures are presented for the HPD only. II. EXPERIMENTAL DETAILS The studies were performed for the PP0275C HPD; delivered by Delft Electronic Products B. V., see Fig. 1. The HPD was equipped with a S20UV photocathode and a quartz window, having the calibrated quantum efficiency of 24 % at 400 nm. The photoelectrons accelerated by 10 kV to 15 kV are electrostatically focused on a 2 mm diameter Si PIN diode working with the reversed bias voltage of 25 V to 100 V. The HPD also includes an internal low noise charge sensitive preamplifier, having noise of 180 rms electrons. The high voltage bias for the HPD of about 15 kV at the photocathode and 75% of that at the focalization electrode was obtained from the DEP high voltage supply delivered together with the tube.
Fig. 1. Schematic drawing of the HPD.
For comparison, a XP2020Q with a calibrated quantum efficiency of 26% at 404 nm was used. Fig. 2 presents a comparison of quantum efficiency characteristics for both the studied tubes. Comparable values in the whole range of wavelengths are seen.
2 source. The position of the 59.6 keV peak was used afterwards to calibrate the energy of the single photoelectron. Fig. 3 presents the energy spectrum measured at 2 μs shaping time constant in the spectroscopy amplifier. A very high energy resolution of 2.3 keV, reflecting an excellent performance of the Si-diode and preamplifier is observed.
Fig. 2. Quantum efficiency characteristics of the tested HPD and XP2020Q.
For the study of the HPD in the scintillation detection NaI(Tl), two samples of CsI(Tl) and LSO crystals of different dimensions were chosen. The crystals were wrapped with several layers of white Teflon tape, except for the NaI(Tl) crystal, which had been assembled by the manufacturer. In all the measurements the crystals were optically coupled to the HPD or PMT using DC 200 silicone grease, whose viscosity is 100 000 cSt. An overview of all the crystals used is given in Table I. TABLE I TESTED SCINTILLATORS
Crystal
Size [mm]
Surface finish
Manufacturer
Peak emission [nm]
Light output [ph/MeV]
CsI(Tl)
5x5x5
ground
Scionix
560
61000
CsI(Tl)
Ø9x9
ground
Bicron
560
61000
NaI(Tl)
Ø10x10
ground
Amcrys-H
420
40000
LSO
14x5x4a)
polished
Russia
420
27000
a)
Fig. 3 Energy spectrum of 59.6 keV γ-rays from a directly in the Si-diode.
241
Am source recorded
Fig. 4 presents the response of the HPD to light pulses of about 3 photons of mean intensity from a laser light pulser. The measurement was done at about 15 kV high voltage at the photocathode and energy of a single photoelectron of about 14 keV was estimated, in relation to the 59.6 keV γ-peak.
coupled to the HPD or XP2020Q by 14 x 5 mm2 face.
In the experiments the signal from the HPD preamplifier was fed to a spectroscopy amplifier (Tennelec TC244). The PC-based multichannel analyzer (Tukan) was used to record energy spectra. In the measurements of single photons the original configuration of the preamplifier was used. In the measurements with the scintillators the preamplifier was modified to reduce its gain by a factor of 3. III. RESULTS A. Single photoelectron study The tests of the HPD was started by an inspection of the response of the Si-diode to 59.6 keV γ-rays from a 241Am
Fig. 4. Single photoelectron response of HPD under illumination by about 3 photons light pulses in average.
Note a very high pulse height resolution of the peaks, similar to that reported earlier [2]. No doubt that the excess noise factor in the measurements at multi photon signals is close to 1.
3 B. HPD and XP2020Q in Scintillation Detection Fig. 5 presents the energy spectrum of 662 keV γ-rays from a 137Cs source detected in the 10 mm in diameter and 10 mm high NaI(Tl) crystal, in a comparison to the single photoelectron spectrum measured with the crystal coupled to the HPD. Note that the single photoelectron peak is very well defined, in spite of a large afterglow of NaI(Tl).
TABLE II COMPARISON OF NUMBER OF PHOTOELECTRONS AND ENERGY RESOLUTIONa) MEASURED WITH HPD AND XP2020Q
HPD Crystal
A poor energy resolution of the 662 keV peak of 9.2% is seen. It is much worse compare to the measurements with the same crystal coupled to the XP2020Q [7] and the Large Area Avalanche Photodiodes [8]. Moreover, the photoelectron number obtained with the HPD was about 40% lower than that observed with the XP2020Q, in spite of comparable quantum efficiency characteristics. Table II summarize results of the measurements of the photoelectron numbers and the energy resolution carried out with the tested scintillators. To increase the accuracy all the measurements were performed three times and an average value used for the analysis. The results show a systematically reduced photoelectron numbers measured with the HPD for all the tested crystals, in relation to those measured with the XP2020Q. In the last column the ratio of the photoelectron numbers is collected. For the 5 mm crystal the ratio of 0.72 is observed, while for the 14 mm long LSO crystal it is reduced to about 0.4. Moreover, the energy resolution is affected more than that arising from a larger statistical spread of a reduced photoelectron number in the HPD. Both the effects are better presented in Figs 6-8. Fig. 6 shows a linear reduction of the photoelectron number in relation to that measured with the XP2020Q, versus the size of the crystal (diameter or length). The last points at 1 mm and 18 mm were adopted from refs [5] and [2], respectively. In both cases the quoted numbers of photoelectrons for NaI(Tl) were normalized to the value of 9000 phe/MeV, a typical quantity for the XP2020Q with NaI(Tl) crystals.
XP2020Q
N [phe/ MeV]
ΔE/E [%]
N [phe/ MeV]
ΔE/E [%]
NHPD/ NXP
CsI(Tl)b)
5x5x5
2300 ±70
8.1 ±0.3
3200 ±100
7.0 ±0.2
0.72 ±0.03
CsI(Tl)b)
Ø9x9
2150 ±70
8.6± 0.3
3600 ±100
6.7 ±0.2
0.59 ±0.03
NaI(Tl)
Ø10 x10
5000 ±150
9.2± 0.3
8900 ±200
6.7 ±0.2
0.56 ±0.03
LSO
4x5x14
2200 ±70
12.1 ±0.4
5700 ±200
8.4 ±0.3
0.39 ±0.02
a) b)
Fig. 5. Energy spectrum of 662 keV γ-rays measured with NaI(Tl) coupled to the HPD in comparison to the single photoelectron peak.
Size [mm]
for 662 keV γ-rays from a 137Cs source at 3 µs shaping time constant
In ref. [5] a 15 mm diameter and 1 mm thick NaI(Tl) was illuminated by a beam of 122 keV γ-rays from a 57Co source, collimated by means of the 1 mm diameter and 38 mm long lead collimator. No doubt that the light was produced in about 1 mm diameter region in the center of the photocathode. Thus the adopted point was placed at this position. Both the points fit well to the measured dependency, independently of the window type, i.e. quartz or YAP. The curve extrapolated to the infinite small size of the crystal cross the y-axis at about 0.9, possibly corresponding to the loss of photoelectrons because of back scattering from the Sidiode.
Fig. 6. Ratio of the photoelectron numbers measured with the HPD and XP2020Q versus size of the crystal (diameter or length).
The results presented in Fig. 6 seem to indicate a poor photoelectron collection in the HPD. High-energy photoelectrons of several keV are difficult to be focalized at the small 2 mm diameter Si-diode [9], even more difficult
4 from external parts of the photocathode. This conclusion is supported by the tests of the focalization process of the HPD by varying the focalization voltage. It shows no influence on the response of the HPD to the light from the NaI(Tl) crystal. In the case of switching off the focalization voltage and leaving it floating or grounded by the output of HV supply, no influence on the energy spectra from the scintillators is observed. The position of the full energy peak, its energy resolution and number of photoelectrons were the same. Moreover, the test of photoelectron number at lower voltage of 8 kV, carried out with the NaI(Tl) crystal showed the same number of 5000 phe/MeV. In contrary to this hypothesis a poor light collection on photocathode in the HPD window was postulated in ref. [2], based on the Monte Carlo simulation. The HPD is equipped with a rather thick plano-concave entrance window, which shape is far from optimal for use for scintillation detectors [2]. According to the simulation about 40% of the light is lost in the HPD window. In conclusion, the use of a window made of BGO scintillator having a much larger refractive index was proposed in [2], which should increase the light collection on the photocathode by about 30%. This principle was used in ref. [5], replacing the quartz window in the HPD by that made of a YAP crystal, with the refractive index of 1.95. However, the reported large improvement of the collected light, reflected in the measured number of photoelectrons and the good energy resolution for 122 keV γ-rays from a 57Co seems to be rather an effect of collimation, see Fig. 6. Thus it does not justify the hypothesis on the light transport in the HPD window. C. Analysis of energy resolution measured with the HPD light readout The energy resolution ∆E/E of the full energy peak measured with a scintillator coupled to a photomultiplier can be written as [7]:
(∆E/E)2 = (δsc)2 + (δp)2 + (δst)2
(1)
where δsc is the intrinsic resolution of the crystal, δp is the transfer resolution and δst is the PMT contribution to the resolution. The statistical uncertainty of the signal from the PMT is described, as:
δst = 2.35 x 1/N1/2 x (1 + ε)1/2
(2)
where N is the number of photoelectrons and ε is the variance of the electron multiplier gain, typically 0.1-0.2 for modern PMTs [7]. In the case of HPD the value ε approaches zero and the statistical uncertainty is described only by the variance of the number of photoelectrons. In the case of the XP2020Q it was determined as 0.1 in [10]. The transfer component δp is described by the variance associated with the probability that a photon from the scintillator results in the arrival of photoelectron at the first dynode and then is fully multiplied by the PMT. The transfer component depends on the quality of the optical coupling of the crystal and PMT, homogeneity of the quantum efficiency
of the photocathode and efficiency of photoelectron collection at the first dynode. In modern scintillation detectors the transfer component is negligible compared to the other components of the energy resolution [7]. The intrinsic resolution of a crystal is associated with the non-proportional response of the scintillator, refs. [7], [10], [11], and many effects such as inhomogeneities in the scintillator causing local variations of the light output and non-uniform reflectivity of the crystal covering. This quantity is independent of the photodetector used [8]. The results of the analysis of the energy resolution measured with the HPD light readout are summarized in Table III. To evaluate the statistical contribution of the photoelectron number, according to eq. (2), a ε equal to zero was assumed. In the column six the intrinsic energy resolutions of the tested crystals are listed, following refs. [8,12]. Then, in the last column the transfer resolution of the scintillator-HPD system, calculated according to eq. (1) is collected. Note the rather large values varying with the size of the scintillators. TABLE III ANALYSIS OF ENERGY RESOLUTION MEASURED WITH HPD FOR 662 KEV γ-RAYS
Crystal
Size [mm]
N [phe /MeV]
ΔE/E [%]
ΔN/N [%]
Intrins. res. [%]
Transfer res. [%]
CsI(Tl)
5x5x5
2300 ±70
8.1±0.3
6.0 ±0.3
4.4 ±0.4a)
3.2±0.6
CsI(Tl)
Ø9x9
2150 ±70
8.6±0.3
6.2 ±0.3
4.4 ±0.4a)
4.0±0.6
NaI(Tl)
Ø10x10
5000 ±150
9.2±0.3
4.0 ±0.2
5.8 ±0.3a)
5.9±0.5
LSO
4x5x14
2200 ±70
12.1 ±0.4
6.2 ±0.3
7.4 ±0.5b)
7.3±0.7
a) b)
following [8], following [12],
This is better presented in Fig. 7, where the evaluated transfer resolution is plotted versus the size of the crystal. A linear dependency of the transfer resolution on the size of crystal is observed, crossing approximately zero for an infinity small crystal. It shows that the poor photoelectron or light collection in the HPD, discussed in sect. III A, reduces not only the number of photoelectrons, but also introduce a distortion of the energy resolution. The transfer resolution should be a constant component, independent of the γ-ray energy and its energy resolution. It is presented in Fig. 8, where the measured energy and the transfer resolutions are plotted versus energy, as measured with the NaI(Tl) crystal. Note the constant value of about 6%. It is worth to mention that in the modern photomultipliers the transfer resolution has a negligible contribution compared to the other components of energy resolution. This is supported by the recently estimated extremely low scintillation resolution of YAP crystal, of 1.3±0.5% [13], which combines
5 both the intrinsic and the transfer resolutions. This comes from a measurement performed for a 20 mm long YAP crystal coupled to a XP2020Q PMT.
Fig. 7. Transfer resolution of scintillator-HPD versus size of the crystal (diameter or length).
beam of the 122 keV γ-rays to the 1 mm diameter region in the center of the crystal. Moreover, such an effect has never been observed in standard PMTs, even for small diameter PMTs with similar curvature of the photocathode. The light from the small crystal of a 5 mm diameter should be collected well and weakly affected by the total internal reflections in the HPD window, because the evaporated photocathode at the window has an optical contact blocking this process. The hypothesis of the reduced photoelectron collection efficiency seems to be supported by two facts. The first one is a linear dependence on the size of the crystal. According to [9] this effect is known in image intensifier devices using a similar focalization system. The second is the lack of influence of the focalization voltage in the HPD, which seems to indicate that this electrode is working mainly as a diaphragm. The voltage at this electrode only slightly improves the spectrum of single photoelectrons. Probably, in fact, both the discussed effects may act on the measured number of photoelectrons and energy resolution, however, mainly for crystals approaching diameter of the photocathode. IV. CONCLUSIONS
Fig. 8. Measured energy resolution and transfer resolution of NaI(Tl) crystal coupled to the HPD versus energy of γ-rays.
However, the main question which remains is what is the origin of the so dramatically reduced number of photoelectrons at larger diameter of the crystals? The hypothesis of the light loss in the window of the HPD is strongly supported by the Monte Carlo simulations [2]. In fact, the window of the HPD is more spherical than that used in conventional photomultipliers, with a radius of 15.15 m. Moreover, the diameter of the window, of 26.5 mm, is larger than that of the photocathode itself, of 18 mm. It makes that a part of the light can be lost, traveling in the window to regions outside of the photocathode. However, the observed improvement of the light collection in the experiment with the HPD equipped with a YAP window is less clear. As it is presented in Fig. 5, it could be an effect of the collimated
The performed study confirmed the unequally good capabilities of the PP0275C HPD for a precise detection of single photons. A high pulse height resolution of peaks up to several photoelectrons was shown. The study of performance of the HPD in the scintillation detection showed serious limitations of this photodetector. A reduced number of photoelectrons, linearly decreasing with the diameter of scintillator, down to about 40% in comparison to the XP2020Q PMT was shown. The same effect seems to be responsible for a large distortion of the energy resolution measured with the different scintillators, accounted to the large transfer resolution in the scintillator-HPD system. Two processes responsible for the reduction of the number of photoelectrons are discussed. The first one is a limitation in the light transport from a crystal to the photocathode in the HPD window. The other one is an effect of reduced photoelectron collection on the small Si-photodiode. It seems to be supported by the lack of influence of the focalization voltage in the HPD on the measured energy spectra. The presented results showed that the studied HPD is optimized for single photon detection. Its application to multi photon detection, particularly from scintillators, is limited because of a much lower photoelectron number and seriously distorted energy resolution. It is hardly possible to use this device for precise studies of the energy resolution in scintillation detectors. This conclusion is supported by similar results of tests of this type of HPD carried out by Schotanus [14]. In contrast, the recent studies of small, 10 mm x 10 mm, different scintillators with a use of a hybrid photodetector, reported by Mares et al [15-17], showed a good agreement with the earlier measurements performed by means of an XP2020Q PMT. However, in these measurements a new HPD
6 developed by Delft Electronics Product B.V. was used, with a 40 mm active diameter of the photocathode on a flat input quartz window. This new design of HPD, in fact, confirmed the conclusion of the present work. V. REFERENCES [1] [2] [3]
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