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Quantum efficiency of cesium iodide photocathodes in the 120–220 nm spectral range traceable to a primary detector standard
Nuclear Instruments and Methods in Physics Research A 438 (1999) 94}103
Quantum e$ciency of cesium iodide photocathodes in the 120}220 nm spectral range traceable to a primary detector standard H. Rabus!,*, U. Kroth!, M. Richter!, G. Ulm!, J. Friese", R. GernhaK user", A. KastenmuK ller", P. Maier-Komor", K. Zeitelhack" !Physikalisch-Technische Bundesanstalt, Abbestra}e 2-12, 10587 Berlin, Germany "Physik-Department E12, Technische Universita( t Mu( nchen, 85747 Garching, Germany
Abstract Di!erently prepared CsI samples have been investigated in the 120}220 nm spectral range for their quantum e$ciency, spatial uniformity and the e!ect of radiation aging. The experiments were performed at the PTB radiometry laboratory at the Berlin synchrotron radiation facility BESSY. A calibrated GaAsP Schottky photodiode was used as transfer detector standard to establish traceability to the primary detector standard, because this type of photodiode } unlike silicon p-on-n photodiodes } proved to be of su$ciently stable response when exposed to vacuum ultraviolet radiation. The paper reviews the experimental procedures that were employed to characterize and calibrate the GaAsP photodiode and reports the results that were obtained on the investigated CsI photocathodes. ( 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Ring-Imaging Cherenkov (RICH) detectors are used as trigger detectors in high energy physics experiments in order to achieve a suppression of unwanted events by 5}6 orders of magnitude. A proper event is identi"ed by a ring image on the photon detector produced by Cherenkov radiation emitted along the particle trajectory by electrons and positrons which are created in the speci"c reaction channel under investigation. The RICH detector for the HADES project [1], presently in
* Corresponding author. Tel.: #49-30-82004 231. E-mail address: [email protected] (H. Rabus)
preparation at Gesellschaft fuK r Schwerionenforschung (GSI) in Darmstadt, is based on a multi-wire chamber with a CaF window and a CsI-coated 2 photocathode, which is developed at the Technische UniversitaK t MuK nchen (TUM). The number of Cherenkov photons that have to be detected in order to e$ciently identify a ring image is about 13. On the other hand, only a few tens of Cherenkov photons are emitted in the spectral range covered by the HADES-RICH (120}220 nm). Thus, the quantum e$ciency of the photocathode material (number of photoelectrons per incident photon) is of crucial importance for the success of all experiments triggered by the RICH detector. Values for the quantum e$ciency of CsI as quoted by di!erent groups in the literature are at
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 6 5 - 8
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variance [2}5]. Commonly attributed to di!erent preparation methods, these discrepancies could also result from the questionable calibration of the reference detectors employed. Therefore, TUM and the Physikalisch-Technische Bundesanstalt (PTB), the German national metrology institute, formed a collaboration to determine the quantum e$ciency of CsI photocathodes with particular emphasis on a well-established traceability to a primary detector standard. This paper reports on the experiments that were performed using the facilities of the PTB radiometry laboratory at the Berlin synchrotron radiation source BESSY. A semiconductor photodiode of su$ciently stable response when exposed to vacuum ultraviolet radiation was calibrated for use as transfer detector standard. Di!erently prepared CsI samples were investigated for their quantum e$ciency, their spatial uniformity and the e!ect of radiation aging. The results of these investigations and the availability of the calibrated transfer detector standard for the in situ characterization of the CsI samples allowed the TUM to further optimize the CsI preparation procedure. The performance of the improved CsI photocathodes and the optimized preparation procedure are described in two accompanying papers in these proceedings [6,7].
2. Calibration chain The primary standard employed is the synchrotron-radiation cryogenic electrical substitution radiometer (SYRES), which is the primary detector standard for the ultraviolet (UV), vacuum ultraviolet (VUV) and soft X-ray spectral ranges [8]. SYRES is a thermal detector with a cavity absorber that is operated close to liquid-helium temperature. The absorber temperature is kept constant by means of an electrical heater that is controlled by a feedback loop from the thermometer that is attached to the absorber. The incident radiant power is given by the di!erence in electrical power in the absence and in the presence of radiation. Radiant power in the 1}10 lW range, corresponding to a photon #ux of about 1012}1013 s~1 in the wavelength range of this study, can be measured
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with a relative uncertainty of about 0.2% [8]. The spectral responsivity or, equivalently, the (external) quantum e$ciency of such radiation detectors that can be operated at the quoted radiant power levels, like for instance semiconductor photodiodes, can be determined with a relative uncertainty well below 1% [8,9]. A determination of the quantum e$ciency of CsI samples by direct comparison to the primary detector standard is unattainable on account of the known decrease of CsI quantum e$ciency [10] already under exposure to photon #uxes several orders of magnitude lower than required for operation of SYRES. Therefore, the use of a transfer detector standard was mandatory in order to bridge the di!erent photon #ux levels. A commercially available GaAsP Schottky-type photodiode with a gold front contact (Hamamatsu G2119) was used for this purpose. This type of photodiode was chosen because of its large dynamic range of linearity [11] and, most important, because of the known instability of di!usion-type silicon p-on-n (Si pn) photodiodes under irradiation at wavelengths below 250 nm [12,13]. Schottky-type photodiodes are generally expected to be less susceptible to radiation damage under prolonged UV or VUV irradiation [14], and in the meantime, a new type of silicon Schottky photodiode has been developed which shows practically no change in spectral responsivity under prolonged exposure to UV or VUV radiation [15]. At the time when the experiments of this study were conducted, GaAsP}Au Schottky photodiodes were the best choice for a nearly stable VUV detector. This is exempli"ed by Fig. 1 which shows the quantum e$ciency, normalized to its initial value, of a GaAsP}Au Schottky diode and an Si pn photodiode (Hamamatsu S1337), respectively, as function of photon exposure at two VUV wavelengths. For both wavelengths, the quantum e$ciency of the Si pn photodiode is seen to change with a faster rate than the quantum e$ciency of the GaAsP diode. Typical photon exposures involved in a one year use as transfer detector standard may be in the order of 1016 cm~2. To this extent, the change in quantum e$ciency is negligible for both photodiodes when irradiated with 170 nm radiation. This
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3. Experimental
Fig. 1. Change of the relative quantum e$ciency at the wavelengths 170 and 120 nm as function of photon exposure for an Si pn di!usion-type photodiode (Hamamatsu S1337) and a GaAsP}Au Schottky-type photodiode (Hamamatsu 2119). The latter type of photodiode has been used as transfer detector standard in the experiments reported in this paper.
is also true of the GaAsP photodiode when exposed to 120 nm radiation, whereas a signi"cant reduction in quantum e$ciency is seen for the Si pn photodiode at this wavelength. It should be noted that if radiation damage is induced by prolonged irradiation at a speci"c wavelength, the quantum e$ciency at all wavelengths is a!ected. Therefore the use of the Si pn photodiode as a reference detector in the VUV is not advisable. The better stability under irradiation of the GaAsP compensates for their known large spatial nonuniformity. Over the entire photosensitive surface, variations of the local quantum e$ciency of up to 20% are encountered, but in the central regions, the quantum e$ciency typically varies by less than 5% [12]. The spatial nonuniformity introduces a signi"cant additional contribution to the uncertainty of the photodiode quantum e$ciency of about 2% (Table 1).
The experiments reported here were conducted at one of the monochromator beamlines in the radiometry laboratory of PTB [16] at the Berlin synchrotron radiation facility BESSY. This particular beamline, equipped with a 1m-153 McPherson-type normal-incidence monochromator, is part of the PTB detector calibration facility for the UV and VUV spectral ranges. Provided with a set of di!erently coated mirrors and re#ection gratings, it delivers small-bandwidth synchrotron radiation of high spectral purity in the wavelength range from 40 to 500 nm [8,17]. At the downstream end, the beamline is divided into three branches installed on top of a pivoting platform that permitted the di!erent experimental stations to be alternately placed in the beam path. The beam divergence at the location of the experiments amounts to 12 mrad, and the beam size at the focal point is 2.8 mm horizontally by 1.8 mm vertically; within this area more than 99% of the total radiant power is found. Several monitor detectors, operating in di!erent spectral ranges, are available to trace the monotonous decrease of the spectral photon #ux with time, which is due to the loss of electrons from the beam circulating in the BESSY storage ring. In normal operation, the stored beam current decreases from about 700 mA to about 300 mA in the time interval between subsequent injections with a 1/e lifetime of about 2}4 h. For the 120}220 nm spectral range of this investigation, normal-incidence optics with aluminum coating and a protective MgF layer was used. An 2 MgF cut-on "lter was placed in front of the mono2 chromator entrance slit to suppress radiation at wavelengths shorter than 110 nm, which would lead to a spectral contamination due to higher di!raction orders of the monochromator grating. Spectral impurities consisting of false light at longer wavelengths were studied using a set of cut-on "lters, which are available behind the monochromator exit slit, and a photodiode at the beamline focal plane as detector. The fraction of the total incident radiant power due to false light turned out to be below 0.2% for wavelengths greater or equal to 130 nm [17].
H. Rabus et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 94}103
Fig. 2. Experimental chamber for the CsI quantum e$ciency measurements.
For the calibration of the GaAsP reference detector, the SYRES radiometer was installed at one of the beamline branches. The photodiode was placed in a vacuum chamber that allowed the full synchrotron radiation beam to fall onto the diode surface and was attached to a second branch of the beamline. By pivoting the platform back and forth, the incident spectral radiant power was measured alternately with the primary detector standard and the photodiode. The widths of the monochromator entrance and exit slits were set to 3 and 2 mm, respectively, resulting in a wavelength bandwidth of about 3.3 nm. The ensuing photon #ux at the focal point is of the order 1013 s~1 for normal operation conditions of BESSY. In the experiments to determine the quantum e$ciency of CsI photocathodes, a special small-size vacuum chamber (Fig. 2) was installed at one of the beamline branches. This chamber was constructed such that it "ts in the glove box of the sample preparation laboratory of TUM. There, the samples could be installed in the chamber while working in a nitrogen atmosphere containing less than 1 ppm of oxygen and water, thus preventing contamination of the CsI surface, which is known to lead to a reduced quantum e$ciency [19]. After insertion of the samples, the chamber was evacuated and shipped to Berlin for the experiments. Inside the vacuum chamber, the samples are moun-
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ted on a turntable, actuated by a rotary feedthrough, so that the eight samples can be positioned alternatively behind the 2 mm diameter entrance aperture. The angle position of the turntable can be reproduced within $0.13, which equals a reproducibility of the sample position with respect to the entrance aperture of $0.05 mm. One of the positions on the turntable was occupied by the semiconductor photodiode used as transfer detector standard. A grid behind the apertures and about 7 mm in front of the samples was used to apply an extraction voltage. At the beamline, the entrance aperture was located at the beamline focal plane. In the horizontal direction, the aperture was over"lled by the incident synchrotron radiation beam. The beam was centered to the aperture within $0.1 mm in the horizontal and within $0.5 mm in the vertical direction. The photocurrents of the diode and the CsI photocathodes were measured with the same Keithley 617 picoamperemeter. The extraction grid was biased to 1500 V, as saturation of the photocurrent of the CsI samples was observed only at voltages exceeding 1200 V. In order to avoid signi"cant radiation damage, the incident photon #ux was limited to about 109 s~1 such that the extracted photocurrent would not exceed 100 pA. To this end a microchannelplate (MCP) mounted on a rotary and linear motion feedthrough was installed in front of the sample chamber, which permitted a convenient scaling of the photon #ux passing through the aperture by about three orders of magnitude. For con"rmation of the quantum e$ciency results and to rule out that the use of the MCP might lead to artifacts, additional measurements with the MCP moved out of the optical path were performed in special operation shifts, in which the storage ring was operated at a reduced electron beam current of below 80 lA. In both cases, the measurement sequence was such that after tuning the monochromator to the desired wavelength, the samples were positioned in the beam one after the other, and their photocurrent was recorded as function of time. For discrimination of transient behavior, at least 20 individual data points under irradiation were recorded, and for the purpose of dark current determination 10 additional readings were taken before opening and
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after closing the beam shutter, respectively. The "rst and the last measurement at each wavelength were done with the photodiode reference detector. After correction for the dark current, all photocurrents were normalized to the simultaneously recorded beam current in the storage ring in order to remove the time dependence of the incident photon #ux. Finally, the quantum e$ciency of the CsI photocathodes was calculated from the ratio of the normalized photocurrents obtained for the CsI samples and for the photodiode, linearly interpolated from the two measurements in the latter case, using the known quantum e$ciency of the reference detector.
4. Characterization of the GaAsP reference detector standard Prior to and after the experiments to determine the quantum e$ciency of the CsI samples, the GaAsP photodiode was calibrated. Fig. 3 shows the obtained (external) quantum e$ciency of the GaAsP Schottky diode as function of wavelength as determined by comparison to the primary detector standard SYRES. The photodiode was positioned such that the beam spot was centered to within $0.5 mm in the photosensitive surface. Data points marked by open diamonds represent the results obtained before the experiments of this study, and the "lled circles correspond to the GaAsP photodiode quantum e$ciency after the experiments were completed. The error bars in the plot represent the absolute standard uncertainty of the photodiode quantum e$ciency. The corresponding relative standard uncertainty amounts to 1.2% in the 120}170 nm spectral range and to 0.7% at wavelengths greater than 170 nm [12]. Obviously, the spectral shape of the photodiode spectral responsivity has changed signi"cantly. For wavelengths below 145 nm, the quantum e$ciency has decreased, while an increase is observed at longer wavelengths. The maximum relative reduction amounts to about 10% at wavelength 125 nm. For wavelengths greater than 160 nm, the relative increase is between 4% and 6%. The change in the quantum e$ciency of the GaAsP reference diode is due to radiation damage
Fig. 3. Quantum e$ciency of the GaAsP}Au Schottky photodiode as determined with the SYRES primary detector standard prior to (open diamonds) and after the experiments ("lled circles).
Fig. 4. Spatial variation of the GaAsP photodiode quantum e$ciency for three di!erent wavelengths along two perpendicular lines through the diode center. For the measurements, the diode was scanned behind a pinhole of 0.25 mm diameter. The peaks and dips that are seen in the vicinity of the center of the diode-sensitive surface, i.e. in the area that was exposed to radiation in the course of the experiments, were initially absent.
e!ects, as is demonstrated in Fig. 4, which shows the spatial variation of the quantum e$ciency for three di!erent VUV wavelengths. For this kind of experiment, a setup was used in which the diode can be scanned in the two directions perpendicular to the beam path, behind a pinhole of 0.25 mm diameter that con"nes the beam size. The "gure shows cuts through the map of the local quantum
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Table 1 Contributions to the relative uncertainty of the GaAsP photodiode quantum e$ciency at four wavelengths. The stated values are standard relative uncertainties (k"1) as evaluated according to Ref. [18]. Relative uncertainty *(QE)/QE at wavelength Source of uncertainty 140 nm Calibration Nonuniformity Radiation ageing Total (sum in quadrature)
160 nm
180 nm
1.2% 0.6% 2.4%
2.0% 0.9% 1.5% 2.5% 2.6%
200 nm
0.7% 1.2% 2.5%
e$ciency along the two lines through the center of the photosensitive surface. The regions in the vicinity of the center of the diode that were exposed to radiation in the course of the experiments are clearly identi"ed by a reduced quantum e$ciency in the case of wavelength 130 nm, and an increased quantum e$ciency for the other two wavelengths. These features were absent when the local quantum e$ciency of the GaAsP diode was initially investigated. The larger beam size in the calibration measurements as well as in the application as reference detector implies an averaging of the local quantum e$ciency. Therefore, the e!ects seen in Fig. 3 are smaller than the local variations seen in Fig. 4. Table 1 summarizes the resulting contributions to the relative uncertainty of the GaAsP Schottky photodiode quantum e$ciency. The "rst row refers to the determination of the quantum e$ciency in the center area of the diode with SYRES. The variation in calibration accuracy at di!erent wavelengths is mainly due to the di!erent stability of the di!erent photon #ux monitors employed in the wavelength ranges above and below 170 nm, respectively. The e!ect of the spatial nonuniformity of the diode is practically wavelength independent in the spectral range of this study and gives a relative uncertainty contribution of about 2%. The contribution from the change in quantum e$ciency varies from 0.6% to 1.5%. In total, the relative uncertainty of the GaAsP photodiode quantum e$ciency when employed as transfer detector stan-
Fig. 5. Initial quantum e$ciency of four di!erently prepared CsI photocathode samples (cf. Table 2).
dard is about 2.5% in the entire wavelength range of this investigation.
5. Quantum e7ciency of the CsI photocathodes Fig. 5 shows the quantum e$ciency of four of the investigated CsI photocathode samples. All samples were produced by vapor deposition in high vacuum, but di!er in their substrate material and preparation temperature (Table 2). Obviously the treatment of the substrate prior to deposition is of crucial importance [6]. The worst quantum e$ciency curve is obtained for sample 1 (diamonds in Fig. 5) for which the CsI was deposited onto a polished stainless steel substrate that was maintained at an elevated temperature of 1003C. The use
Table 2 Substrate material and its temperature during the preparation of the CsI photocathode samples investigated in this study. In all cases electron-beam heating was used for evaporation. Substrate Sample
Material
Temperature (3C)
1 2,5 4,6 7
Polished VA VA # RSG G10# RSG G10# RSG
100 100 60 25
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of bare G10-Ni/Au as substrate (sample 7, plus signs) gives about 15% improvement on average. The best quantum e$ciency is obtained when the substrate is coated with resin-stabilized graphite (RSG) prior to deposition of CsI. The total improvement as compared to the polished stainless steel substrate amounts to almost a factor of 2 on average. When the RSG coating is employed, the nature of the substrate is practically unimportant. The quantum e$ciency of sample 5 (open boxes), where stainless steel was used as the substrate, is only by less than 2% on average worse than the best quantum e$ciency curve obtained for sample 4 ("lled circles) that was deposited on G10-Ni/Au. This di!erence is slightly smaller than the di!erence in quantum e$ciency of sample 4 or 5 to the respective second sample that was produced by the same preparation procedure (Table 2). The quantum e$ciency curves of these two samples are omitted in Fig. 5 for lucidity. The error bars that are attached to the plot symbols in Fig. 5 indicate the uncertainty of the respective absolute quantum e$ciency value. In almost all of the spectral range of this study, the relative uncertainty of the quantum e$ciency remains below 10%, in the 130}180 nm wavelength range even below 5%. For a couple of wavelengths, the contributions to relative uncertainty of the quantum e$ciency are listed in Table 3. The "rst row refers to the uncertainty of the GaAsP photodiode quantum e$ciency as given in Table 1. This row also comprises the non-linearity of the diode, which is a negligible e!ect of order 0.01%, however. The uncertainty contribution originating from the measurement of the photocurrent is dominated by the statistics of the repeated current read-out at "xed wavelength which also a!ects the dark current correction. The relative calibration uncertainty of the picoamperemeter is about 0.1%. The row labeled `radiant #ux normalizationa comprises the statistics of the reference signal and the change in the normalized photodiode signal between the two diode measurements, which actually dominates this uncertainty contribution. The dominant source of uncertainty, at least at wavelengths longer than 170 nm, turns out to be the reproducibility of the quantum e$ciency deter-
Table 3 Contributions to the relative uncertainty of the CsI photocathodes quantum e$ciency at four wavelengths. The stated values are standard relative uncertainties (k"1) as evaluated according to Ref. [18]. Relative uncertainty *(QE)/QE at wavelength Source of uncertainty
140 nm 160 nm 180 nm 200 nm
Transfer detector standard Photocurrent measurement (calibration, statistics, dark current correction)
2.4% 0.4%
2.5% 0.4%
2.6% 0.4%
2.5% 0.6%
Radiant #ux normalization 0.7% Reproducibility of quantum 1.9% e$ciency
0.7% 2.3%
0.7% 4.5%
0.8% 8.8%
Total (sum in quadrature)
3.5%
5.4%
9.2%
3.2%
mination. The values shown in the table are derived from the standard deviation of at least four repetitions of the quantum e$ciency measurements for all samples. The comparatively large scatter of the data points for the same sample at the same wavelength is attributed to the spatial nonuniformity of the quantum e$ciency on the surfaces of the CsI photocathodes and of the GaAsP reference detector (Fig. 4). Due to the spatial nonuniformity, a di!erent irradiance pro"le or a di!erent location of the irradiated area on the sample surface results in a di!erent quantum e$ciency value. For initially highly uniform samples this e!ect can be still of negligible magnitude as is demonstrated by Fig. 6 which shows the results of repeated determinations of the quantum e$ciency at 150 nm wavelength for sample 5. The "rst four values show a relative standard deviation of only 0.23%. When the last three data points were taken, the accumulated photon exposure was already high enough to induce detectable radiation aging. As only the surface area exposed to radiation is a!ected, the radiation damage induces a spatial nonuniformity. Therefore, the monotonous behavior in this case is not typical and rather correlated to the fact that realignments of the sample chamber with respect to the beam were performed at this particular wavelength 150 nm.
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Fig. 6. Results of a repeated determination of the quantum e$ciency of sample 5 at wavelength 150 nm. The solid line indicates the instrumentally weighted average over the data points, and the dashed lines indicate the standard deviation which amounts to 0.23% of the average. The decrease in quantum e$ciency for the last three measurements is due to radiation aging.
6. Radiation aging and spatial uniformity of the CsI photocathodes After the experiments on the quantum e$ciency, two of the investigated CsI samples were subjected to prolonged exposure to high photon #ux levels in order to study the e!ect of severe radiation aging. These measurements were performed at wavelength 150 nm. The MCP was removed from the beam path so that a photon #ux density of order 5]1014 cm~2 s~1 was achieved and the initial photocurrent was about 300 nA. The decreasing quantum e$ciency of the two samples is plotted in the upper part of Fig. 7 as function of the photon exposure. For comparison, the total photon exposure of a photocathode used in the RICH detector of the HADES experiment is estimated to be of the order 1017 cm~2 per year of operation. The radiation aging of the quantum e$ciency is commonly attributed to damage induced by the #ow of electrical charge in the CsI bulk [20]. This interpretation is supported by the lower part of Fig. 7 which shows the quantum e$ciency as a function of the extracted charge per unit area. An almost linear dependence is observed up to about 3 mC/cm2 of extracted charge per unit area. Fig. 7 furthermore demonstrates that the aging behavior
Fig. 7. Decrease of quantum e$ciency at wavelength 150 nm as function of accumulated photon exposure (top) and versus the extracted charged (bottom) for the CsI photocathode samples 5 and 7.
obviously depends on the sample preparation. The initially higher quantum e$ciency of sample 5 (solid line) as compared to sample 7 (dashed) decreases much faster so that already after an exposure that is equivalent to about 2 months of operation in the HADES RICH the quantum e$ciency of sample 5 is below that of sample 7. For a radiant exposure expected for one year of implementation, the quantum e$ciency of both samples has decreased by more than a factor of 2. For sample 7, the radiation aging experiment was divided into several exposure steps. After each step, a quick and less accurate determination of the quantum e$ciency curve in the entire spectral range of this study was performed. In this case, the wavelength of the monochromator was scanned while recording the photocurrent of the CsI photocathode and the GaAsP photodiode, respectively, in two consecutive measurements. The resulting quantum e$ciency curves have a large uncertainty
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Fig. 8. Quantum e$ciency of sample 7 as function of wavelength after several exposure steps.
as far as absolute values are concerned, the spectral shape, however, is reliable. As is obvious from Fig. 8, the aging induced by prolonged exposure to radiation at 150 nm wavelength leads to a decrease in quantum e$ciency at all wavelengths. Before and after the radiation aging experiments, all samples were probed for spatial nonuniformity of the quantum e$ciency. For this kind of examination, a pinhole of 50 lm diameter was placed about 10 cm in front of the entrance aperture. For the two wavelengths 170 nm and 190 nm, the resulting con"ned beam was scanned across each of the seven samples and the diode, respectively, by rotating the turntable in 13 steps, which corresponds to about 0.55 mm steps. Like in the quantum e$ciency measurements, several readings of the photocurrent were recorded at each position. The spatial variation of the quantum e$ciency of sample 5 at wavelength 190 nm is shown in Fig. 9. Initially sample 5 had a rather uniform local quantum e$ciency that varied by less than 1% over the cathode surface except in the vicinity of the dip marking the central area that was exposed to radiation in the course of the quantum e$ciency measurements. After extraction of almost 4.5 mC/cm2 of charge per unit area from the sample, the quantum e$ciency was drastically reduced in the area that was subjected to prolonged exposure, almost vanishing in the center, while no change was observed in the unexposed areas. The irradiated area was clearly perceivable in a visual inspection of the sample after the end of the experi-
Fig. 9. Spatial variation of the quantum e$ciency at wavelength 190 nm for the CsI photocathode sample 5 prior to and after prolonged exposure to VUV radiation. For this experiment, the sample was scanned behind a pinhole of 50 lm diameter de"ning the synchrotron radiation beam size.
ments which indicates that a modi"cation of the bulk material occurred during the exposure to intense VUV radiation.
7. Conclusions As a conclusion, the quantum e$ciency of CsI photocathodes have been determined for the "rst time with a measurement scheme which guarantees traceability to a primary radiometric standard. The relative uncertainties achieved are in the few percent range, which constitutes a signi"cant progress over previous work. This improved accuracy could only be achieved by using a GaAsP Schottky type photodiode, which is su$ciently stable under prolonged exposure to VUV radiation, as transfer detector standard.
References [1] HADES collaboration, A proposal for Di-Lepton spectroscopy at GSI, Darmstadt, Germany, 1993. [2] J. SeH guinot, G. Charpak, Y. Giomataris, V. Peskov, J. Tischhauser, T. Ypsilantis, Nucl. Instr. and Meth. A 297 (1990) 133. [3] CERN RD26 collaboration Nucl. Instr. and Meth. A 360 (1995) 411.
H. Rabus et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 94}103 [4] P. Maier-Komor, B.B. Bauer, J. Friese, R. GernhaK user, P. Kienle, H.J. KoK rner, G. Montermann, K. Zeitelhack, Nucl. Instr. and Meth. A 362 (1995) 183. [5] A. Breskin, Nucl. Instr. and Meth. A 371 (1996) 116. [6] J. Friese, R. GernhaK user, J. Homolka, A. KastenmuK ller, P. Maier-Komor, M. Peter, K. Zeitelhack, P. Kienle, H.J. KoK rner, Nucl. Instr. and Meth. A 438 (1999) 86. [7] R. GernhaK user, J. Friese, J. Homolka, A. KastenmuK ller, P. Kienle, H.J. KoK rner, P. Maier-Komor, M. Peter, K. Zeitelhack, Nucl. Instr. and Meth. A 438 (1999) 104. [8] H. Rabus, V. Persch, G. Ulm, Appl. Opt 36 (1997) 5421. [9] H. Rabus, F. Scholze, R. Thornagel, G. Ulm, Nucl. Instr. and Meth. A 377 (1996) 209. [10] C. Lu, Z. Cheng, K.T. McDonald, D.R. Marlow, E.J. Prebys, R.L. Wixted, Nucl. Instr. and Meth. A 366 (1995) 60. [11] A.D. Wilson, H. Lyall, Appl. Opt. 26 (1986) 4530, 4540. [12] A. Lau-FraK mbs, PhD Thesis, TU Berlin, 1995, unpublished.
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[13] R. Goebel, R. KoK hler, R. Pello, Metrologia 32 (1995/1996) 515. [14] M. Razeghi, A. Rogalski, J. Appl. Phys 79 (1996) 7433. [15] K. Solt, H. Melchior, U. Kroth, P. Kuschnerus, V. Persch, H. Rabus, M. Richter, G. Ulm, Appl. Phys. Lett 69 (1996) 3662. [16] G. Ulm, B. Wende, Rev. Sci. Instrum 66 (1995) 2244. [17] A. Lau-FraK mbs, U. Kroth, H. Rabus, E. Tegeler, G. Ulm, Rev. Sci. Instrum 66 (1995) 2324. [18] International Organization for Standardization (ISO), Guide to the expression of uncertainty in measurement, ISBN 92-67-10188-9, Geneva, Switzerland, 1995. [19] Ryoji Enomoto, Takayuki Sumiyoshi, Yoshio Fujita, Nucl. Instr. Meth. A 343 (1994) 117. [20] D.F. Anderson, S. Kwan, V. Peskov, B. Hoeneisen, Nucl. Instr. and Meth. A 323 (1992) 626.
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Quantum e$ciency of cesium iodide photocathodes in the 120}220 nm spectral range traceable to a primary detector standard H. Rabus!,*, U. Kroth!, M. Richter!, G. Ulm!, J. Friese", R. GernhaK user", A. KastenmuK ller", P. Maier-Komor", K. Zeitelhack" !Physikalisch-Technische Bundesanstalt, Abbestra}e 2-12, 10587 Berlin, Germany "Physik-Department E12, Technische Universita( t Mu( nchen, 85747 Garching, Germany
Abstract Di!erently prepared CsI samples have been investigated in the 120}220 nm spectral range for their quantum e$ciency, spatial uniformity and the e!ect of radiation aging. The experiments were performed at the PTB radiometry laboratory at the Berlin synchrotron radiation facility BESSY. A calibrated GaAsP Schottky photodiode was used as transfer detector standard to establish traceability to the primary detector standard, because this type of photodiode } unlike silicon p-on-n photodiodes } proved to be of su$ciently stable response when exposed to vacuum ultraviolet radiation. The paper reviews the experimental procedures that were employed to characterize and calibrate the GaAsP photodiode and reports the results that were obtained on the investigated CsI photocathodes. ( 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Ring-Imaging Cherenkov (RICH) detectors are used as trigger detectors in high energy physics experiments in order to achieve a suppression of unwanted events by 5}6 orders of magnitude. A proper event is identi"ed by a ring image on the photon detector produced by Cherenkov radiation emitted along the particle trajectory by electrons and positrons which are created in the speci"c reaction channel under investigation. The RICH detector for the HADES project [1], presently in
* Corresponding author. Tel.: #49-30-82004 231. E-mail address: [email protected] (H. Rabus)
preparation at Gesellschaft fuK r Schwerionenforschung (GSI) in Darmstadt, is based on a multi-wire chamber with a CaF window and a CsI-coated 2 photocathode, which is developed at the Technische UniversitaK t MuK nchen (TUM). The number of Cherenkov photons that have to be detected in order to e$ciently identify a ring image is about 13. On the other hand, only a few tens of Cherenkov photons are emitted in the spectral range covered by the HADES-RICH (120}220 nm). Thus, the quantum e$ciency of the photocathode material (number of photoelectrons per incident photon) is of crucial importance for the success of all experiments triggered by the RICH detector. Values for the quantum e$ciency of CsI as quoted by di!erent groups in the literature are at
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 6 5 - 8
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variance [2}5]. Commonly attributed to di!erent preparation methods, these discrepancies could also result from the questionable calibration of the reference detectors employed. Therefore, TUM and the Physikalisch-Technische Bundesanstalt (PTB), the German national metrology institute, formed a collaboration to determine the quantum e$ciency of CsI photocathodes with particular emphasis on a well-established traceability to a primary detector standard. This paper reports on the experiments that were performed using the facilities of the PTB radiometry laboratory at the Berlin synchrotron radiation source BESSY. A semiconductor photodiode of su$ciently stable response when exposed to vacuum ultraviolet radiation was calibrated for use as transfer detector standard. Di!erently prepared CsI samples were investigated for their quantum e$ciency, their spatial uniformity and the e!ect of radiation aging. The results of these investigations and the availability of the calibrated transfer detector standard for the in situ characterization of the CsI samples allowed the TUM to further optimize the CsI preparation procedure. The performance of the improved CsI photocathodes and the optimized preparation procedure are described in two accompanying papers in these proceedings [6,7].
2. Calibration chain The primary standard employed is the synchrotron-radiation cryogenic electrical substitution radiometer (SYRES), which is the primary detector standard for the ultraviolet (UV), vacuum ultraviolet (VUV) and soft X-ray spectral ranges [8]. SYRES is a thermal detector with a cavity absorber that is operated close to liquid-helium temperature. The absorber temperature is kept constant by means of an electrical heater that is controlled by a feedback loop from the thermometer that is attached to the absorber. The incident radiant power is given by the di!erence in electrical power in the absence and in the presence of radiation. Radiant power in the 1}10 lW range, corresponding to a photon #ux of about 1012}1013 s~1 in the wavelength range of this study, can be measured
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with a relative uncertainty of about 0.2% [8]. The spectral responsivity or, equivalently, the (external) quantum e$ciency of such radiation detectors that can be operated at the quoted radiant power levels, like for instance semiconductor photodiodes, can be determined with a relative uncertainty well below 1% [8,9]. A determination of the quantum e$ciency of CsI samples by direct comparison to the primary detector standard is unattainable on account of the known decrease of CsI quantum e$ciency [10] already under exposure to photon #uxes several orders of magnitude lower than required for operation of SYRES. Therefore, the use of a transfer detector standard was mandatory in order to bridge the di!erent photon #ux levels. A commercially available GaAsP Schottky-type photodiode with a gold front contact (Hamamatsu G2119) was used for this purpose. This type of photodiode was chosen because of its large dynamic range of linearity [11] and, most important, because of the known instability of di!usion-type silicon p-on-n (Si pn) photodiodes under irradiation at wavelengths below 250 nm [12,13]. Schottky-type photodiodes are generally expected to be less susceptible to radiation damage under prolonged UV or VUV irradiation [14], and in the meantime, a new type of silicon Schottky photodiode has been developed which shows practically no change in spectral responsivity under prolonged exposure to UV or VUV radiation [15]. At the time when the experiments of this study were conducted, GaAsP}Au Schottky photodiodes were the best choice for a nearly stable VUV detector. This is exempli"ed by Fig. 1 which shows the quantum e$ciency, normalized to its initial value, of a GaAsP}Au Schottky diode and an Si pn photodiode (Hamamatsu S1337), respectively, as function of photon exposure at two VUV wavelengths. For both wavelengths, the quantum e$ciency of the Si pn photodiode is seen to change with a faster rate than the quantum e$ciency of the GaAsP diode. Typical photon exposures involved in a one year use as transfer detector standard may be in the order of 1016 cm~2. To this extent, the change in quantum e$ciency is negligible for both photodiodes when irradiated with 170 nm radiation. This
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3. Experimental
Fig. 1. Change of the relative quantum e$ciency at the wavelengths 170 and 120 nm as function of photon exposure for an Si pn di!usion-type photodiode (Hamamatsu S1337) and a GaAsP}Au Schottky-type photodiode (Hamamatsu 2119). The latter type of photodiode has been used as transfer detector standard in the experiments reported in this paper.
is also true of the GaAsP photodiode when exposed to 120 nm radiation, whereas a signi"cant reduction in quantum e$ciency is seen for the Si pn photodiode at this wavelength. It should be noted that if radiation damage is induced by prolonged irradiation at a speci"c wavelength, the quantum e$ciency at all wavelengths is a!ected. Therefore the use of the Si pn photodiode as a reference detector in the VUV is not advisable. The better stability under irradiation of the GaAsP compensates for their known large spatial nonuniformity. Over the entire photosensitive surface, variations of the local quantum e$ciency of up to 20% are encountered, but in the central regions, the quantum e$ciency typically varies by less than 5% [12]. The spatial nonuniformity introduces a signi"cant additional contribution to the uncertainty of the photodiode quantum e$ciency of about 2% (Table 1).
The experiments reported here were conducted at one of the monochromator beamlines in the radiometry laboratory of PTB [16] at the Berlin synchrotron radiation facility BESSY. This particular beamline, equipped with a 1m-153 McPherson-type normal-incidence monochromator, is part of the PTB detector calibration facility for the UV and VUV spectral ranges. Provided with a set of di!erently coated mirrors and re#ection gratings, it delivers small-bandwidth synchrotron radiation of high spectral purity in the wavelength range from 40 to 500 nm [8,17]. At the downstream end, the beamline is divided into three branches installed on top of a pivoting platform that permitted the di!erent experimental stations to be alternately placed in the beam path. The beam divergence at the location of the experiments amounts to 12 mrad, and the beam size at the focal point is 2.8 mm horizontally by 1.8 mm vertically; within this area more than 99% of the total radiant power is found. Several monitor detectors, operating in di!erent spectral ranges, are available to trace the monotonous decrease of the spectral photon #ux with time, which is due to the loss of electrons from the beam circulating in the BESSY storage ring. In normal operation, the stored beam current decreases from about 700 mA to about 300 mA in the time interval between subsequent injections with a 1/e lifetime of about 2}4 h. For the 120}220 nm spectral range of this investigation, normal-incidence optics with aluminum coating and a protective MgF layer was used. An 2 MgF cut-on "lter was placed in front of the mono2 chromator entrance slit to suppress radiation at wavelengths shorter than 110 nm, which would lead to a spectral contamination due to higher di!raction orders of the monochromator grating. Spectral impurities consisting of false light at longer wavelengths were studied using a set of cut-on "lters, which are available behind the monochromator exit slit, and a photodiode at the beamline focal plane as detector. The fraction of the total incident radiant power due to false light turned out to be below 0.2% for wavelengths greater or equal to 130 nm [17].
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Fig. 2. Experimental chamber for the CsI quantum e$ciency measurements.
For the calibration of the GaAsP reference detector, the SYRES radiometer was installed at one of the beamline branches. The photodiode was placed in a vacuum chamber that allowed the full synchrotron radiation beam to fall onto the diode surface and was attached to a second branch of the beamline. By pivoting the platform back and forth, the incident spectral radiant power was measured alternately with the primary detector standard and the photodiode. The widths of the monochromator entrance and exit slits were set to 3 and 2 mm, respectively, resulting in a wavelength bandwidth of about 3.3 nm. The ensuing photon #ux at the focal point is of the order 1013 s~1 for normal operation conditions of BESSY. In the experiments to determine the quantum e$ciency of CsI photocathodes, a special small-size vacuum chamber (Fig. 2) was installed at one of the beamline branches. This chamber was constructed such that it "ts in the glove box of the sample preparation laboratory of TUM. There, the samples could be installed in the chamber while working in a nitrogen atmosphere containing less than 1 ppm of oxygen and water, thus preventing contamination of the CsI surface, which is known to lead to a reduced quantum e$ciency [19]. After insertion of the samples, the chamber was evacuated and shipped to Berlin for the experiments. Inside the vacuum chamber, the samples are moun-
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ted on a turntable, actuated by a rotary feedthrough, so that the eight samples can be positioned alternatively behind the 2 mm diameter entrance aperture. The angle position of the turntable can be reproduced within $0.13, which equals a reproducibility of the sample position with respect to the entrance aperture of $0.05 mm. One of the positions on the turntable was occupied by the semiconductor photodiode used as transfer detector standard. A grid behind the apertures and about 7 mm in front of the samples was used to apply an extraction voltage. At the beamline, the entrance aperture was located at the beamline focal plane. In the horizontal direction, the aperture was over"lled by the incident synchrotron radiation beam. The beam was centered to the aperture within $0.1 mm in the horizontal and within $0.5 mm in the vertical direction. The photocurrents of the diode and the CsI photocathodes were measured with the same Keithley 617 picoamperemeter. The extraction grid was biased to 1500 V, as saturation of the photocurrent of the CsI samples was observed only at voltages exceeding 1200 V. In order to avoid signi"cant radiation damage, the incident photon #ux was limited to about 109 s~1 such that the extracted photocurrent would not exceed 100 pA. To this end a microchannelplate (MCP) mounted on a rotary and linear motion feedthrough was installed in front of the sample chamber, which permitted a convenient scaling of the photon #ux passing through the aperture by about three orders of magnitude. For con"rmation of the quantum e$ciency results and to rule out that the use of the MCP might lead to artifacts, additional measurements with the MCP moved out of the optical path were performed in special operation shifts, in which the storage ring was operated at a reduced electron beam current of below 80 lA. In both cases, the measurement sequence was such that after tuning the monochromator to the desired wavelength, the samples were positioned in the beam one after the other, and their photocurrent was recorded as function of time. For discrimination of transient behavior, at least 20 individual data points under irradiation were recorded, and for the purpose of dark current determination 10 additional readings were taken before opening and
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after closing the beam shutter, respectively. The "rst and the last measurement at each wavelength were done with the photodiode reference detector. After correction for the dark current, all photocurrents were normalized to the simultaneously recorded beam current in the storage ring in order to remove the time dependence of the incident photon #ux. Finally, the quantum e$ciency of the CsI photocathodes was calculated from the ratio of the normalized photocurrents obtained for the CsI samples and for the photodiode, linearly interpolated from the two measurements in the latter case, using the known quantum e$ciency of the reference detector.
4. Characterization of the GaAsP reference detector standard Prior to and after the experiments to determine the quantum e$ciency of the CsI samples, the GaAsP photodiode was calibrated. Fig. 3 shows the obtained (external) quantum e$ciency of the GaAsP Schottky diode as function of wavelength as determined by comparison to the primary detector standard SYRES. The photodiode was positioned such that the beam spot was centered to within $0.5 mm in the photosensitive surface. Data points marked by open diamonds represent the results obtained before the experiments of this study, and the "lled circles correspond to the GaAsP photodiode quantum e$ciency after the experiments were completed. The error bars in the plot represent the absolute standard uncertainty of the photodiode quantum e$ciency. The corresponding relative standard uncertainty amounts to 1.2% in the 120}170 nm spectral range and to 0.7% at wavelengths greater than 170 nm [12]. Obviously, the spectral shape of the photodiode spectral responsivity has changed signi"cantly. For wavelengths below 145 nm, the quantum e$ciency has decreased, while an increase is observed at longer wavelengths. The maximum relative reduction amounts to about 10% at wavelength 125 nm. For wavelengths greater than 160 nm, the relative increase is between 4% and 6%. The change in the quantum e$ciency of the GaAsP reference diode is due to radiation damage
Fig. 3. Quantum e$ciency of the GaAsP}Au Schottky photodiode as determined with the SYRES primary detector standard prior to (open diamonds) and after the experiments ("lled circles).
Fig. 4. Spatial variation of the GaAsP photodiode quantum e$ciency for three di!erent wavelengths along two perpendicular lines through the diode center. For the measurements, the diode was scanned behind a pinhole of 0.25 mm diameter. The peaks and dips that are seen in the vicinity of the center of the diode-sensitive surface, i.e. in the area that was exposed to radiation in the course of the experiments, were initially absent.
e!ects, as is demonstrated in Fig. 4, which shows the spatial variation of the quantum e$ciency for three di!erent VUV wavelengths. For this kind of experiment, a setup was used in which the diode can be scanned in the two directions perpendicular to the beam path, behind a pinhole of 0.25 mm diameter that con"nes the beam size. The "gure shows cuts through the map of the local quantum
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Table 1 Contributions to the relative uncertainty of the GaAsP photodiode quantum e$ciency at four wavelengths. The stated values are standard relative uncertainties (k"1) as evaluated according to Ref. [18]. Relative uncertainty *(QE)/QE at wavelength Source of uncertainty 140 nm Calibration Nonuniformity Radiation ageing Total (sum in quadrature)
160 nm
180 nm
1.2% 0.6% 2.4%
2.0% 0.9% 1.5% 2.5% 2.6%
200 nm
0.7% 1.2% 2.5%
e$ciency along the two lines through the center of the photosensitive surface. The regions in the vicinity of the center of the diode that were exposed to radiation in the course of the experiments are clearly identi"ed by a reduced quantum e$ciency in the case of wavelength 130 nm, and an increased quantum e$ciency for the other two wavelengths. These features were absent when the local quantum e$ciency of the GaAsP diode was initially investigated. The larger beam size in the calibration measurements as well as in the application as reference detector implies an averaging of the local quantum e$ciency. Therefore, the e!ects seen in Fig. 3 are smaller than the local variations seen in Fig. 4. Table 1 summarizes the resulting contributions to the relative uncertainty of the GaAsP Schottky photodiode quantum e$ciency. The "rst row refers to the determination of the quantum e$ciency in the center area of the diode with SYRES. The variation in calibration accuracy at di!erent wavelengths is mainly due to the di!erent stability of the di!erent photon #ux monitors employed in the wavelength ranges above and below 170 nm, respectively. The e!ect of the spatial nonuniformity of the diode is practically wavelength independent in the spectral range of this study and gives a relative uncertainty contribution of about 2%. The contribution from the change in quantum e$ciency varies from 0.6% to 1.5%. In total, the relative uncertainty of the GaAsP photodiode quantum e$ciency when employed as transfer detector stan-
Fig. 5. Initial quantum e$ciency of four di!erently prepared CsI photocathode samples (cf. Table 2).
dard is about 2.5% in the entire wavelength range of this investigation.
5. Quantum e7ciency of the CsI photocathodes Fig. 5 shows the quantum e$ciency of four of the investigated CsI photocathode samples. All samples were produced by vapor deposition in high vacuum, but di!er in their substrate material and preparation temperature (Table 2). Obviously the treatment of the substrate prior to deposition is of crucial importance [6]. The worst quantum e$ciency curve is obtained for sample 1 (diamonds in Fig. 5) for which the CsI was deposited onto a polished stainless steel substrate that was maintained at an elevated temperature of 1003C. The use
Table 2 Substrate material and its temperature during the preparation of the CsI photocathode samples investigated in this study. In all cases electron-beam heating was used for evaporation. Substrate Sample
Material
Temperature (3C)
1 2,5 4,6 7
Polished VA VA # RSG G10# RSG G10# RSG
100 100 60 25
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of bare G10-Ni/Au as substrate (sample 7, plus signs) gives about 15% improvement on average. The best quantum e$ciency is obtained when the substrate is coated with resin-stabilized graphite (RSG) prior to deposition of CsI. The total improvement as compared to the polished stainless steel substrate amounts to almost a factor of 2 on average. When the RSG coating is employed, the nature of the substrate is practically unimportant. The quantum e$ciency of sample 5 (open boxes), where stainless steel was used as the substrate, is only by less than 2% on average worse than the best quantum e$ciency curve obtained for sample 4 ("lled circles) that was deposited on G10-Ni/Au. This di!erence is slightly smaller than the di!erence in quantum e$ciency of sample 4 or 5 to the respective second sample that was produced by the same preparation procedure (Table 2). The quantum e$ciency curves of these two samples are omitted in Fig. 5 for lucidity. The error bars that are attached to the plot symbols in Fig. 5 indicate the uncertainty of the respective absolute quantum e$ciency value. In almost all of the spectral range of this study, the relative uncertainty of the quantum e$ciency remains below 10%, in the 130}180 nm wavelength range even below 5%. For a couple of wavelengths, the contributions to relative uncertainty of the quantum e$ciency are listed in Table 3. The "rst row refers to the uncertainty of the GaAsP photodiode quantum e$ciency as given in Table 1. This row also comprises the non-linearity of the diode, which is a negligible e!ect of order 0.01%, however. The uncertainty contribution originating from the measurement of the photocurrent is dominated by the statistics of the repeated current read-out at "xed wavelength which also a!ects the dark current correction. The relative calibration uncertainty of the picoamperemeter is about 0.1%. The row labeled `radiant #ux normalizationa comprises the statistics of the reference signal and the change in the normalized photodiode signal between the two diode measurements, which actually dominates this uncertainty contribution. The dominant source of uncertainty, at least at wavelengths longer than 170 nm, turns out to be the reproducibility of the quantum e$ciency deter-
Table 3 Contributions to the relative uncertainty of the CsI photocathodes quantum e$ciency at four wavelengths. The stated values are standard relative uncertainties (k"1) as evaluated according to Ref. [18]. Relative uncertainty *(QE)/QE at wavelength Source of uncertainty
140 nm 160 nm 180 nm 200 nm
Transfer detector standard Photocurrent measurement (calibration, statistics, dark current correction)
2.4% 0.4%
2.5% 0.4%
2.6% 0.4%
2.5% 0.6%
Radiant #ux normalization 0.7% Reproducibility of quantum 1.9% e$ciency
0.7% 2.3%
0.7% 4.5%
0.8% 8.8%
Total (sum in quadrature)
3.5%
5.4%
9.2%
3.2%
mination. The values shown in the table are derived from the standard deviation of at least four repetitions of the quantum e$ciency measurements for all samples. The comparatively large scatter of the data points for the same sample at the same wavelength is attributed to the spatial nonuniformity of the quantum e$ciency on the surfaces of the CsI photocathodes and of the GaAsP reference detector (Fig. 4). Due to the spatial nonuniformity, a di!erent irradiance pro"le or a di!erent location of the irradiated area on the sample surface results in a di!erent quantum e$ciency value. For initially highly uniform samples this e!ect can be still of negligible magnitude as is demonstrated by Fig. 6 which shows the results of repeated determinations of the quantum e$ciency at 150 nm wavelength for sample 5. The "rst four values show a relative standard deviation of only 0.23%. When the last three data points were taken, the accumulated photon exposure was already high enough to induce detectable radiation aging. As only the surface area exposed to radiation is a!ected, the radiation damage induces a spatial nonuniformity. Therefore, the monotonous behavior in this case is not typical and rather correlated to the fact that realignments of the sample chamber with respect to the beam were performed at this particular wavelength 150 nm.
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Fig. 6. Results of a repeated determination of the quantum e$ciency of sample 5 at wavelength 150 nm. The solid line indicates the instrumentally weighted average over the data points, and the dashed lines indicate the standard deviation which amounts to 0.23% of the average. The decrease in quantum e$ciency for the last three measurements is due to radiation aging.
6. Radiation aging and spatial uniformity of the CsI photocathodes After the experiments on the quantum e$ciency, two of the investigated CsI samples were subjected to prolonged exposure to high photon #ux levels in order to study the e!ect of severe radiation aging. These measurements were performed at wavelength 150 nm. The MCP was removed from the beam path so that a photon #ux density of order 5]1014 cm~2 s~1 was achieved and the initial photocurrent was about 300 nA. The decreasing quantum e$ciency of the two samples is plotted in the upper part of Fig. 7 as function of the photon exposure. For comparison, the total photon exposure of a photocathode used in the RICH detector of the HADES experiment is estimated to be of the order 1017 cm~2 per year of operation. The radiation aging of the quantum e$ciency is commonly attributed to damage induced by the #ow of electrical charge in the CsI bulk [20]. This interpretation is supported by the lower part of Fig. 7 which shows the quantum e$ciency as a function of the extracted charge per unit area. An almost linear dependence is observed up to about 3 mC/cm2 of extracted charge per unit area. Fig. 7 furthermore demonstrates that the aging behavior
Fig. 7. Decrease of quantum e$ciency at wavelength 150 nm as function of accumulated photon exposure (top) and versus the extracted charged (bottom) for the CsI photocathode samples 5 and 7.
obviously depends on the sample preparation. The initially higher quantum e$ciency of sample 5 (solid line) as compared to sample 7 (dashed) decreases much faster so that already after an exposure that is equivalent to about 2 months of operation in the HADES RICH the quantum e$ciency of sample 5 is below that of sample 7. For a radiant exposure expected for one year of implementation, the quantum e$ciency of both samples has decreased by more than a factor of 2. For sample 7, the radiation aging experiment was divided into several exposure steps. After each step, a quick and less accurate determination of the quantum e$ciency curve in the entire spectral range of this study was performed. In this case, the wavelength of the monochromator was scanned while recording the photocurrent of the CsI photocathode and the GaAsP photodiode, respectively, in two consecutive measurements. The resulting quantum e$ciency curves have a large uncertainty
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Fig. 8. Quantum e$ciency of sample 7 as function of wavelength after several exposure steps.
as far as absolute values are concerned, the spectral shape, however, is reliable. As is obvious from Fig. 8, the aging induced by prolonged exposure to radiation at 150 nm wavelength leads to a decrease in quantum e$ciency at all wavelengths. Before and after the radiation aging experiments, all samples were probed for spatial nonuniformity of the quantum e$ciency. For this kind of examination, a pinhole of 50 lm diameter was placed about 10 cm in front of the entrance aperture. For the two wavelengths 170 nm and 190 nm, the resulting con"ned beam was scanned across each of the seven samples and the diode, respectively, by rotating the turntable in 13 steps, which corresponds to about 0.55 mm steps. Like in the quantum e$ciency measurements, several readings of the photocurrent were recorded at each position. The spatial variation of the quantum e$ciency of sample 5 at wavelength 190 nm is shown in Fig. 9. Initially sample 5 had a rather uniform local quantum e$ciency that varied by less than 1% over the cathode surface except in the vicinity of the dip marking the central area that was exposed to radiation in the course of the quantum e$ciency measurements. After extraction of almost 4.5 mC/cm2 of charge per unit area from the sample, the quantum e$ciency was drastically reduced in the area that was subjected to prolonged exposure, almost vanishing in the center, while no change was observed in the unexposed areas. The irradiated area was clearly perceivable in a visual inspection of the sample after the end of the experi-
Fig. 9. Spatial variation of the quantum e$ciency at wavelength 190 nm for the CsI photocathode sample 5 prior to and after prolonged exposure to VUV radiation. For this experiment, the sample was scanned behind a pinhole of 50 lm diameter de"ning the synchrotron radiation beam size.
ments which indicates that a modi"cation of the bulk material occurred during the exposure to intense VUV radiation.
7. Conclusions As a conclusion, the quantum e$ciency of CsI photocathodes have been determined for the "rst time with a measurement scheme which guarantees traceability to a primary radiometric standard. The relative uncertainties achieved are in the few percent range, which constitutes a signi"cant progress over previous work. This improved accuracy could only be achieved by using a GaAsP Schottky type photodiode, which is su$ciently stable under prolonged exposure to VUV radiation, as transfer detector standard.
References [1] HADES collaboration, A proposal for Di-Lepton spectroscopy at GSI, Darmstadt, Germany, 1993. [2] J. SeH guinot, G. Charpak, Y. Giomataris, V. Peskov, J. Tischhauser, T. Ypsilantis, Nucl. Instr. and Meth. A 297 (1990) 133. [3] CERN RD26 collaboration Nucl. Instr. and Meth. A 360 (1995) 411.
H. Rabus et al. / Nuclear Instruments and Methods in Physics Research A 438 (1999) 94}103 [4] P. Maier-Komor, B.B. Bauer, J. Friese, R. GernhaK user, P. Kienle, H.J. KoK rner, G. Montermann, K. Zeitelhack, Nucl. Instr. and Meth. A 362 (1995) 183. [5] A. Breskin, Nucl. Instr. and Meth. A 371 (1996) 116. [6] J. Friese, R. GernhaK user, J. Homolka, A. KastenmuK ller, P. Maier-Komor, M. Peter, K. Zeitelhack, P. Kienle, H.J. KoK rner, Nucl. Instr. and Meth. A 438 (1999) 86. [7] R. GernhaK user, J. Friese, J. Homolka, A. KastenmuK ller, P. Kienle, H.J. KoK rner, P. Maier-Komor, M. Peter, K. Zeitelhack, Nucl. Instr. and Meth. A 438 (1999) 104. [8] H. Rabus, V. Persch, G. Ulm, Appl. Opt 36 (1997) 5421. [9] H. Rabus, F. Scholze, R. Thornagel, G. Ulm, Nucl. Instr. and Meth. A 377 (1996) 209. [10] C. Lu, Z. Cheng, K.T. McDonald, D.R. Marlow, E.J. Prebys, R.L. Wixted, Nucl. Instr. and Meth. A 366 (1995) 60. [11] A.D. Wilson, H. Lyall, Appl. Opt. 26 (1986) 4530, 4540. [12] A. Lau-FraK mbs, PhD Thesis, TU Berlin, 1995, unpublished.
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[13] R. Goebel, R. KoK hler, R. Pello, Metrologia 32 (1995/1996) 515. [14] M. Razeghi, A. Rogalski, J. Appl. Phys 79 (1996) 7433. [15] K. Solt, H. Melchior, U. Kroth, P. Kuschnerus, V. Persch, H. Rabus, M. Richter, G. Ulm, Appl. Phys. Lett 69 (1996) 3662. [16] G. Ulm, B. Wende, Rev. Sci. Instrum 66 (1995) 2244. [17] A. Lau-FraK mbs, U. Kroth, H. Rabus, E. Tegeler, G. Ulm, Rev. Sci. Instrum 66 (1995) 2324. [18] International Organization for Standardization (ISO), Guide to the expression of uncertainty in measurement, ISBN 92-67-10188-9, Geneva, Switzerland, 1995. [19] Ryoji Enomoto, Takayuki Sumiyoshi, Yoshio Fujita, Nucl. Instr. Meth. A 343 (1994) 117. [20] D.F. Anderson, S. Kwan, V. Peskov, B. Hoeneisen, Nucl. Instr. and Meth. A 323 (1992) 626.
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