CsBr and CsI UV photocathodes: new results on quantum e$ciency and aging

Transcription

1 Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 CsBr and CsI UV photocathodes: new results on quantum e$ciency and aging B.K. Singh*, E. Shefer, A. Breskin, R. Chechik, N. Avraham Department of Particle Physics, The Weizmann Institute of Science, Rehovot, Israel Received 17 February 2000; received in revised form 4 April 2000; accepted 6 April 2000 Abstract We report on the photoemission properties of 300 As thick transmissive- and 5000 As thick re#ective UV-sensitive CsBr photocathodes. Following post-evaporation heat treatment at 703C the absolute quantum e$ciency is 35% at 150 nm, with a red boundary cut-o! at about 195 nm. Extensive aging studies of CsBr and CsI photocathodes, under high photon #ux and under ion bombardment in gas avalanche multiplication mode, were carried out for the "rst time without exposure to air. The results are compared with the previously published data on CsI aging and the methodology of the aging tests is discussed in details Elsevier Science B.V. All rights reserved. PACS: i; Fv; x; n Keywords: UV-photocathodes; CsBr photocathodes; CsBr photocathode aging; CsI photocathode aging; Photon detectors 1. Introduction * Corresponding author. Tel.: # ; fax: # address: fnsingh@wis.weizmann.ac.il (B.K. Singh). In recent years we have seen considerable activity in the "eld of photon imaging detectors, combining solid photocathodes and gaseous electron multipliers [1]. Such devices, which are sensitive to single photons, can reach dimensions of a square meter [2] and can operate at very high magnetic "elds [3]. They may have applications in various "elds, as for example, the readout of large arrays of scintillators and scintillating "bers, as well as of gas scintillators in medical imaging and space instrumentation. In particular, they are applied in particle and astroparticle physics, for particle identi"cation by the Ring Imaging Cherenkov (RICH) technique [4]. In the UV spectral range, CsI photocathodes (see review [5]), currently employed in vacuum- and gas-operated imaging detectors, have the bestknown quantum e$ciency (40% at 150 nm) and relatively good stability for short exposure to air. Their high quantum response is due to the good electron transport and emission properties, typical of alkali halides. CVD diamond "lms have also been investigated recently, showing rather good photoemission properties (12% at 140 nm), but at a much more restricted spectral range [6]. CsBr, though known for a few decades as a solar-blind photocathode [7], has not received much attention, due to its much poorer photoemission yield compared to that of CsI. Recently, there has been a renewed interest in CsBr as a protective coating "lm /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S ( 0 0 ) X

2 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} for alkali}antimonide photocathodes [8]. The coating of such photocathodes with a few hundred angstrom thick CsI or CsBr "lms was found to protect them against exposure to oxygen, paving the way towards their use in gas avalanche detectors for visible light [9,10]. In this work, we have investigated the preparation method and photoemission properties of transmissive (300 As thick) and re#ective (5000 As thick) CsBr photocathodes. Post-evaporation heat treatment as well as the e!ect of exposure to ambient air were studied. Particular emphasis was placed in this work on the aging of CsBr "lms under the impact of photons and ions. To date, there has not been a clear understanding of aging processes of photocathodes. Although a large collection of data exists in the literature, mainly for CsI "lms, many discrepancies are seen among the measurements by di!erent investigators. We believe that this is due to our ignorance about the exact nature of the aging mechanism and the relevant experimental parameters that should be followed during the aging test. Some possible explanations and a compilation of existing data are reviewed in Ref. [5] and discussed in more detail below. We present here the results of systematic aging studies of CsBr photocathodes under photon and avalanche-ion bombardment, and compare them with new data of CsI photocathodes aging carried out exactly under the same conditions. The current tests were carried out for the "rst time without exposing the "lms to air and include the recording of relative photocurrent as well as the absolute photoyield. In the following, we discuss the importance of the aging study methodology. 2. Experimental technique The experimental set-up includes a high-vacuum evaporation chamber (pumped with a cryogenic pump to 10 Torr), coupled via a CaF window to a vacuum ultra violet (VUV) monochromator (MC), equipped with a 30 W D lamp (see Fig. 1). An important feature of the set-up is that after the sample preparation, its quantum e$ciency (QE) can be measured in situ, by rotating and displacing it from the evaporation site to the measurement location. The QE was measured in a re#ective mode, under vacuum or in CH, % pure. A positive voltage was applied to a mesh electrode Fig. 1. A schematic view of the experimental set-up. The photocathodes are deposited (deposition site) and investigated (measurement site) in the vacuum chamber without exposure to air. The photocathode history is followed by continuously monitoring the photocurrent and also by measuring the absolute QE at time intervals.

3 366 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 placed at a distance of 3.5 mm from the photocathode surface; the photocurrent, induced by monochromatic UV photons, was recorded from the photocathode. In all QE measurements, the photon #ux at the lamp's peak (160 nm) was of the order of 810 ph/s on a cathode surface of about 10mm. The absolute QE value is derived from the ratio of the current measured from the photocathode to the current from a calibrated Hamamatsu 1460R photomultiplier (Cal. PMT). This was done by alternatively directing the UV beam to both, by a rotating mirror. This PMT was calibrated against a NIST vacuum-photodiode [11] and was operated in a photodiode collection mode (gain 1) with #80 V on its "rst dynode. The stability of the MC D lamp was monitored throughout the experiments by a second reference photomultiplier (Ref. PMT), of the same type, and the measured photocurrent values were corrected correspondingly. The photocathode materials, CsI and CsBr (ALFA, ultra-pure quality) were deposited from molybdenum and tantalum evaporation boats, respectively, onto a stainless-steel (SS) substrate placed 250 mm above the evaporation boats. The alkali}halide powders were melted and outgassed under a shutter prior to evaporation. The photocathode "lms (300 and 5000 As CsBr and CsI) were deposited on the polished stainless-steel substrates, pre-coated with a 1000 As thick Al layer. The CsBr evaporation rate was of 0.2}10 As /s and 20}70 As /s for the 300 and 5000 As thick "lms, respectively. After the measurement of the QE of an `as-evaporateda photocathode, it was heated under vacuum, with the help of a built-in water heat exchanger, to 703C, for a few hours. For photonaging studies, carried out under vacuum and under gas #ow (CH at 1 atm pressure), we used another 30 W D lamp, mounted at the top of the mirror box (with the mirror removed), directly illuminating the photocathode at high photon #ux (see Fig. 1). For aging studies under positive ion bombardment (under gas avalanche), we used the MC light at 160 nm wavelength. In these measurements, the UV-induced photoelectrons emitted from the photocathode were multiplied in a parallel-plate (PP) avalanche mode by applying a high voltage to the mesh (placed here at a reduced distance of 1.5 mm above the photocathode). The absolute QE Fig. 2. Post evaporation heat enhancement of the photoyield of CsBr photocathodes. (a) Absolute QE of 300 As CsBr, measured as deposited and after 2 and 4 h at 703C. (b) Absolute QE of 5000 As CsBr measured as deposited and after 3 and 6 h at 703C. The data of Taft and Philipp [7] is also shown. spectra were measured occasionally without multiplication, either under vacuum or in gas. In ionaging studies, the chamber was operating in #ow mode, with 50 Torr of methane. During the measurements the gas pressure and the temperature variations were carefully monitored. 3. Results 3.1. Quantum ezciency Fig. 2 shows the absolute QE as a function of wavelength for 300 and 5000 As thick CsBr photocathodes. The QE was "rst measured immediately after evaporation (referred to `as-evaporateda in

4 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} Fig. 3. The decay of the QE of 5000 As CsBr under exposure to air inside and outside the chamber. Fig. 2). Thereafter the photocathode temperature was gradually raised to 703C, resulting in an enhancement of the QE values, which saturated after 4 and 6 h for thin and thick photocathodes, respectively. For both "lm thicknesses we observe a strong e!ect of heat treatment, which is larger at longer wavelength. At 160 nm the QE is almost doubled for both thin and thick "lms, typically reaching values of 15 and 17%, respectively. A shift in the photoemission threshold (red boundary) by about 0.7 ev can be observed in Fig. 2. The CsBr QE spectrum measured by Taft and Philipp [7] is also shown in the "gure and is very similar to our `as-evaporateda data. Our best QE data for heatenhanced CsBr "lms are shown in Fig. 3, reaching about 27% at 160 nm Exposure to air Moisture plays an important role in the decay of the photoemission properties of hygroscopic photocathode materials. This e!ect has been investigated in detail for uncoated [12}16] and coated [17,18] CsI photocathodes. It was lately shown that the morphology of thin CsI and CsBr "lms is strongly a!ected by humidity, which could explain part of their decay [19]. We present here new results on the decay in photoemission properties of CsBr photocathodes. Most of the measurements were performed inside the evaporation chamber, at room temperature. Air was introduced into the chamber, for di!erent time periods, followed by pumping and QE measurement in vacuum. Absolute QE spectra for a 5000 As thick CsBr photocathode are shown in Fig. 3. It was observed that a short-term exposure (20 min) to air inside the chamber does not deteriorate the QE value; on the contrary, the QE slightly improves at long wavelengths. However, a 25% drop in QE was observed after 9 h of exposure to humid air inside the chamber. The exposure of the same photocathode for 40 min outside the chamber, resulted in a further 30% decay in QE. The slower decay within the evaporation chamber, also observed for CsI [20], is most probably due to a getter e!ect of CsBr deposited on the chamber walls. The slight increase in CsBr QE after a short exposure to air may be due to an adsorption of a monolayer of water molecules, which by their electric dipole reduce the electron a$nity, as discussed in detail in Refs. [18,21] Aging Aging studies, which are very time consuming, are always carried out under accelerated conditions, namely incident #ux of photons or ions which is by several orders of magnitude larger than the realistic one. Therefore, some additional processes may be provoked, such as charging-up or non-linear e!ects, which could distort the results. The conclusion on the photocathode aging, based solely on linear extrapolation of #ux, should therefore be carefully examined. Moreover, in the present work we have followed the decay of the photocathode by measurements of photocurrent and of absolute photoyield in vacuum or in gas. In part of the cases, there is no agreement between the results obtained from the two types of measurements. We have full con"- dence in the absolute QE results, always measured under the same controlled conditions. The odd behavior of the photocurrent measurements will be discussed at length. In the present aging study, we used di!erent aging conditions, which may be classi"ed into four di!erent phases, as de"ned in Table 1. Some more technical details are also given in Table 2.

5 368 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 Table 1 De"nition of photon- and ion-aging studies conditions Phase/type Photon #ux (Photon/mm s) Gas/pressure Gain Photocathode I Photon aging in vacuum &10 Vac, 10 Torr * Thick and thin CsBr II Photon aging in gas &10 CH, 1 atm 1 Thick and thin CsBr III Ion aging at high gain &10 CH, 50 Torr 10 Thick and thin CsBr/CsI IV Ion aging at low gain &10 CH, 50 Torr 10 Thick and thin CsBr/CsI Table 2 Summary of photon- and ion-aging studies of CsBr and CsI photocathodes. Given are the experimental conditions (see Table 1), the Fig. no. showing the data and the accumulated charge corresponding to twenty-percent loss (TPL) of QE. We also remark on consistency between photocurrent and QE measurements. Phases I and II are photon-aging; phases III and IV are ion-aging. The photon #ux for each data point was extracted from the recorded photocurrent and the measured absolute QE Photocathode /thickness (As ) Phase Flux TPL of QE (μc/mm) (photon/mm s) (pa/mm) Photocurrent vs. QE consistency Fig. no. CsBr/300 As I Yes 4 CsBr/5000 As I Yes 4 CsBr/300 As II '230 No 5 CsBr/5000 As II '130 No 5 CsBr/300 As III (14% loss) No 7 CsBr/300 As IV Yes 7 CsBr/5000 As III Stable No 8 CsBr/5000 As IV Yes 8 CsI/300 As III Yes 9 CsI/300 As IV Yes 9 CsI/5000 As III Yes 10 CsI/5000 As IV (15% loss) Partial Photon aging Photon-aging studies were "rst carried out in Phase I, with intense photon #ux under vacuum (10 Torr). Fig. 4a shows the relative photocurrent as a function of total accumulated charge for both 300 and 5000 As thick CsBr photocathodes; the respective initial photon #ux were 610 and 510 photon/mm s. In Fig. 4a we observe a similar initial decay rate for both "lms and then a faster QE decay for the thinner "lm. The decay rate seems to decrease with increasing accumulated charge, for both. A twenty-percent loss (TPL) in relative photocurrent was observed after 7 and 14 μc/mm for the 300 and 5000 As thick photocathodes, respectively. Following the above aging process we re-measured the QE spectra of the aged photocathodes in situ. In Fig. 4b we display the absolute QE spectra measured for the 300 As thick CsBr photocathode before and at the end of the photon aging. The drop in QE by 58% at 160 nm is consistent with the drop of &57% seen for the photocurrent measurements in Fig. 4a. Similarly, in Fig. 4c we show the absolute QE spectra evolution of a 5000 As thick CsBr photocathode. A QE loss of 54% is observed at 160 nm, which is again consistent with the relative 55% drop of the photocurrent in Fig. 4a. Adi!erent behavior was observed in Phase II photon aging of thick and thin CsBr photocathode, in 1 atm of methane. The aging of 5000 As CsBr

6 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} Fig. 4. Photon-induced aging of CsBr photocathodes in vacuum (Phase I). (a) Relative photocurrent as a function of accumulated charge for 300 and 5000 As CsBr photocathodes. (b) The absolute QE spectrum of a 300 As CsBr, measured in vacuum before and at the end of the photon-induced aging test. (c) Same as (b) for 5000 As CsBr. Fig. 5. Photon-induced aging of CsBr photocathode in 1 atm CH, gain"1 (Phase II). (a) Relative photocurrent as a function of accumulated charge for 5000 As CsBr. (b) The absolute QE spectra measured in vacuum before and at the end of the aging test. (c) Same as (b) for 300 As CsBr.

7 370 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 photocathode was carried out under an incident photon #ux of 410 photon/mm s (Fig. 5a). For this large #ux, a decrease of 10% in the photocurrent was initially observed, followed by a plateau up to an accumulated charge of about 50 μc/mm and a further slow decrease up to 127 μc/mm. After this aging process we again re-measured the absolute QE of the photocathode in vacuum (Fig. 5b), observing no degradation of the QE at 160 nm. Similarly, we studied the photon aging of 300 As CsBr photocathode at 1 atm pressure of methane with no charge multiplication. The incident photon #ux was about &3 10 photon/mm s. We observed a steep rise in the relative photocurrent, to about 145% of the initial value, followed by a slow decrease to 140% of the initial value after accumulated charge of 87 μc/mm (not shown). At this point, the absolute QE was remeasured in gas as shown in Fig. 5c. We record no enhancement in QE, in contradiction with the observed rise in photocurrent. We continued the aging of this photocathode up to an accumulated charge of &230 μc/mm. The photocurrent "rst dropped sharply and then stayed at about 130% of the initial value (not shown). Absolute QE measurements were again done at intermediate points of accumulated charges of 189 and 230 μc/mm; once againnochangeinabsoluteqewasfound Ion aging of CsBr and CsI photocathodes As pointed out above, accelerated ion-induced aging should be performed in order to obtain the results over a reasonable time-scale. This may be achieved by carrying out the measurements under high #ux or high gain, leading to high count rate (although quite limited due to upcharging), or by choosing operation conditions known to invoke accelerated aging. The last occurs, for example, under a parallel plate (PP) avalanche mode, in which all the avalanche ions are back drifting and sputtering the photocathode at high velocity. Choosing, for example, low gas pressure and high electric "eld (namely high gain) even further intensi- "es the photocathode damage. In the present work, we used PP avalanche mode in CH, at 50 Torr under various chamber gains. Slower aging is therefore expected at atmospheric pressure under di!erent multiplication geometries. Fig. 6. The total gain/voltage curve at 50 Torr CH of the PP comprising a polished SS substrate cathode located 1.5 mm below the mesh anode. We "rst measured the absolute gain curve by recording the UV-induced photocurrent from a SS cathode substrate as a function of voltage (Fig. 6). Gains of 10 and 10 were reached at 620 and 500 V, respectively, with 50 Torr of methane. The aging studies of phases III and IV under gas multiplication were carried out under these conditions, with the SS photocathode replaced by our CsBr or CsI photocathodes. The photocathode was irradiated by UV light at 160 nm. In each aging test, we recorded continuously the current from the photocathode, and in addition we measured the full absolute QE spectrum at given time intervals CsBr. The results obtained with 300 As thick CsBr photocathode under gas gain of 10 (phase III) are shown in Fig. 7a. The incident photon #ux was photon/mm s. We observe a two-component decay; "rst the relative photocurrent drops to 70% of its initial value after an accumulated charge of 13 μc/mm. Thereafter, it drops to &20% of its initial value after an accumulated charge of 158 μc/mm. At the beginning and at the end of this part of the aging process we measured the absolute QE spectrum in vacuum (lines 1 and 2 in Fig. 7c). We found that the drop of QE at 160 nm does not follow the 80% drop recorded in the photocurrent and it is of only 14%. We de"ne the relative quantum e$ciency (RQE) as the ratio of the absolute QE at 160 nm at any point

8 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} Fig. 7. Ion-induced aging of 300 As CsBr photocathode in 50 Torr CH. (a) Relative photocurrent as a function of accumulated charge at gains 10 and 10. RQE values at 160 nm are also shown. (b) An expanded view of the relative photocurrent decay and RQE values at 160 nm at gain"10. (c) The absolute QE spectra measured in vacuum before and at the end of the ioninduced aging tests at gains 10 (curves 1 and 2) and 10 (curves 2 and 3). in time to its value at the beginning of the aging process. The RQE values at points 1 and 2 are as well shown in Figs. 7a and b, as open circles 1 and 2. The aging measurements of the same photocathode were then continued at a reduced gain of 10 (phase IV), in 50 Torr of CH. The incident photon #ux was photon/mm s. The results are shown in Fig. 7a (above 171 μc/mm) and an expanded view of this part is given in Fig. 7b. A sharp drop of 50% in the relative photocurrent is seen after an additional accumulated charge of only 4 μc/mm. The absolute QE, measured under vacuum after this aging step, is shown in Fig. 7c (dotted line marked 3) and the RQE as open circle 3 in Fig. 7b; this time the results were consistent (&47% decay) with the relative drop of photocurrent at a gain of 10. The same aging test was done on a 5000 As thick CsBr "lm, in a reverse order. The results are shown in Fig. 8a. At a gain of 10 (phase IV) the incident photon #ux was 1.410photon/mm s. The relative photocurrent shows an initial increase of about 12% of its initial value, followed by a drop to 83% after an accumulated charge of 13 μc/mm. The absolute QE values (Fig. 8b) were measured this time in gas (yielding slightly lower absolute values as compared to vacuum, due to the backscattering e!ect [22]) along the phase IV aging step; the RQE (open circles 1}4 in Fig. 8a) are well consistent with the photocurrent measurement. Following these measurements, the gas gain was raised to 10 and the aging process was continued (phase III) on the same thick photocathode, at an incident photon #ux of photon/mm s. As shown in the Fig. 8a we recorded a fast rise in the relative photocurrent, followed by a gradual decrease, to about 61% of its initial value, after an additional accumulated charge of 160 μc/mm. The absolute QE spectra (Fig. 8c) show no increase or deterioration whatsoever, up to the total accumulated charge of 174 μc/mm. The RQE evolution (open circles 4}9 in Fig. 8a), based on the absolute QE measurements, is very di!erent compared to that of the photocurrent. The strong inconsistency between photocurrent and absolute QE measurements under some conditions and their good agreement under other

9 372 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 Fig. 9. Ion-induced aging of 300 As CsI photocathode in 50 Torr CH and gains of 10 and 10 as a function of accumulated charge. RQE values at 160 nm are also shown. conditions is puzzling, requiring further studies, as discussed in Section 4. Fig. 8. Ion-induced aging of 5000 As CsBr photocathode in 50 Torr CH. (a) Relative photocurrent and RQE values at 160 nm as a function of accumulated charge at gains 10 and 10. (b) The absolute QE spectra measured in vacuum and in gas before and after the aging at gain 10. (c) Same as (b) at gain CsI. In order to carry out a reliable comparison between the aging processes of CsBr and CsI photocathodes, the aging measurements have to be carried out exactly under the same conditions and protocols. We cannot make use of any of the published data on CsI, as in each case some experimental parameters are di!erent than in the present study. Therefore, we performed two sets of measurements, on 300 and 5000 As thick CsI photocathodes, under identical conditions to those of CsBr. The results for a 300 As CsI photocathode are shown in Fig. 9, for an initial gain of 10, followed by a gain of 10. At the gain of 10 the incident photon #ux was &1.310 photon/ mm s. A smooth decrease in photocurrent was observed, up to an accumulated charge of 83 μc/mm, where it reached about 68% of its initial value. Further aging of the same aged photocathode, at a gain of 10 and under an incident photon #ux of photon/mm s, resulted in a faster decay (Fig. 9). A relative loss of 25% in the photocurrent was reached for an additional accumulated charge of 23 μc/mm. The RQE evolved in an excellent accordance with the photocurrent data (Fig. 9). Fig. 10a displays the variation of the relative photocurrent for a 5000 As thick CsI photocathode,

10 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} Fig. 10. Ion-induced aging of 5000 As CsI photocathode in 50 Torr CH. (a) Relative photocurrent and RQE values at 160 nm as a function of accumulated charge with gain"10. (b) The absolute QE spectra measured in gas at di!erent values of accumulated charge. Fig. 11. Ion-induced aging of 5000 As CsI photocathode in 50 Torr CH. (a) Relative photocurrent and RQE values at 160 nm as a function of accumulated charge at gain"10. (b) The absolute QE spectra measured in gas at di!erent values of accumulated charge. aged at a gain of 10. The incident photon #ux was photon/mm s. We observed a fast decay to about 79% of its initial photocurrent value, followed by a gradual decrease to 57% after an accumulated charge &109 μc/mm. The drop in the absolute QE was measured along the aging process (Fig. 10b); the RQE evolution at 160 nm, shown in Fig. 10a, is fully consistent with that of the photocurrent. Aging results of a second 5000 As thick CsI photocathode, at a gain of 10, are shown in Fig. 11a; the incident photon #ux was photon/mm s. We observed an initial fast drop of the photocurrent, of almost 40%, followed by a rise and then a slow decrease. Absolute QE measurements along the aging study do not reproduce the variations in the photocurrent; RQE data at 160 nm is shown in Fig. 11a; it indicates only a 15% decay in QE, after an accumulated charge of 26 μc/mm. 4. Discussion 4.1. Quantum ezciency and heat enhancement The QE of `as-evaporateda CsBr "lms is consistent with the published data of Taft and

11 374 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 Phillipp [7]. A post-evaporation heating of CsBr photocathodes at about 703C in vacuum considerably enhances their QE, both for 300 and 5000 As thick "lms. A similar enhancement was previously observed for CsI, CuI and NaI [23]. The heat enhancement is a permanent modi"cation of the QE, measured at room temperature. It should be distinguished from a di!erent observation, of an increase in photocurrent when the photocathode temperature is raised [24,25], which is a temporary e!ect. We are tempted to ascribe the QE enhancement to the removal of water from the surface, as some of the previously mentioned authors did, but further considerations may contradict this hypothesis. Firstly, it is known that e$cient removal of water from the surface usually requires temperatures above 1003C and therefore we are indeed not sure that water is removed in the present process. Secondly, the same heat enhancement e!ect is observed on various materials with a very di!erent a$nity to water. Moreover, the opposite phenomenon was demonstrated on hygroscopic "lms such as LiF and NaF, whereby a monolayer of water was shown to enhance photoemission, due to a dipole moment created on the surface, e!ectively reducing the electron a$nity [18,21]. A di!erent explanation for the heat enhancement could be due to bromine desorption during the heating process. This explanation follows the approach presented in Ref. [23] for interpreting the heat enhancement observed in alkali}iodine photocathodes (CsI, CuI and NaI). It is known that bromides decompose more easily than other halides; an example is the decomposition of Ag(Br) in photoemulsion under exposure to visible light. We may assume that part of the CsBr decomposes during the evaporation and, due to the higher vapor pressure of bromine compared to cesium, this could result in a bromine-enriched deposited "lm. The fact supporting this idea is that there exists a stable compound of CsBr [26]. When the deposited "lm is heated, the excess bromine is presumably released, which enhances the photoelectron emission probability by improving the lattice structure of the "lm. The heat enhancement observed in this work has a similar spectral dependence to the one observed in Ref.[23]forCsI,CuIandNaIphotocathodes.Interestingly, the enhancement in QE is accompanied by an extension of the photocathode sensitivity to longer wavelengths, which is consistent with the hypothesis of improved electron transport and reduced surface barrier Exposure to air The thick CsBr "lms were found to be rather stable under short exposure to air. 20 min in air inside the chamber did not a!ect the QE; 9 h in air inside the chamber reduced the QE by about 25%; additional 40 min in air outside the chamber further reduced the QE by &30%. This behavior of slower decay inside the evaporation chamber is very similar to the one observed for CsI photocathodes [20]; it is probably due to a `gettera e!ect resulting from alkali}halide deposits on the chamber walls. It has recently been demonstrated [19], using scanning electron microscope (SEM) analysis, that the morphology of thick and thin CsBr "lms is di!erent. It was shown that the grain size is smaller in thin "lms, although it is not clear whether this fact in#uences the photoemission performance of the "lms. However, the authors of Ref. [19] show that a 750 As thick CsBr "lm has a continuous morphology whereas a thin CsBr "lm of 200 As is discontinuous. The authors also demonstrated that, both thin and thick CsBr "lms, when exposed to humidity undergo a drastic morphological transformation, the small grains coalesce into large, separated grains. This is believed to cause the QE decay in air, by the considerable reduction in surface coverage Aging The data from the present study of aging under photon and avalanche-ion #ux is summarized in Table Photon aging Aging of CsBr photocathodes in vacuum under intense photon #ux (Phase 1) occurs faster for thinner "lms. Currently, we do not have an explanation for this phenomenon. The drop in photocurrent, measured along the aging experiment, is consistent with the drop in the QE, measured at intervals along the aging process. The aging of CsBr photocathodes under intense photon #ux in 1 atm CH

12 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} (phase 2) showed practically no degradation in QE up to accumulated charges of 230 and 130 μc/mm for 300 and 5000 As thick "lms, respectively. This observation is in disagreement with the photocurrent measurements during the same aging process, reading an increase to 186% and a drop to 60% of the initial values, for the thin and thick samples, respectively. These results are still not explained. It is interesting to note that a similar trend, of prolonged lifetime under gas as compared to vacuum, was observed by Anderson et al. [24] for CsI photocathodes (see Table 3 and further discussion below) Ion aging CsBr photocathodes aged under gas multiplication (50 Torr CH ) in PP geometry (Phases III and IV) showed a faster decay for 300 As than for 5000 As thick "lms. They also showed a large di!erence in the QE decay rate for di!erent gas gains. A twentypercent loss (TPL) of QE was found at 1.5 and 13 μc/mm for thin and thick photocathodes, respectively, at a gain of 10, in agreement with the photocurrent measurements. A TPL of QE was found at considerably larger accumulated charge values, 171 and ''174 μc/mm, respectively for thin and thick photocathodes, at a gain of 10. This is in disagreement with the photocurrent measurements that showed a faster drop for the thinner "lm and an increase followed by a drop for the thicker one. This behavior is not quite understood. In principle, the disagreement between absolute QE measurements and photocurrent measurements could be related to a possible change of the detector gain (the photocurrent is a product of the QE and the gain). However, unlike thin multiplying anode wires known to increase in diameter due to polymerization under intense irradiation that results in decrease of gain, a PP geometry is known to be stable. In addition, the above disagreement does not persist, but appears only in some cases and not in others. We may argue that at a gain of 10 the chamber had some transient instabilities due to defects in the cathode or to local upcharging, inducing current #uctuations (Such #uctuations were also reported by Va'vra et al. [27] and by Krizan (private communication)). This could presumably explain the discrepancy between true QE variations and the measured photocurrent variations. At a gain of 10 such instabilities probably do not exist and hence the QE and current measurements agreement. It is interesting that a thin "lm ages faster than a thick one also under ion bombardment. The nature of the ion aging is however not clear. Obviously, the avalanche ions sputter the cathode surface and cause modi"cation of its surface. However, the faster decay under gain of 10 compared to that at 10 seems to contradict our intuition, because we would have expected larger sputtering damage to the photocathode under higher electric "eld (more energetic ions). The experimental data may indicate that ion sputtering also rejuvenates the aged photocathode surface, and under some conditions the rejuvenation rate could exceed that of the aging. But unless this hypothesis is further investigated, for example by systematically varying the gas type and pressure and the gas}ions energy, we could not draw any real conclusion on this matter. Rejuvenation of CsI photocathodes was discussed by Anderson et al. [24]. The same ion-induced aging process was carried out under identical conditions on CsI photocathodes of 300 and 5000 As thickness. For both photocathodes there seems to be a faster decay at the lower gain although the di!erence is smaller compared to that discussed for CsBr. A good agreement exists between photocurrent and QE measurements at the higher gain but only a limited agreement is seen at the lower gain. We may compare our results with previously published data on aging of CsI photocathodes, summarized in Table 3 (which is an extension of Table 2 from Ref. [5]). We should note that unlike the present measurements, which were made in situ, all studies quoted in this table were carried out on samples that were exposed to air for a short time prior to the aging process. In Table 3 values of TPL of photocurrent for photon aging of CsI in vacuum vary from 0.1 to 8 μc/mm and in gas they are consistently larger. Anderson et al. [24] indeed measured both in vacuum and in gas, "nding the same trend, similar to our observation on CsBr. Moreover, they found a dependency on the gas type and pressure. They interpreted the photon-induced QE decay by the

13 376 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 Table 3 A summary of photon and ion-induced aging studies of CsI photocathodes in vacuum and gas Author Thickness (As )/substrate Exposed to air Flux Detector Gas, pressure Gain TPL of photocurrent (photon/ (pa/mm) (μc/mm) mm s) QE consistent/ remarks Dangendorf [14] '2000 As /Al 1 min 10 (185 nm) 210 PP CH 20 Torr 1 2 Not provided Torr Same Torr Same Torr Same Anderson et al. [24] 5000/Al 10 min Unknown (180 nm) &70 PP vacuum 1 &0.5 Not provided Same &70 C H 20 Torr 1 4 Same Same &70 CH 20 Torr 1 4 Same Same &70 i-c H 20 Torr 1 4 Same Same 600 i-c H 20 Torr 1 27 Same Same Unknown i-c H 20 Torr Same Lu et al. [25] 5000/Al Shortly Unknown(195 nm) Unknown MW Vacuum (two components) RQE, consistent Same C H 20 Torr (190 nm) RQE, consistent Unknown(180 nm) C H 20 Torr (190 nm) two Only RQE components Same C H 20 Torr (190 nm) two components Krizan et al. [13] 5000/Cu Yes 10(180 nm) 200 MW CH 1atm RQE, consistent 5000/Cu#Sn/Pb Same 200 CH 1atm Same 9000/Cu#Sn/Pb Same 200 CH 1atm Same Va'vra et al. [27] 5000/SS#Al 2}5min 1.210(185 nm) 300 MW CH 1 atm 1 20 Not provided 5000/SS#Al 10(185nm) 16 CH 1atm 10 1 Abs. QE, consistent 5000/Cu#Sn/Pb Same CH 1atm 10 7 Same 5000/Cu#Ni/Au Same CH 1atm Same Rabus et al. [28] 5000/SS#RSG Yes 510 (150nm) 300 PP Vacuum 1 8 Only abs. QE 5000/Cu#Ni/Au #RSG Same Same Vacuum 1 8 Same This work 5000/SS#Al No 1.210(160 nm) 330 PP CH,50Torr Abs. QE, consistent Same 31 CH,50Torr (15% loss) Abs. QE, partial cons. RSG"resin-stabilized graphite.

14 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364} photolysis process, whereby neutral Iodine atoms are created and evaporated from the surface, leaving it rich with Cs. This process, proposed by Dangendorf et al. [14] and studied with X-rayinduced spectroscopy, was not entirely con"rmed. Anderson et al. [24] provide indirect evidence for this process by the recovery of the QE after heating, which presumably evaporates the excess of Cs and returns the surface to its original composition. Under gaseous environment, this process is considerably slowed down, but Anderson et al. do not provide an explanation. We may assume that the gas molecules hitting the surface supply the missing negative charge and thus prevent the Iodine evaporation process. Alternatively, we may speculate that the hydrocarbonic gas is polymerized on the surface and prevents the evaporation of the volatile species. Such a hypothesis could be checked by investigating the aging under other, `non-aginga gases, such as Ar/CO. We believe that the same mechanism, of surface modi"cation by evaporation of one atomic species, could be assumed in the CsBr case. Similar to our data on CsBr, Lu et al. [25] observed a two-component decay, which they attribute to surface (fast) and bulk (slow) aging phases, however, they do not provide direct evidence for this hypothesis. These authors have also observed, in similarity to our data on CsBr, a faster decay at longer wavelengths. Finally, we note that there also seems to be a dependence of the decay rate on the substrate material. Previous data on Ion-induced aging of CsI under gas multiplication of 10 also shows a large spread of TPL photocurrent values, from 1 to 100 μc/mm, which depend on gas type and on sample preparation details. Some authors (Va'vra [27] and Krizan (private communication)) report on a #uctuating (by up to a factor 1.4) behavior of the photocurrent during measurements, and Anderson et al. [24] consistently observe an initial increase of photocurrent by 5}10% before observing a decrease. The sample preparation (substrate) seems to be a very important factor in this aging process. The comparison between photocurrent measurements and that of absolute (or relative) QE is not complete, but seems to be quite consistent whenever exists. Varying decay slopes were observed by Krizan [13] and Lu [25], and a more complicated behavior (initial increase) is indicated in the data of Anderson et al. [24]. We may conclude that although none of the previously published data on CsI can be exactly compared to our results on CsI or CsBr, as they di!er by various experimental parameters (vacuum/gas-type pressure, spectral range of the aging light, light #ux and current density, exposure to air prior to the aging study, photocathode deposition technique, the substrate material and its preparation, etc.), some gross similarities do exist. The photon aging and the ion-induced aging results show a large spread of decay rates, which can be expected in view of the di!erent experimental conditions. In addition, it is not even clear which are the relevant parameters that should be controlled in order to have a reproducible set of results. An example is the sample temperature or the photon #ux, which seem to be very relevant to the aging process [24] but were not controlled in the same way and even sometimes disregarded by the di!erent authors. Another example is gas impurities (e.g. water and oxygen), that might cause a decay of the photocathode by chemically reacting with its surface and thus modifying the electron escape probability [13]. The large spread in the results may also be related to the fact that most of the studies, which are very time consuming, including the present one, (and except those of Anderson et al. [24]) were carried out on a single sample per experimental condition. The sample-to-sample variations are unknown, which makes it impossible to accurately assess any level of agreement between di!erent sets of data. Finally, we would like to point out that the mechanism, or rather mechanisms, responsible for the photocathode aging are not yet fully understood. The aging probably occurs due to an accumulation of several processes, occurring on the surface and in the subsurface layers, which change the electronic structure of the material. There are speculations about the role of charge transport through the layer: according to one hypothesis, in materials of ionic lattice such as CsI and CsBr it may lead to displacement of ions, i.e. build-up of lattice defects. According to another assumption mentioned above, neutralized species (I, Br) are created and are evaporated from the surface, thus modifying the surface stoichiometry and the

15 378 B.K. Singh et al. / Nuclear Instruments and Methods in Physics Research A 454 (2000) 364}378 electron a$nity. Some discussion of the charge transport and its consequences was given by Va'vra et al. [27]. We observe a large di!erence in the TPL values under gains of 10 and 10 in the case of CsBr, but a much smaller di!erence for CsI under the same conditions. This could indeed be related to their di!erent electrical properties } higher resistivity of CsBr compared to CsI. The substrate material and its preparation certainly play a role in the aging process, but this parameter too is not quite clear. We refer the reader to Refs. [12,19] for further information. The role of the gas in slowing down the photocathode decay was mentioned above and the reader may refer to Anderson et al. [24] for more information. 5. Summary We have presented new data on the absolute QE of semitransparent and re#ective CsBr photocathodes. Following a post-evaporation heat treatment, these photocathodes reach 35% QE at 150 nm. They may be attractive for some applications as solar-blind photocathodes. We have presented a new systematic study of the evolution of the QE of CsBr and CsI photocathodes, under photon and avalanche-ion bombardment, carried out for the "rst time in situ. In these aging tests we have measured the photocurrent in a continuous way, and the absolute QE at several time intervals. It is quite clear from our CsBr studies that the photocathode decay deduced from photocurrent measurements is not necessarily equivalent to that obtained by measuring the absolute QE. The "rst is easier to perform, but, in our opinion, it has to be backed by at least a relative QE measurement, in vacuum or in gas, under well-controlled conditions. We have discussed in detail the various phenomena observed in aging studies in this work as well as in previous works on CsI, and indicated some possible physical processes responsible for the experimental observations. However, the whole aging process is still not understood and requires further studies, preferably done in combination with surface analysis that will point more clearly at the nature of the material structure and surface modi"cation. Acknowledgements The work was partly supported by the Israel Science Foundation. A. Breskin is the W.P. Reuther Professor of Research in the peaceful uses of atomic energy. References [1] A. Breskin et al., Nucl. Instr. and Meth. A 442 (2000) 58, and references therein. [2] A. Di Mauro et al., Nucl. Instr. and Meth. A 433 (1999) 190. [3] F. Piuz et al., Nucl. Instr. and Meth. A 433 (1999) 178. [4] J. Seguinot, T. Ypsilantis, Nucl. Instr. and Meth. A 343 (1994) 1, and references therein. [5] A. Breskin, Nucl. Instr. and Meth. A 371 (1996) 116, and references therein. [6] A. Laikhtman et al., Appl. Phys. Lett. 84 (1998) [7] E.A. Taft, H.R. Philipp, J. Phys. Chem. Solids 3 (1957) 1. [8] A. Buzulutskov et al., Nucl. Instr. and Meth. A 400 (1997) 173. [9] E. Shefer et al., Nucl. Instr. and Meth. A 419 (1998) 612, and references therein. [10] E. Shefer et al., Nucl. Instr. and Meth. A 433 (1999) 502. [11] A. Breskin et al., Nucl. Instr. and Meth. A 343 (1994) 159. [12] J. Almeida et al., Nucl. Instr. and Meth. A 367 (1995) 337. [13] P. Krizan et al., IJS Report, IJS-DP-7087, October [14] V. Dangendorf et al., Nucl. Instr. and Meth. A 308 (1991) 519. [15] G.W. Fraser et al., ESA SP-356 (1992) 97. [16] C. Lu, K.T. McDonald, Nucl. Instr. and Meth. A 343 (1994) 135. [17] A. Buzulutskov et al., J. App. phys. 77 (1995) [18] A. Buzulutskov, A. Breskin, R. Chechik, Nucl. Instr. and Meth. A 372 (1996) 572. [19] T. Boutboul et al., Nucl. Instr. and Meth. A 438 (1999) 409. [20] A. Buzulutskov et al., Nucl. Instr. and Meth. A 371 (1996) 147. [21] A. Buzulutskov, A. Breskin, R. Chechik, J. Appl. Phys. 81 (1997) 466. [22] A. Di Mauro et al., Nucl. Instr. and Meth. A 371 (1995) 137. [23] A. Buzulutskov et al., Nucl. Instr. and Meth. A 366 (1995) 410. [24] D.F. Anderson et al., Nucl. Instr. and Meth. A 323 (1992) 626. [25] C. Lu et al., Nucl. Instr. and Meth. A 366 (1995) 60. [26] Handbook of Chemistry and Physics, R.C. Weast (Ed.), Chemical Rubber Co., Cleveland, 1972, p. 13}81. [27] J. Va'vra, A. Breskin, A. Buzulutskov, R. Chechik, E. Shefer, Nucl. Instr. and Meth. A 387 (1997) 154. [28] H. Rabus, U. Kroth, M. Richter, G. Ulm, J. Friese, R. Gernhauser, A. Kastenmuller, P. Maier-Komor, K. Zeitelhack, Nucl. Instr. and Meth. A 438 (1999) 94.