The dependence of quantum efficiency of alkali halide photocathodes on the radiation incidence angle

Transcription

1 Proceedings SPIE, vol. 3765, Denver Colorado (1999) The dependence of quantum efficiency of alkali halide photocathodes on the radiation incidence angle A.S. Tremsin, O.H.W. Siegmund Experimental Astrophysics Group Space Sciences Laboratory UC Berkeley Berkeley, CA 9472 ABSTRACT We studied variation of quantum efficiencies of CsI, KBr and KI-evaporated reflective planar photocathodes with the angle of radiation incidence (±55 degrees) in the spectral range of nm. The photocurrent increases with the photon incidence angle for short wavelengths (by as much as ~6% at 55 o for 25-3 nm illumination), while it changes only ~5% at wavelengths ~9-115 nm. The theoretical calculations of the photocathode angular response based on the absorption length of the photons and the escape length of the photoelectrons are in a relatively good agreement with the measured data. A detailed study of the detection efficiency angular variation for the microchannel plates with CsI, KI and KBr photocathodes in the spectral range of nm is also presented. Heat annealing of the planar photocathodes did not result in any significant variation in their angular response. Keywords: Alkali Halide Photocathodes, Quantum Efficiency, Microchannel Plates 1. INTRODUCTION Alkali halide films have proved to be efficient photoconverters in the far ultraviolet and soft x-ray range 1,2,3. Their high quantum efficiency (QE) and a relatively high stability when exposed to ambient air allows these photocathodes to be widely used in x-ray and UV photon imaging devices. However, for detectors operating in a wide range of input angles, variation of photocathode quantum efficiency with the incidence angle may result in significant nonlinearities in the detector performance (e.g. Refs. 2,3,4 ). As reported previously by Mine et.al 5, the quantum efficiency of a flat CsI photocathode decreased with an increase of the photon incidence angle in the spectral range of nm. The photocurrent dropped by as much as 3% for an incidence angle of 55 o at a wavelength of 21 nm. However, the theoretical predictions of Fraser 6 and Lu 7, based on the absorption length of UV photons and the escape length of photoelectrons, are in contradiction with the results of Ref. 1. On the other hand, Lopez and Conde 8 observed a 3% increase in QE for a photocathode with 9- and 12- degree wedges relative to a flat one, although the efficiency was not measured as a function of wavelength. The efficiency of a flat cesium-telluride photocathode for 265 nm radiation was also observed to increase by as much as 2-3% at 3 o incidence relative to normal 4. Furthermore, a number of angular response studies were performed on photocathode-coated microchannel plate (MCP) detectors 2,3,9,1, and the sensitivity of interchannel web area was also observed to increase with the incidence angle (for example, see Fig.16 in Ref. 9 ). The above mentioned disagreements in the results on the photocathode angular response may be explained by the differences in the polycrystalline structure of the evaporated alkali halide films, revealed by the recent microscopic surface studies 11,12. The characteristics of this structure depend on the type of substrate and deposition procedure, e.g. the grain size may vary between.3 and 3 µm. This paper presents a systematic study of the angular dependence of the quantum efficiencies of CsI, KI and KBrevaporated photocathodes in the reflective mode and the spectral range of nm (Section 2), thus for CsI extending the - 1 -

2 previous studies 5 to shorter wavelengths. The results of our measurements are compared to the theoretical calculations based on the QE model suggested by Fraser 6, Section 2.4. In Section 3 we present our measurements of the QE angular dependence for photocathode-coated microchannel plates. In this case the angular variation of quantum efficiencies of both photocathode and MCP itself contribute to changes in detection efficiency. The absolute QE values versus the incidence angle are presented for CsI, KI and KBr-coated MCPs in the spectral range of nm. 2. PLANAR REFLECTIVE PHOTOCATHODES 2.1. Photocathode preparation The photocathodes were vacuum deposited on 4x4 cm 2 polished stainless steel substrates. A high purity (99.999%) photocathode material was evaporated at a rate of 3 Å sec -1 in a vacuum system at 1-6 Torr. Prior to deposition, the material was heated behind a shutter for several hours to outgas any absorbed water. During the evaporation the substrates were rotated at.5 rev/sec. All coatings were ~8 Å thick. After the deposition, the chamber was purged with dry nitrogen and then the photocathodes were exposed to air (with a relative humidity of <5%) for several minutes during the transfer and installation into the calibration chamber. Fig. 1. A schematic view of the experimental setup Measurement technique Once the substrates have been coated, they were mounted on the input of a custom-made electrometer with a sensitivity of about 5x1-16 A. A 9% transmissive nickel mesh was installed ~3 mm in front of the cathodes for the photoelectron collection. The electrometer with the samples was then mounted on a manipulator with a rotation stage, which allowed rotation of ±6 degrees with ~1 degree accuracy, Fig. 1. All the reported measurements were performed at pressures of about 2x1-7 Torr. A positive voltage of ~5 V, which was initially verified to be sufficient for the plateau regime of the photoelectron collection, was applied to the mesh. Monochromatic radiation (25-2 nm, with a beam of 4 mm in diameter at - 2 -

3 the photocathode) was provided by a gas discharge hollow cathode source in combination with a 1 m grazing incidence monochromator. The absolute quantum efficiency was determined from the ratio of the observed photocurrent from the samples to the flux measured by the NIST-calibrated standard photodiodes. The angular dependence of the mesh transparency was calibrated prior to all QE measurements and was subsequently used to normalize the incidence flux variation. The measurements with each photocathode material were repeated on several samples in order to eliminate the effects of different deposition runs Experimental results Fig. 2 shows the absolute values of the photocathode quantum efficiencies obtained at normal incidence. Following these measurements, some CsI and KBr samples were heated to 6-7 degrees for several hours in order to study the heat enhancement of photocathode sensitivity observed by several groups 13,14. After the samples cooled down to room temperature, the QE measurements were repeated (our more detailed study on the QE variation after heat treatment is reported elsewhere 15 ), and no significant change in the photocathodes angular response was found. QEs of KI and KBr photocathodes remained almost unchanged, while CsI photocathode QE increased by ~2% at 18 nm. The latter fact is in disagreement with Refs. 13,14, where a substantial increase of CsI QE was observed. On the other hand, Kononenko et. al. 12 also did not observe any enhancement. The recent data of Boutboul et. al. 16,17 showed that heat annealing did not result in any variation of the absorption length and the electron escape length in CsI photocathodes. This suggests that post-evaporation heating either lowered the electron affinity or induced the removal of residual water and/or other molecules from the surface, which improved the electron emission. However, the desorption of water molecules can not solely explain the QE enhancement, otherwise KI, which is the most sensitive to water vapor exposure among the materials studied, should have exhibited the largest increase in sensitivity. We believe that the discrepancy in the heat enhancement effect may be attributed to the differences in the deposition techniques, namely the quality of vacuum in the deposition chamber (in our case it was an order of magnitude higher, 1-6 Torr, than in the experiments of Buzulutskov et. al. 13, 1-7 Torr), and pre-deposition treatment (Fraser et. al. 14 used substrate baking and plasma discharge cleaning). 1 Quantum Efficiency CsI KI KBr Wavelength (A) Fig. 2. The quantum efficiency of CsI, KBr and KI planar reflective photocathodes (8 Å thick, stainless steel substrate) as a function of wavelength for normal incidence. The separate data points in Fig. 3.a-Fig. 3.c show the results of our angular QE variation measurements, normalized to the efficiency at normal incidence. The quantum efficiency increases substantially with angle for 256 and 34 Å wavelengths (and also 43 Å for CsI), reaching the value of about 6% at incidence angles of 55 o. However, at longer wavelengths the efficiency does not increase as much and even drops by 5-1% for KI and KBr cathodes at 1152 Å. The angular variation of detection efficiency could not be measured in our experiments for longer wavelengths due to the fact that the absolute value of the photocurrent was then beyond the sensitivity of our electrometer

4 Relative photocurrent variation (%) CsI Å 43 Å 48 Å 52 Å 1152 Å 1339 Å Angle of incidence (degrees) Fig. 3.a. The angular variation of CsI photocathode quantum efficiency at different wavelengths normalized to the efficiency at normal incidence. The separate points comprise the data measured with 8 Å thick photocathodes evaporated on stainless steel substrates. The curves represent the results of calculations with the parameters described in the text (Section 2.4). Relative photocurrent variation (%) KI Å 34 Å 43 Å 52 Å 834 Å 988 Å 1152 Å Angle of incidence (degrees) Fig. 3.b. The angular variation of KI photocathode quantum efficiency at different wavelengths normalized to the efficiency at normal incidence. The separate points comprise the data measured with 8 Å thick photocathodes evaporated on stainless steel substrates. The curves represent the results of calculations with the parameters described in the text (Section 2.4)

5 Relative photocurrent variation (%) KBr Å 34 Å 43 Å 52 Å 834 Å 988 Å 1152 Å 1339 Å Angle of incidence (degrees) Fig. 3.c. The angular variation of KBr photocathode quantum efficiency at different wavelengths normalized to the efficiency at normal incidence. The separate points comprise the data measured with 8 Å thick photocathodes evaporated on stainless steel substrates. The curves represent the results of calculations with the parameters described in the text (Section 2.4) Calculations and discussion The curves in Fig. 3.a-Fig. 3.c represent the angular QE dependence calculated using the model of Fraser 6, which for the normalized angular QE variation is expressed as follows: QE( α) 1 R( α) 1+ µ Ls = QE( normal incidence) 1 R( normal) cos( α ) + µ L s µ 1 1 exp T + cos( α ) Ls 1 1 exp T µ + Ls where α and α' are, respectively, the incidence and refraction angles relative to the photocathode normal, R(α) is the specular reflection coefficient, µ is the linear absorption coefficient of the photocathode material, L s is the secondary electron escape length and T is the photocathode thickness. The value of R(α) was calculated according to Fresnel reflection coefficient equations (e.g. Ref. 18 ) using the complex refraction index values (n,k) for KBr, KI and CsI from Refs. 19,2. We used the relation between α' and α defined in Ref. 18. µ was calculated from µ = 4 ð k / ë, where k is the imaginary part of the refraction index, ë is the radiation wavelength. It should be noted, that the refraction index constants in Ref. 2 were reported for a bulk material rather than evaporated films of polycrystalline structure, which may lead to a discrepancy in the optical constants 21. The linear absorption coefficient µ (and corresponding k) for CsI at 34 Å was taken from Ref. 22 as 3x1 4 cm -1, and the missing in Ref. 2 values of refraction index for CsI at 48 and 52 Å were interpolated from other wavelengths. The escape length L s for CsI at 1339 Å was measured and theoretically predicted to be about ~32 Å 17,23, and at 34 Å estimated to be about ~26 Å 3. L s values at 43, 48 and 52 Å were chosen so as to fit experimental data - 3, 3 and 32 Å, respectively. At those wavelengths both single photoelectron and multiple photoelectron events are present, which correspond to different electron energies and thus different escape lengths L s. Therefore the values of L s should be considered as averaged estimates. The photoelectron escape length L s for KBr was estimated in Ref. 24 to be of the order of hundreds of angstroms. The values which fit our experimental data are as follows: (1) - 5 -

6 L s = 88, 15, 38, 34, 28, 3, 12 and 3 Å for 256, 34, 43, 52, 834, 988, 1152 and 1339 Å radiation, respectively. In case of KI photocathode we used L s = 1, 1, 25, 35, 3, 35 and 3 Å for 256, 34, 43, 52, 834, 988 and 1152 Å photons, respectively. The strong angular dependence of QE at wavelengths shorter than ~4 Å, accompanied by a drop in absolute values as compared to longer-wavelength radiation, Fig. 2, can be explained by the increase of the photon absorption length. For wavelengths longer than ~4 Å, the angular variation of sensitivity decreases with wavelength, provided the absorption length does not change significantly, except for the cases corresponding to the photon energies of ~ n the bandgap + the electron affinity (e.g. 834 Å at KBr, which is close to 2xthe bandgap + the electron affinity, 15.6eV or 795 Å). The absolute value of quantum efficiency also drops at these energies 24, since the excited low-energy photoelectrons have a lower escape probability 23. It should be noted here, that the calculations of angular QE variation of evaporated alkali halide photocathodes from formula (1) with crystal optical constants can not be extended to very large incidence angles relative to normal. In that case the results of calculations are very sensitive to the reflection coefficient values, which should be calculated with evaporated film (rather than crystal) optical constants. The use of the optical constants measured with a crystal material may lead to a large discrepancy between the experimental and calculated data, since the evaporated layers feature polycrystalline granular structure, and therefore at large incidence angles (relative to normal) their reflectivity is very different from that of a crystal. 3. PHOTOCATHODE-COATED MCP ANGULAR RESPONSE Alkali halide opaque photocathodes are commonly used for improvement of quantum detection efficiency (QDE) of microchannel plates. A wide range of materials for MCP coating has been studied to date, providing an optimal selection of the photocathode material for specific spectral range requirements. In this section we continue the investigation of the QDE of a photocathode-coated MCP as a function of the incident radiation angle 2,3,9,1,24. The optimization of GALEX 25 and COS NASA flight detectors in order to meet the sensitivity requirements became a motivation for the present work, as was the recent availability of MCPs with a relatively high bare-qe, which required evaluation of their angular response. Besides, we intended to compare the results of our planar reflective photocathode studies with the data obtained with MCPs, in particular the contribution of the interchannel web photoelectrons to the total MCP QDE. From the previously reported measurements (e.g. Ref. 1 ), it is well known that the proportion of this contribution generally increases with the increase of photon wavelength from EUV to VUV. In our experiments we measured the sensitivity of both MCP channels and the interchannel web area as a function of the incident radiation angle. In these studies we used Photonis MCPs, 33 mm in diameter, 1 µm pores on 12.5 µm centers, 8:1 L/D, resistance ~3 MΩ, 13 o channel bias, stacked in Z configuration. The front MCP in each triple stack was coated with a CsI, KI or KBr ~9 Å thick photocathode. The photocathode deposition procedure was similar to that used for the planar sample preparation (Section 2.1). The same QE calibration chamber was used for MCP tests. A calibrated 9% transmittance photoelectron repelling mesh was placed in front of the MCPs. During the total QE measurements, a negative voltage of 2-3V relative to MCP input was applied to the grid, which provided the electric field repelling the photoelectrons from the inter-channel web area back to the channels. Changing this voltage to a small positive value relative to the MCP input provides an electric field which accelerates the web photoelectrons towards the mesh, thus excluding the contribution of interchannel photoelectrons to the total sensitivity. This allowed us to separately study the MCP angular response for the photocathode inside MCP channels and for the photocathode on the interchannel web area. The detector was mounted on a four-axis manipulator table, which permitted both rotational and translational movements. All the results of angular dependence presented below were normalized to the value of the repelling mesh transmittance, which varies with the detector rotation. The microchannel axis orientation was determined by finding the detector position where the count rate drops to the minimum value. This corresponds to a situation where the photons strike the channel wall far from the MCP input, resulting in the detector gain drop, which, in turn, leads to the situation where a number of events fall below the low level discriminator threshold

7 Fig. 4 shows the absolute QDE values for CsI, KI and KBr-coated MCPs measured at 17 degrees relative to MCP normal (3 degrees relative to channel axis). QDE as a function of the incident radiation angle is presented in Fig CsI KI KBr Wavelength (A) Fig. 4. The absolute quantum detection efficiency of CsI, KBr and KI-coated microchannel plates as a function of wavelength measured at 17 degrees relative to MCP normal (3 degrees relative to channel axis) CsI Å 34 Å 1152 Å 1335 Å 156 Å Fig. 5.a. The quantum efficiency versus the graze angle to the channel axis for a 9 Å thick CsI opaque photocathode on a microcahnel plate. The legend shows the radiation wavelengths corresponding to each curve

8 .6.5 KI Å 34 Å Fig. 5.b. The quantum efficiency versus the graze angle to the channel axis for a 9 Å thick KI opaque photocathode on a microcahnel plate. The legend shows the radiation wavelengths corresponding to each curve Å 736 Å 1152 Å 1335 Å 156 Å 171 Å KBr Fig. 5.c. The quantum efficiency versus the graze angle to the channel axis for a 9 Å thick KBr opaque photocathode on a microcahnel plate. The legend shows the radiation wavelengths corresponding to each curve. It is clearly seen from these figures, that for different photon energies the maximum sensitivity of the detector is achieved at different incidence angles. The short wavelength angular dependence exhibits QDE peak at angles of 15-2 degrees relative to the channel axis (for 25-3 nm photons), while the longer energy photons are detected with the most - 8 -

9 efficiency at ~3 degrees. Typical measured contributions of the interchannel web photoelectrons to the total QDE are presented in Fig. 6. Although we could observe the angular variation of the web sensitivity, the accuracy of our web photoelectron contribution measurements was not sufficient for the quantitative comparison with the results obtained for the planar photocathode samples. The planar sample data suggests that the largest angular variation of the web contribution should be observed at small wavelengths ( Å). However, at these wavelengths the proportion of the interchannel photoelectrons contributing to the total detection efficiency is rather small and that imposes stricter requirements on the accuracy of the measurements, which was not high enough in our experiments CsI, 256 Å Total Channels Web Total Channels Web CsI, 171 Å Fig. 6.a. Dependence of the quantum efficiency on the graze angle to the microchannel axis for a CsI photocathode (9 Å thick) at wavelengths of 256 and 171 Å Total Channels Web KBr, 256 Å Total Channels Web KBr, 156 Å Fig. 6.b. Dependence of the quantum efficiency on the graze angle to the microchannel axis for a KBr photocathode (9 Å thick) at wavelengths of 256 and 156 Å

10 4. CONCLUSIONS The results of the current study of the quantum efficiency variation with the angle of radiation incidence for CsI, KBr and KI-evaporated photocathodes show that for wavelengths shorter than ~4 Å the QE increases substantially with the incidence angle. The increase in QE is less pronounced between 4 and 9 Å and the quantum efficiency even drops with angle by ~1% in KI and KBr cathodes for 1152 Å photons. The model of QE angular dependence based on the absorption length of photons and the escape length of photoelectrons appears to be in a good agreement with the measured data, although the photoelectron escape lengths L s for the evaporated films yet have to be measured in order to verify the parameters used in our calculations. Our further studies of the photocathode-coated MCP quantum detection efficiency variation with the angle of radiation incidence determined the optimal input angles for the spectral range of Å. Based on these results, the microchannel plates with 19 degree (maximum available from the manufacturer) channel bias were chosen for the future NASA space flight GALEX 25 and COS missions. 5. REFERENCES 1. A. Breskin, "CsI UV photocathodes: history and mystery", Nucl. Instr. and Meth. A 371, (1996). 2. D.G. Simons, G.W. Fraser, P.A.J. De Korte, J.F. Pearson, and L. de Jong, "UV and XUV quantum detection efficiencies of CsI-coated microchannel plates", Nucl. Instr. and Meth. A 261, (1987). 3. S.R. Jelinsky, O.H.W. Siegmund, and J.A. Mir, "Progress in soft x-ray and UV photocathodes", Proc. SPIE 288, (1996). 4. S.M. Johnson, Jr.,"Ultraviolet angular response of cesium-telluride photocathodes", Appl. Opt. 31, (1992). 5. Ph. Mine, G. Vasileiadis, G. Malamud, D. Vartsky, A. Breskin, A. Buzulutskov, and R. Chechik, "Incident angle effect on the quantum efficiency of CsI photocathodes", Nucl. Instr. and Meth. A 36, 43-1 (1995). 6. G.W. Fraser, "The characterisation of soft X-ray photocathodes in the wavelength band 1-3 AA", Nucl. Instr. and Meth. 26, (1983). 7. C. Lu, K.T. McDonald, "Properties of reflective and semitransparent CsI photocathodes", Nucl. Instr. and Meth. A 343, (1994). 8. J.A.M. Lopez, C.A.N. Conde, "VUV efficiency of chevron type CsI vacuum photocathodes", Journal of Optics 24, (1993). 9. G.W. Fraser, M.A. Barstow, J.F. Pearson, M.J. Whiteley, and M. Lewis, "The soft X-ray detection efficiency of coated microchannel plates", Nucl. Instr. and Meth. A 224, (1984). 1. O.H.W. Siegmund, E. Everman, J.V. Vallerga, and M. Lampton, "Extreme ultraviolet quantum efficiency of opaque alkali halide photocathodes on microchannel plates", Proc. SPIE 868, (1987). 11. J. Almeida, A. Braem, A. Breskin, A. Buzulutskov et. al., "Microanalysis surface studies and photoemission properties of CsI photocathodes", Nucl. Instr. and Meth. A 367, (1995). 12. W. Kononenko, N.S. Lockyer, "Substrate studies of cesium-iodide photocathodes", Nucl. Instr. and Meth. A 371, (1996). 13. A. Buzulutskov, A. Breskin, and R. Chechik, "Heat enhancement of the photoyield from CsI, NaI and CuI photocathodes", Nucl. Instr. and Meth. A 366, 41-2 (1995). 14. G.W. Fraser, S.E. Pearce, J.F. Pearson, V.N. Schemelev, A.P. Pavlov, and A.S. Shulakov, "Thermally annealed soft X- ray photocathodes", Nucl. Instr. and Meth. A 381, (1996). 15. A.S. Tremsin, O.H.W. Siegmund, "Heat enhancement of radiation resistivity of evaporated CsI, KI and KBr photocathodes", Proc. 2nd International Conference "New Developments in Photodetection", Beaune, France, June 21-25, 1999 (to be published in Nucl. Instr. and Meth. A). 16. T. Boutboul, A. Breskin, R. Chechik, A. Braem, and G. Lion, "Ultraviolet photoabsorption measurements in alkali iodide and cesium bromide evaporated films", J. Appl. Phys. 83, (1998). 17. T. Boutboul, A. Akkerman, A. Breskin, and R. Chechik, "Escape length of ultraviolet induced photoelectrons in alkali iodide and CsBr evaporated films: Measurements and modeling", J. Appl. Phys. 84, (1998). 18. B.L. Henke, "Ultrasoft X-ray reflection refraction and production of photoelectrons (1-1-eV region)", Phys. Rev. A2, (1972)

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