Caesium bromide X-ray photocathodes

Nuclear Instruments and Methods in Physics Research A256 (1987) 401-405 North-Holland, Amsterdam

401

Letter to the Editor

CAESIUM B R O M I D E X-RAY P H O T O C A T H O D E S G.W. FRASER, J.F. PEARSON

a n d J.E. L E E S

X-ray Astronomy Group, Department of Physics, University of Leicester, Leicester LE1 7RH, England Received 3 November 1986

The soft X-ray detection efficiencies of CsI- and CsBr-coated microchannel plates are compared. CsBr is the more efficient photocathode material for X-ray wavelengths - 20-100 ,~.

The soft X-ray response of CsI photocathodes has recently received considerable attention [1-5]. CsI has come to be regarded as an optimum photocathode material for a number of applications, not least the coating of microchannel plate (MCP) detectors for X-ray and X U V astronomy [3-5]. Recently, however, She Yong-zheng et al. [6] have reported relative photocurrent measurements from normal density CsI and CsBr transmission photocathodes which indicate that CsBr is the more efficient photocathode in the 8.3-44.7 ,~ band. In this letter we present the first absolute efficiency measurements from a CsBr-coated M C P and compare them with measurements previously obtained from a channel plate identically coated with CsI [3]. Table 1 compares the physical properties of CsI and CsBr with those of CsC1. Caesium chloride was initially included in our experimental comparisons by virtue of the high photoyields reported by Eliseenko et al. [8] and modelled in ref. [2]. Trial evaporations of CsC1 into glass substrates, however, t u m e d opaque much more rapidly than films of either CsI or CsBr on exposure to laboratory air. Because of this more pronounced susceptibility to water vapour (note the final column of table 1) no M C P was coated with CsC1 in the current investigation. As indicated by the melting points and boiling points

of table 1. CsBr and CsI exhibit very similar evaporation characteristics. Heating currents of 55-65 A were sufficient in both cases to produce the desired (low) evaporation rates (20-40 ,~ s - l ) . The IR spectroscopy grade CsBr * was preheated in a lidded molybdenum boat to drive off water vapour. After drying, 14000 ,~ CsBr was deposited onto one-half of the rotating ( - 1 Hz), heated (100 o ) surface of a vacuum-baked [3] Mullard * * M C P with zero degree bias angle channels. The plate diameter was 36 mm. The coating angle (subtended by the centre of the boat at the centre of the MCP) was 4 °. The diameter of the coated semicircle was 30 mm. The CsBr photocathode, therefore, was geometrically identical to the CsI coating evaluated in ref. [3]. After coating, the CsBr-coated plate was calibrated as the front plate of the two-stage M C P detector described in ref. [3] and elsewhere. Handling of the coated plates always took place in an atmosphere of dry nitrogen. Figs. l a - e compare absolute quantum efficiency measurements for both the coated and uncoated halves * Procured from BDH Ltd., Broom Road, Poole, Dorset BH12 4NN, England. ** Mullard Ltd., 4, New Road, Mitcham, Surrey CR4 4XY, England.

Table 1 Physical properties of CsCI, CsBr and CsI Material

Density [g/cm 3]

Effective atomic number

Melting point [ o C]

Boiling point [ o C]

Solubility (g/100 g H 20) (at 25 o C)

CsCI CsBr CsI

3.99 4.44 4.51

47.0 47.5 54.0

645 636 621

1290 1300 1280

189 124 a) 92 b)

a~ Interpolated from data at 0.90°C. b) Interpolated from data at 0.61°C. 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Fig. 1. (a) Measured q u a n t u m detection efficiency vs angle of X-ray incidence. Si K X-rays, X = 7.1 ,~. Filled circles: CsBr-coated MCP. Open circles: uncoated MCP. Crosses: CsI-coated M C P [3]. (b) As fig. la; A1 K X-rays, X = 8.3 A. (c) Cu L X-rays; k = 13.3 ]k. (d) C K X-rays; X = 44.7 ,~. (e) B K X-rays; X = 67 ,~.

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of the present MCP with the CsI yields of ref. [3]. Measurements of Q~ were made at five wavelen~gths (?,) - 67 A (B K), 44.7 A (C K), 13.3 A (Cu L), 8.3 A (A1 K) and 7.1 A (Si K) - generated by appropriate coating and filtering of an electron bombarded anode. An argon-methane single-wire proportional counter was used as a reference detector [3]. The efficiency of the CsBr-coated channels is a factor of 2 higher than that of the CsI-coated plate at large angles of X-ray incidence, O, at the longer wavelengths. This is in agreement with She Yong-zheng et al. [6]. At 8.3 and 13.3 A, however, the present (pulse) quantum yields, measured in reflection, contradict the transmission photocurrent data of

ref. [6]; our measurements indicate a slight superiority for CsI over CsBr, rather than vice versa. For CsBr at 44.7 and 67 A, the measured peak efficiencies approach the open area of the MCPs (63%). No field-defining grid was used in the present experiments to collect electrons released from the interchannel web [5]. X-ray photoemission from the alkali halides is dominated by the emission of low-energy ( - eV) electrons [1,2]. This secondary electron yield may be characterised by three parameters, in addition to the X-ray linear absorption coefficient of the material, ~. We denote these parameters Ps(0), Ls, and c [2]. P~(0) and L s together define the probability of electron escape

404

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Table 2 Secondary electron parameters of CsCI, CsBr and Csl M aterial

P~(0)

L~ [,~]

e [eV]

Ref.

Comments

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0.24 a)

240

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211

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0.20 a)

215

7

[2]

Estimates from modelling of electron-induced secondary electron emission and X-ray photoyield data Modelling of electron-induced secondary electron emission data. Value for ~ is a lower limit; equal to band gap plus electron affinity As for CsC1. Band gap plus electron affinity, 6.4 eV

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Table 2 compares secondary electron p a r a m e t e r values for CsI (and CsC1) derived in the X-ray photoemission analysis of ref. [2] with those for CsBr derived by Grais a n d Bastawros [9], using similar methods. The secondary electron parameters of CsI and CsBr are seen to be rather similar. M u c h greater differences (times

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G. W. Fraser et al. / CsBr X-ray photocathodes

three) are observed between the calculated linear absorption coefficients of the two materials (fig. 2). In fact, the measured wavelength dependence of the largeangle efficiency ratio, Q~[CsBr]/Qc[CsI], follows rather closely the calculated trend of/x[CsBr]/~[CsI]. Figs. la, b, for example, show clearly the influence of the bromine L edges. For Si K X-rays, above the bromine LII and LIII edges in energy, the CsBr photoyields exceed those of CsI. For A1 K X-rays, below all the bromine L edges, both the linear absorption coefficient and measured efficiency of the CsBroPhOtocathode are less than for CsI. Longward of 20 A (iodine MIV, MV edges), the much larger absorption coefficient of CsBr translates into the much increased efficiencies seen in figs. ld, e. Based on the measured 70-250 A absorption coefficients of Cardona et al. [10], CsBr should in fact be the better photocathode of the two materials out to 100 ]k. At UV wavelength [11], longward of 1700 A, the response of CsBr is known to fall much more rapidly than that of CsI (compare c values in table 2). In summary, we have shown that CsBr is a soft X-ray photocathode of comparable sensitivity (and, in the limited experiments performed so far, comparable stability) to CsI. In the 20-100 A band, in fact, CsBr is clearly the more efficient of the two. Given its relative UV insensitivity, noted above, CsBr may become the coating material of choice for certain microchannel plate applications in astronomy.

405

Acknowledgement The authors wish to acknowledge the financial support of the Science and Engineering Research Council. References [1] B.L. Henke, J.P. Knauer and K. Premaratne, J. Appl. Phys. 52 (1981) 1509. [2] G.W. Fraser, Nucl. Instr. and Meth. 206 (1983) 251 and 265. [3] G.W. Fraser and J.F. Pearson, Nucl. Instr. and Meth. A 219 (1984) 199. [4] C. Martin and S. Bowyer, Appl. Opt. 21 (1982) 4206. [5] M.A. Barstow, G.W. Fraser and S.R. Milward, Proc. SPIE 597 (1985) 352. [6] She Yong-zheng, Yang Xiaowen and Ding Yishan, Adv. Electron. Electron Phys. 64B (1985) 541. [7] Handbook of Chemistry and Physics, 65th ed., R.D. Weast (CRC Press, Boca Raton, FL, 1985). [8] L.G. Eliseenko, V.N. Shchemelev and M.A. Rumsch, Sov. Phys. Tech. Phys. 13 (1968) 122. [9] K.1. Grais and A.M. Bastawros, J. Appl. Phys. 53 (1982) 5239. [10] M. Cardona, R. Haensel, D.W. Lynch and B. Sonntag, Phys. Rev. B2 (1970) 1117. [11] E.A. Taft and H.R. Phillipp, J. Phys. Chem. Sol. 3 (1957) 1. [12] D.T. Cromer and D. Liberman, LASL report LA-4403 (1970).