Nuclear Instruments and Methods in Physics Research A344 (1994) 435-437North-Holland

Letter to the Editor

Measurement of photomultiplier quantum efficiency

P. Besson, Ph. Bourgeois, P . Garganne, J.P . RobertCE Saclay, DAPNIA /SED, 91191 Gef sur Yvette Cedex, France

L. Giry, Y. VitelLaboratoire des Plasmas Denses, Uneversite P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France

(Received 21 October 1993)

The absolute quantum efficiency of two Philips XP2020Q photomulttpliers and one Hamamatsu R2059 photomultiplier aremeasured in the VUV range for three wavelength values, (193, 201, and 253 nm). We used a deuterium lamp for which the absoluteradiance has been calibrated ; the wavelength range is selected by means of optical filters .

During the last few years, we have studied CsIphotocathodes coupled to gaseous detectors with theaim to use these photoconverters in fast RICH detec-tors . In particular, we have measured the absolutequantum efficiency of these photocathodes and theexperimental method is described in ref. [1] .

In this paper, we point out the necessity of knowingwith precision the number of photons impinging on theCsI photocathode . This value is obtained by means of aphotomultiplier (Philips XP2020Q), for which we mea-sure the gain G by recording the single photoelectronspectrum [1,2] . It is also necessary to know the quan-tum efficiency s of this photomultiplier; this parameterwas up to now taken from the manufacturer's typicaldata curves [2,31 . The collection efficiency r7 of the firststage was assumed to be 100% .

Recently, a publication by Dorenbos et al . [4], con-cerning the PMTs Philips XP2020Q, and HamamatsuR2059, showed a considerable difference in the spec-tral range of interest (i .e . 185-220 nm) between themanufacturer's typical curve and experimental resultsobtained by this group using a calibrated tube (ThornEMI 9426). In particular, we note at A = 190 nm, adiscrepancy greater than 100% for the XP2020Q PMT.If we use this value, all the results of ref. [1] would bedramatically changed. Therefore we decided to makean absolute measurement of our PMT's quantum effi-ciency.

The main part of the setup, shown in Fig. 1 consistsof a deuterium lamp for which the absolute spectralradiance d3P/dSdA d .f2 has been calibrated [5] forthe central part of the light source (Fig . 2). An imageof this central part is formed by means of a CaFZ lens(f = 100 mm) on a very small collimator (0 = 100 p,m) .

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NUCLEARINSTRUMENTS&METHODSIN PHYSICSRESEARCH

Section A

Fig . 1 . Schematic diagram of the experimental setup .

The solid angle .f2 is determined by the diaphragmplaced near the CaF2 lens (O = 2.5 mm), and is ,f2 =1.39 x 10-4 sr . A bandwidth optical filter (ORIEL) isplaced between the collimator and the PMT. In addi-

Fig . 2 . Radiance of the central part of the deuterium lamp vswavelength .

436

N =SlS

a2- f AP(A)t l(A)t f(A) dA

[photons/s],hc n ,

C20.8-

Ec 0.6-0L

0.21

0.0 ,120

r140 160

P. Besson et at. INucl. Instr. and Meth. in Phys. Res. A 344 (1994) 435-437

tion, and to make sure that nothing but the mean lightbeam hits the PMT, ashutter was inserted between thelens and the collimator. For every experimental pointwith the shutter closed, the PMT current was negligi-ble or zero . The spectral transmissions of the filtersand the CaF2 lens were measured in our laboratory,and are shown in Figs . 3a and 3b . All the elements ofthis optical system were aligned with a laser beam, andfinally, the collimator position was adjusted accordingto the maximum PMT current, in order to ensure aperfect coincidence with the image of the lamp . Allthese devices were installed in an air-tight chamberfilled with nitrogen at atmospheric pressure in order tominimize photon absorption .

For such a setup, we can calculate the photon fluxincident on the PMT in the filter range (A 1 , A Z). It isgiven by :

where: f1 = solid angle, S = collimator angle, P(A) =spectral radiance of the lamp, t l(A) = lens transmis-sion, t f(A) = filter transmission, h = Planck's constant,

200 220 240Wavelength (nm)

Fig. 3. (a) Measured transmission of the filters used forwavelength selection . (b) Measured transmission of the CaF2

lens .

E

0 .30

0.20

0.101

0 .00160 180 200 220 240 260 280

Wavelength (nm)

E

0 .20

0 .10-

(b) R2059

R

E1) = IPMT/NGe .

(A) = f~ 2AN(A) dA/fA2N(A) dA .a 1

XP2020Q

0.00 1180 200 220 240 260 280

Wavelength (nm)

Fig. 4. (a) Quantum efficiency of XP2020Q PMT. Presentwork: (o) XP2020QW, (X) XP2020Q(2). Curve a: resultsfrom ref. [41, curve b: Philips data . (b) Quantum efficiency ofR2059 PMT: Present work : (o) HV = 2500 V, (X)HV = 2400V. Curve a: results from ref. [41, curve b: Hamamatsu curve .

and c = light velocity. The average wavelength for agiven filter is given by :

We therefore obtain the product E71 for the averagewavelength, and for the three filters, by measuring thePMT current IPMT . It is given by :

Where G is the gain of the PMT (measured by thesingle photoelectron method), and e is the electroncharge.We have tested three PMTs with this setup (two

Philips XP2020Q and one Hamamatsu R2059), withthree filters centered on 193, 201, and 253 run. Theresults obtained from these measurements are shownin Fig. 4a (XP2020Q) and 4b (R2059).

Concerning the R2059 tube measurements (Fig . 4b),and in the wavelength range around 200 nm, there is

P. Besson et al. /Nucl. Instr. and Meth . in Phys. Res. A 344 (1994) 435-437

Fig . 5 . Ratio e(XP2020Q)/e(R2059) . Curve a : results fromref. [4], curve b : Manufacturer's data, curve c : present workwith relative measurement . o : present work with absolute

measurement from Figs . 4a and 4b .

no considerable difference between the Hamamatsutypical curve, the results of ref. [4], and our valueswhich must be raised somewhat since we measure theproduct e77 and not directly the value of e . The experi-mental points R2059(1) and R2059(2) are obtainedwith the same tube, but for two different HV values,2500 V (1) and 2400 V (2) . As for the XP2020Qsamples, our measurement of the product e?7 is slightlylower than the Philips typical curve which can be

437

attributed to a value of q on the order of 80% .Anyway, we do not confirm the tremendous increasementioned for small wavelengths by Dorenbos et al .

Furthermore, as shown in Fig. 5, we have plottedthe ratio QE(XP2020Q)/ QE(R2059) obtained by arelative measurement . For doing this, we use a pulsednitrogen-filled UV lamp ; the wavelength is selectedwith a monochromator (Jobin-Yvon model H2O-FUV);the two PMTs are illuminated successively by means ofan optical quartz fiber (SEDI ref . HCG550), while asecond fiber connected to a reference PMT allows oneto monitor the light yield fluctuations of the lamp [1] .We note in the wavelength range of 185-220 nm agood agreement between our values and those given bythe manufacturers, while the results from ref. [4] differby a factor of two .

References

[1] R. Aleksan, P . Besson, Ph . Bourgeois, P . Garganne, J .P .Robert, Nucl . Instr . and Meth . A 340 (1994) 293 .

[2] Data handbook Photomultipliers, Philips Components .[3] Photomultiplier tubes, Hamamatsu Photonics .[41 P . Dorenbos, J.T.M . de Haas, R. Visser, C.W.E . van Eijk

and R.W . Hollander, Nucl . Instr. and Meth . A 325 (1993)367 .

[5] Institut National de Metrologie, CNAM, Paris.