Nuclear Instruments and Methods in Physics Research A 392 (1997) 310-314 NUCLEAR

ELSEVIER

INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH

SecrtonA

A LaF, : Nd( 10%) scintillation detector with microgap read-out for the detection of y-rays

gas chamber

J. van der MareP**, V.R. Born”, C.W.E. van Eijk”, R.W. Hollander”, P.M. Sarrob

a Radiation Technolog)> Group, Facula of Applied Ph.wics. Delft Unioersity of Technology, c/o IRI, Mekelweg 15. NL-2629 JB De& Netherlands

b De& Institute of Microelectronics and Submicrontechnologv, Delft Unioersity of‘ Technology, De@ Netherlands

Abstract A LaF3: Nd(lO%) scintillator crystal has been placed in a microgap gas chamber to obtain a position-sensitive

detector for y-rays. Directly on the crystal a thin layer of nickel is evaporated, and on top of that a thin semi-transparant Csl photocathode. II-rays absorbed in the scintillator crystal produce a UV-light flash, which liberates electrons in the photocathode. These electrons are multiplied and detected in the microgap chamber. By comparing the spectrum

measured when this detector is irradiated with 511 keV y-rays with a spectrum that is computed in a Monte Carlo simulation, it is concluded that the probability that a UV-light photon created in the scintillator produces a photo-

electron in the photocathode. is about 2.5%.

1. Introduction

The Microgap Chamber (MGC) was introduced in 1993 by Angelini et al. [l] as an alternative to the

Microstrip Gas Chamber (MSGC). Both types of gas- eous detectors drew a lot of attention for applications in high-energy physics for the accurate position-sensitive detection of charged particles. Also the detection of (soft)

X-rays was investigated [2,3]. A problem in detecting X-rays is that the detection medium (the gas) has a low

density which results in a poor efficiency for higher-en- ergy X-rays. Attempts have been made to increase the efficiency by using high-Z gases like xenon and by press- urizing the gas [4]. However, for the detection of y-rays with energies above 100 keV this is not sufficient.

Inorganic scintillators are efficient media for the ab- sorption of y-rays. The light pulses from a scintillator can be measured with the help of a photo-sensitive material.

Caesium iodide (CsI) is an oxide-resistent photo-sensitive material that can be used for a photocathode in a gaseous detector. Its combination with a multi-wire proportional chamber (MWPC). MSGC or MGC yields a position- sensitive photodetector that can cover large areas at a reasonable cost. Reflective CsI photocathodes are inves- tigated quite thoroughly in MWPCs (see e.g. Refs. [5,6]). However, in MGCs and MSGCs it is difficult to use

*Corresponding author.

reflective photocathodes. Only semi-transparent photo- cathodes can be used. With a semi-transparent photo- cathode, also a good optical coupling between the scintillator and the photocathode can be established. Some research has been performed on its semi-transpar- ant photocathodes [7,S] and on its application in MSGCs [9] and MGCs [lO.ll]. Because of the better stability of the MGC with respect to the MSGC, the

MGC is more appropriate. In this research, a lanthanum fluoride scintillator crys-

tal with a semi-transparent CsI photocathode on top of it, is coupled to an MGC. The lanthanum fluoride crystal is doped with 10% neodynium (LaF,:Nd(lO%)). The emission spectrum of this scintillator matches the sensi- tivity of CsI rather well. It is placed in an MGC to test its ability to detect ;+rays. The combination of microgap

plane and photocathode is called a microgap photomul- tiplier (MGPM). Monte Carlo simulations have been performed to be able to explain the measured spectra.

2. Set-up

The set-up of the total test detector is schematically shown in Fig. 1. It consists of the following main parts: scintillator, photocathode, and microgap plane.

The photocathode is placed on the scintillator. The distance between the photocathode and the microgap plane is 4mm. The whole detector is mounted in an

0168-9002/97/$17.00 Copyright c; 1997 Elsevier Science B.V. All rights reserved PII SO168-9002(97)00237-4

J. van der Mare1 et al. 1 Nucl. Instr. and Meth. in Phys. Rex A 392 (1997) 310-314

otocathode

Fig. 1. Schematical view of the detector set-up.

aluminium box with a beryllium entrance window.

On the back side of the box read-out electronics are

mounted.

2.1. Scintillator and photocathode

The LaF, : Nd( 10%) crystal is one out of a series doped with various concentrations of Nd3+ ranging from 0.1 to 15 mol%. This crystal has the highest light yield

in the VUV of this series [12]. The strongest emission lies around 173nm and has an intensity of 290 f 60 photons/MeV. There are also emission lines in the visible

part of the electromagnetic spectrum. The total yield of photons is 570 & 120 photons/MeV. In Fig. 2 the emis- sion spectrum as well as the quantum efficiency of

a semi-transparent CsI photocathode made on quartz are shown. The decay time of the scintillator is 5-6ns. Due to impurities, the crystal has some intrinsic activity.

The crystal has a diameter of 10 mm and a thickness of 2 mm. It is glued in a stainless-steel disk using Torr Seal (see Fig. 3). Because CsI has a high resistivity, it cannot be deposited directly onto the surface of the scintillator. The photocathode would be charged up by the emission of photoelectrons and would show an unstable behav-

iour. To provide some conductivity, a ring of NiCr is evaporated on the SS disk just overlapping the transition from metal to crystal. Over the crystal and the disk, a layer with a thickness of 5nm nickel is evaporated,

which has a transmission of =60%. On top of this the final photocathode is deposited: a layer of 12.5 nm CsI ‘. Data on the quantum efficiency of this particular photo- cathode are not available.

In the detector box, the disk is placed on an aluminium carrier with conducting rubber strips to allow a good electrical contact and to prevent damage of the photo- cathode surface.

’ The metal films and the CsI are evaporated by Delft Elec- tronische Producten in Roden. The Netherlands.

150 200 250 300

wavelength (nm)

Fig. 2. The emission spectrum of the LaF, : Nd( 10%) scintilla- tion crystal (from Ref.[lZ]) and the quantum efficiency of CsI (from Ref.171). The quantum efficiency is of the CsI only (after correction for absorption in the carrier material).

Ton Seal , , LaF,:Nd(lO%)

NiCr / l5 nzZ$ nm Csi

Fig. 3. The detailed lay-out of the scintillator and the photo- cathode. The units are in mm.

2.2. Microgap plane

The microgap plane is made on a silicon wafer and has a size of 1 x 1 cm’. The silicon is first passivated with a 0.7um thick thermally grown oxide layer. Onto this, aluminium cathode strips are placed. The silicon dioxide

strips between the anode and cathode have a thickness of 3 urn. The microgap plane can be read out in two dimen- sions: both anode and cathode have a pitch of 200um. A more thorough description of the MG planes can be found in Refs. [13,14].

In this research the MGPM is operated with a mixture of 60% neon and 40% dimethylether (DME). This gas provides the highest gas gain [3] and due to the low Z of the atoms gives little interaction with the y-rays. The microgap plane is operated, in most cases, at an anode voltage of 440V (cathode grounded) and a drift field of 2.5 kV/cm or 5 kV/cm. The gas gain is, respectively, 2.5 x lo3 or 3.0 x 103.

IX. SCINTILLATORS

312 J. mm der Maw1 et al. :I Nucl. IIISII.. and Meth. in PII~s. Res. A 392 (1997) 310-314

0 5 10 15 20 25 Primary electrons

Fig. 4. Pulse height spectrum measured with the MGPM with and without ‘2Na source.

3. Singles measurements with 22Na

The MGPM with scintillator has been irradiated with a ““Na source. The spectrum is shown in Fig. 4. The pulse height has been converted into the number of primary electrons leaving the photocathode. For com-

parison, the spectrum registered without a source is plot- ted as well. This spectrum is due to the intrinsic activity

of the scintillator. The spectra can be clearly distin- guished. In the spectrum with source, two parts can be

distinguished besides the noise peak: from 3 electrons to 12 electrons and from 12 electrons to about 100 electrons. The last part is ascribed to ionisation by fast electrons from the walls of the gas volume and from the gas itself that are the result of interactions of ;+rays. The first part is from the scintillation light.

4. Coincidence measurements with 22Na

4.1. Set-up

A “Na source produces y-rays of 1275 keV and posi- trons that annihilate into two collinearly emitted 511 keV y-quanta. To eliminate the contribution of the intrinsic activity of the scintillator and to lower the noise contri- bution of the electronics, the MGPM is placed in a coin- cidence set-up with a photomultiplier tube with a 2” NaI : Tl scintillator on top of it. The NaI : Tl crystal is shielded by a lead shield in which a hole with a diameter of 1 cm is made. When the photomultiplier registers a 511 keV y-quantum, the gate for the MGPM is opened. If the gate is open and the MGPM gives a signal above a threshold, this event is placed in a pulse height spec- trum. The PMT is read-out with a charge sensitive pre- amplifier, which is coupled to a spectroscopy amplifier with a shaping time of 1.5 ps. Because of the low signal- to-noise ratio of the MGPM it was difficult to adjust the

0 5 10 15 20 25

Primary electrons

Fig. 5. Pulse height spectrum measured with the MGPM in

a coincidence set-up with a “Na source. For comparison the

spectrum in which the MGPM and the gate PMT are not

aligned is shown.

gate accurately. Therefore, a wide gate of 5 ps was used to be sure that the MGPM signal fell within the gate.

4.2. Measurements

The spectra measured in the coincidence set-up are

plotted in Fig. 5. Also in these measurements a long tail can be seen in the spectrum, again from ionisation by fast electrons in the gas. From the situation in which the MGPM and the gate PMT are not aligned a spectrum is plotted as well. The electronic noise in the detector system is responsible for this peak. The spectra can be distinguished clearly from each other. When the ‘2Na source is removed, no gate signal is created, so the spectrum remains empty. It can also be seen in Fig. 5 that a higher drift field results in a somewhat

higher number of photoelectrons from the photocathode. This is in agreement with the observations of Breskin [15]. The measurement times for all measurements are

the same.

5. Monte Carlo simulations

Monte Carlo simulations have been performed to be able to estimate the light conversion efficiency. The simu- lation program GEANT developed at CERN is used as the basis for these simulations.

The geometry shown in Fig. 3 is used to compute the distribution of the energy absorbed in the scintillator. In the real set-up a gate-PMT was used which was posi- tioned at a relatively large distance from the LaF3 scintil- lator, so, only absorbtion events in the centre of this crystal were registered. In the simulations, y’s with an energy of 511 keV are shot into the direction of the center of the crystal.

J van der Mare1 et al. 1 Nucl. Instr. and Meth. in Phys. Rex A 392 (1997) 310-314 313

It is assumed that 290photons/MeV are produced in

the scintillator. For every energy loss step during the absorption process of the y-ray in the scintillator the

mean number of photons is computed. With this number a Poisson distribution is made from which a random number is drawn. The random number is the number of photons produced in that step. For every ;‘-ray absorp- tion event the number of photons is summed.

From the interaction points the produced photons

travel in all directions. Just a part of them goes directly in the direction of the photocathode. Then the probability

that they pass the Ni-film and reach the CsI is = 60% (the transmission of the Ni). The probability that a photon is converted into an electron in the photocathode is z 20% (see Fig. 2). Most photons that go in other directions are lost. Taking all these factors into account a conver- sion efficiency between 3% and 6% can be expected for a fresh photocathode. During the measurements the quantum efficiency of the photocathode decreased to about 60%, so the real conversion efficiency will be even lower.

The creation of a photoelectron by an individual photon, the conversion efficiency (the probability that

a photon reaches the photocathode multiplied by the quantum efficiency of the photocathode), can be seen as a binomial process with a relatively low probability due to the circumstances mentioned before. For the simulation a conversion efficiency of 2.5% is as- sumed.

The photoelectrons are multiplied in the MGPM. Ac- cording to Bellazzini [16], the statistics of a single elec- tron avalanche follow a Polya distribution with a vari- ance of 0.39. Every photoelectron is amplified separately. The spectrum presented in Fig. 6 shows the primary electron distribution including the statististics in the am-

plification.

Primary electrons

Fig. 6. Spectrum of the MGPM in a coincidence set-up with the noise subtracted and the spectrum computed by the Monte

Carlo simulation.

6. Discussion and conclusion

In Fig. 6, the spectrum measured with the MGPM in the coincidence set-up with the noise subtracted is plot- ted as well. Electronic noise has not been included in the simulations. It particularly influences the low channels in the spectrum. In Fig. 6 a gap can be seen in the spectrum of the MGPM at the position of the noise peak. Small differences in the measurement times can be the reason

for this. If the MGPM spectrum of Fig. 6 is thought to be

continuous at the position of the gap, it has roughly the same shape as the spectrum from the Monte Carlo simu- lation. It can be concluded that the conversion efficiency of 2.5% chosen for the simulation is in the same order of

magnitude as that of the measurement. The electronic noise is emphatically present in the

spectra of the MGPM. The signal-to-noise ratio of this device can be improved by applying higher gas gains and less noisy electronics. Using a shorter gate signal can

result in slightly lesser noise, but is difficult to adjust

accurately because of the low signal-to-noise ratio. A reduction can also be obtained by lowering the detector capacitance. Higher gain and lower detector

capacitance require modification of the microgap plane design and are subjects in our ongoing research on MGCs.

Considering the small number of electrons at the photocathode, the position resolution that can be achieved will be of the order of magnitude of the light spread in the crystal at the photocathode plane, i.e. of the order of magnitude of the thickness of the scintillation crystal.

It has been proven that the MGPM with a CsI

photocathode can be used to read-out LaF, : Nd(lO%) scintillation crystal. However, the light yield of the LaF3: Nd(lO%) scintillator is much too low for many applications.

References

111

PI

131

c41 c51 C61 c71

@I

F. Angelini et al., Nucl. Instr. and Meth. A 335 (1993)

69.

C. Budtz-Jorgensen et al., Nucl. Instr. and Meth. A 310

(1991) 82.

J. van der Mare1 et al., Proc. Int. Workshop on Micro-

Strip Gas Chambers, Lyon, France, 30 November-2

December, 1995. eds. D. Contardo and F. Sauli (Institut de

Physique Nucleaire de Lyon, 1995) p. 33. SE. Baru et al., Ref. [3]. p. 27.

F. Piuz, Nucl. Instr. and Meth. A 371 (1996) 96.

A. Breskin, Nucl. Instr. and Meth. A 371 (1996) 116. C. Lu et al., Nucl. Instr. and Meth. A 343 (1994)

135. G. Charpak et al., Nucl. Instr. and Meth. A 333 (1993)

391.

IX. SCINTILLATORS

314 J. VCIII dev Marel et al. / Nucl. Instr. arid Meth. in Phw. Res. A 392 (19Y7) 310-313

[9] K. Zeitelhack et al.. Nucl. Instr. and Meth. A 371 (1996)

57.

[lo] J. van der Mare1 et al., Proc. Int. Conf. on Inorganic

Scintillators and their Applications (SCINT95), Delft, The

Netherlands, August 1995, eds. P. Dorenbos and C.W.E.

van Eijk (Delft University Press, 1996) p. 583.

[ll] F. Angelini et al., Nucl. Instr. and Meth. A 371 (1996)

358.

[12] P. Dorenbos et al., J. Luminescence 69 (1996) 229.

1131 J. van der Mare1 et al., Proc. Int. Workshop on Micro-

Strip Gas Chambers, Legnaro, Italy. October 1994, eds.

G. Della Mea and F. Sauli (Edizioni Progetto, Padova,

1995) p. 100.

[14] J. van der Mare1 et al., Nucl. Instr. and Meth. A 367 (1995)

181.

[15] A. Breskin et al., Nucl. Instr. and Meth. A 344 (1994) 537.

[16] R. Bellazzini et al., La Rivista del Nuovo Cimento 17

(1994) 1.