240 Nuclear Instruments and Methods in Physics Research A307 (1991) 240-246 North-Holland

A test of large BaF 2 crystals as an electromagnetic calorimeter

G. Graham and H. Yamamoto The Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA

Received 4 March 1991

Two large BaF 2 crystals (3.6 x 3.6 x 25 cm 3 and 3 x 3 x 20 cm 3) have been tested for use in an electromagnetic calorimeter. Energy resolution and non-linearity critically depend on the uniformity of photon acceptance along the length of a crystal. It is shown that uniformity can be achieved through careful control of wrapping and optical coupling between crystal and photomultipher. Results of a ray-tracing Monte Carlo reproduce the acceptance curves well in various conditions. Also, the timing resolution for minimum ionizing muons has been measured to be 161 _+ 8 ps.

1. Introduction

Recently BaF 2 as a high-densi ty scinti l lat ing mater ia l for an electromagnetic calorimeter has been a t t rac t ing considerable a t ten t ion because of its fast scint i l lat ion componen t [1] with a very small t empera ture depen- dence [2] and its rad ia t ion hardness [3]. In addi t ion to the fast emission c o m p o n e n t (0.6 ns decay time) at 220 nm, however, it has a slow componen t (620 ns decay time) at 310 nm which needs to be suppressed in order to avoid accidental pile-ups in a high-rate envi ronment . This can be effectively accomplished by using a solar- b l ind photomul t ip l ie r which is sensitive mainly to the fast componen t [4].

W h e n blocks of scinti l lator are laid a long the direc- t ion of the beam, and light is collected by photomul t i - pliers coupled to an end of each block, it is impor t an t tha t the light collection efficiency be un i fo rm as a funct ion of longi tudinal position. This is because the

longi tudina l shower f luc tua t ion (about I radia t ion length for electrons and more for pho tons) couples to the non-uni formi ty , giving rise to a cons tan t te rm in energy resolut ion which often is a domina t ing factor at high energies [5]. For the lead-glass array of Fermi lab E731, the effective absorp t ion of abou t 3% per radia t ion length resulted in a 2.5% cons tan t te rm [6]. The main purpose of this s tudy is to f ind out what determines fight accep- tance as a funct ion of posi t ion in scinti l lat ing crystal and to control it. One way is to use a col l imated flux of gamma radia t ion f rom a source such as ~37Cs. This method, however, is no t suited for precision measure- men t for the following reasons. First, at 0.66 MeV, the C o m p t o n effect domina tes the photoelectr ic effect by 6 to 1, and the mean free pa th of scat tered pho tons is several centimeters . This means tha t the energy deposit is no t well localized even after coll imation. Second, the q u a n t u m efficiency of C s - T e pho toca thode is only about 16% at 220 n m [7], so tha t together with the use of a

End View Top View

B F2

I1

/ - - mu;n ~ L i n e a r motion nlatrorm/

Fig. 1. Top and end views of the experimental setup. The muon telescope, consisting of two plastic scintillator "trigger counters", is free to move along the length of the crystal.

0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

G. Graham, H. Yamamoto / Large B a F e crystals as an e.m. calorimeter 241

narrow gate, the photoelectric peak is often too broad to measure the location accurately. These problems are solved by using minimum-ionizing muons. Muons are abundant in the meson area at Fermilab even outside beam areas whenever any of the meson beams are running. The flux is highly parallel and a muon tele- scope can localize the energy deposit precisely along the length of the block. Also, the energy deposit is ap- proximately 20 MeV and monochromatic. The intensity is high enough so that it is possible to make a measure- ment with better than 1% accuracy every few minutes.

2. Experimental setup

Fig. 1 shows the test setup which was placed in one of the control rooms in the meson area at Fermilab. The muon telescope was made of a pair of plastic scintilla- tors, each with a face of 2.8 x 2.8 cm 2 and a thickness of 1 cm, and was attached to a l inear-motion platform. The BaF 2 crystal was coupled to a 1.125 in. diameter Hamamatsu R1802 photomultiplier with a C s - T e solar-blind photocathode and a quartz window. Even though the photomultiplier is "solar-bl ind", a substan- tial level of noise was observed when it was operated with the photocathode exposed to room lamps. There- fore the whole setup was contained in a dark box. The position of the telescope along the crystal was precisely measured by a ruler attached to the platform which extended outside of the box.

Charge pulses from R1802 were analyzed by a LeCroy qvt module interfaced to a minicomputer (mi- croVAX). The standard integration time was 60 ns. Fig. 2 shows a typical pulse height distribution. The width is dominated by the Landau distribution of ionization. By taking an average within a window of 2.5 standard deviations around the peak, typical statistics of about

(xlO 3) 25

20

~o15

o o10

5 : L 0 ~' ' I [ . . . . . [IJll

20 40 60 80 I00 120

PULSE HEIGHT

Fig. 2. A typical pulse height distribution from the BaF 2. The arrow indicates the pedestal.

400 events per two spills resulted in a measurement of 0.8% light yield.

Time resolution of the BaF 2 assembly was extracted by measuring the resolutions of three time intervals oa2, olB, and o2B, where oaB is the standard deviation of the time interval between the signal of the first telescope counter and that of the BaF 2 counter, etc. The time resolution of BaF 2 o B can be obtained using the relation

2o 2 = o12. + o 2 - O 2 1 2 ,

which is correct even if the distributions are non-Oaus- sian as long as the timing fluctuations of each of the counters are independent. We obtained olB = 317 + 5 ps, a2B = 213 + 2 ps, and o12 = 319 + 6 ps; this implies that o B = 161 + 8 ps, where the errors are statistical only and no correction by pulse height was made.

We have tested two BaF 2 crystals: one (manufac- tured by Engelhard) had a dimension of 3.0 x 3.0 x 20.0 cm 3, and the other (supplied by Optovac) 3.6 x 3.6 x 25.0 cm 3. Most of the permutations of the following configurations were studied: The sides of the crystals were either bare or wrapped with white Teflon tape; the end away from the photomult ipl ier was either bare or wrapped with Teflon; and the coupling between the crystals and the photomult ipl ier was either an air gap or UV-t ransmit t ing grease (General Electric Viscasil 600M). The Teflon tape is a good UV-reflecting material and was used at least in double layers to ensure good reflectivity.

3. Ray-tracing Monte Carlo

The ray-tracing Monte Carlo we used simulates the relevant phenomena in the crystal. First, a scintillation point is picked according to a uniform distribution within the fiducial volume in the crystal spanned by the muon telescope. The direction of a scintillation photon is generated uniformly in 4~ and the photon is propa- gated through the crystal with a fixed bulk attenuation length. Snell's law and Fresnel 's equations are used to calculate the behavior of each photon at the boundaries of the crystal. The average of the two polarization states is used to determine the coefficient of reflection (fig. 3); as can be seen in this figure, it is quite satisfactory for qualitative arguments to approximate it with a step function. The Monte Carlo simulates the action of Teflon wrapping by assuming a constant reflectance and randomizat ion of the direction of a photon when it is reflected. Partial optical coupling of the wrapping to the crystal is also taken into account. The photomulti- plier is assumed to be coupled to the crystal by a thin gap of air or optical grease. The indices of reflection for the grease and photomult ipl ier window are also simu- lated. A photon is counted when it reaches the photo- cathode.

242 G. Graham, H. Yamamoto / Large BaF 2 crystals as an e.m. calorimeter

4. Measurements

4.1. Bare crystal

Figs. 4a and 4b show the response as a function of distance from photomult ipl ier for the two BaF 2 blocks. There is no wrapping on the sides or the ends of the crystals, and the coupling between the photomult ipl ier and the block is an air gap in each case. The two solid lines in each plot correspond to the cases in which one

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O(RAD IANS) Fig. 3. Coefficient of reflection as a function of incident angle using Fresnel's equations. The lines (a) and (b) correspond to two polarizations, and the dotted line is the average of the two.

R e g i o n ~ ~

Reg i on C

nx

~ Regi on A

Regi on B Fig. 5. Octant depicting the possible direction n of a photon in the crystal. Regions A, B and C correspond to photons that are not totally internally reflected by surfaces perpendicular to the

x, y, and z directions respectively.

end of block is next to or away f rom the photomult i- plier. The discrepancy between the curves indicates non-uniformity of bulk absorpt ion along the crystal.

The bare crystal with air gap provides a reliable configuration to measure the true absorption length of crystal. In order to unders tand this and other configura- tions, we will discuss the pho ton collection process in some detail.

The direction n of scintillation photons is generated uniformly in 4v sr, and because of symmetry we will consider only one octant with nx, ny, n z all positive (fig. 5). With this convention, a pho ton does not change its position on the sphere no matter how many times it

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c m )

Fig. 4. Pulse height yield as a function of distance of muon telescope from the photomultiplier face for bare crystals manufactured by (a) Harshaw and (b) Optovac; the photomultiplier was coupled with an air gap. The two lines in each plot correspond to the cases in which either one end of a crystal or the other is coupled to the photomultiplier. The dashed lines are Monte Carlo simulations

described in the text.

G. Graham, H. Yamamoto / Large BaF 2 crystals as an e.m. calorimeter 243

is reflected from the walls of crystals, since a reflection only changes the sign of the corresponding component of n. Thus, the photon cannot escape from the crystal through walls perpendicular to x if n x < cos 8 c at gen- eration, where 8c is the min imum angle at which total internal reflection occurs as predicted from Snell's law. Similarly, if ny (or nz) < cos 0c, then it cannot escape through walls perpendicular to y (or z). Thus, photons in the shaded area (region D) in fig. 5 are permanently trapped inside the crystal (until they are absorbed or scattered).

Among photons in the region where n x > cos 0¢ (re- gion A), the ones that start away from the photomulti- plier (nx > 0) mainly exit at the far end. This means that to the first order, only the photons in the region A that start toward the photomultiplier can be detected. Therefore, the effective attenuation length is expected to be approximately (nx ) times the bulk absorption length where ( n x ) is the average of n x taken over the detected photons. This was verified by the ray-tracing Monte Carlo. The Monte Carlo fit (dotted lines in fig. 4) gave a bulk absorption length of 115 cm for the Optovac crystal, and 55 cm for the Engelhard crystal.

One parameter of interest is the ratio of slow to fast components as a function of the scintillation position. Fig. 6 shows the ratio of pulse height with 1/~s gate to that with 40 ns gate as a function of distance from the photomultiplier for the Engelhard crystal. The ratio s low/fas t slightly increases as the distance from the photomultiplier is increased, indicating that the fast component is more absorbed than the slow component. The fraction of the slow component varies from 8 to 9% for the standard 60 ns gate. In the following sections, only the measurements on the Optovac crystal are pre- sented even though measurements are performed on both crystals with qualitatively consistent results.

4.2. Teflon wrapping with air-gap coupling

Fig. 7 shows response curves for cases in which the crystal was wrapped with Teflon and coupled to the

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x (cm) Fig. 6. Ratio of pulse height (1 I~s to 40 ns) as a function of distance of the muon telescope from the photomultiplier face

for the Englehard crystal.

102

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0 OPEN END

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x ( c m ) Fig. 7. Pulse height yield as a function of distance of muon telescope from the photomultiplier face for the case where the crystal sides are wrapped in Teflon, and the photomultiplier

was coupled with an air gap.

photomultiplier with an air gap; one is with open end and the other with a Teflon-wrapped end. When the end is not wrapped, the response is similar to that of a bare crystal. This can be understood as follows: if the Teflon is not optically touching the crystal, then pho- tons generated in the directions where it can escape from side walls (i.e. regions B or C of fig. 5) will always stay in the same region even after it is reflected in a random direction by Teflon and then reenters the crystal. Thus, once generated in regions B or C, the photon can never move to region A where it can be detected by the photomultiplier. Without end wrapping and with an air gap between the photomultiplier and the crystal, the response curve for the case where the side is wrapped with Teflon is similar to that for the case of a bare crystal. In reality, the white Teflon tape is like a woven basket with a structure scale of a few tens of microns. When this is pressed against the crystal it could develop a partial optical coupling. We will come back to this point later.

On the other hand, the photons that are reflected by the Teflon at the far end and that reenter the crystal will always stay within region A. Therefore, they are likely to exit the photomultiplier end. Thus, the ad- dition of end wrapping substantially increases the light output (by about a factor of 1.7 at the center of the block). Also, the response becomes much flatter. In order to understand this, response curves for photons that are reflected at the end and those that are not are shown separately in fig. 8. For the photons that do not hit the far end, the photon acceptance decreases as the

244 G. Graham, H. Yamamoto / Large BaF e crystals as an e.m. calorimeter

distance from the photomultiplier increases. For the photons that hit the far end, however, the acceptance is greater as one moves further from the photomultiplier. This is due to the fact that for those photons the pass length required to reach the photomultiplier is shorter as the generation point moves further away from the photomultiplier. It is this component that is enhanced by the end wrapping.

4.3. Teflon wrapping with grease coupling

The index of refraction of the UV-transmitt ing grease between the crystal and the photomultiplier is close to those of BaF 2 (1.63) and the quartz window (1.53) [8]. This allows photons that are otherwise totally internally reflected at the photomultiplier end to reach the photo- cathode. When the far end is wrapped with Teflon, photons reflected back into the crystal at that end will always stay within region A, and are therefore likely to reach the photocathode with or without the grease. This means that the grease does not significantly affect the acceptance of those photons. On the other hand, pho- tons that are otherwise trapped inside (region D) or exit from the sides (regions B and C) can be detected when there is a grease coupling at the photomultiplier. These photons have a response curve which is steeply peaked toward the photomultiplier quite independently of

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Fig. 8. Monte Carlo simulation of pulse height yield as a function of distance of muon telescope from the photomulti- plier face for the case with the crystal sides and far end wrapped in Teflon, and the photomultiplier was coupled with an air gap. The solid circles (open circles) correspond to photons that are (are not) reflected from the far end. The difference in the signs of the slopes indicates that one can control the overall slope by suppressing one or the other of

these components.

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I TEFLON SIDES, VISCASIL AT PMT • FAR END WRAPED IN TEFLON o OPEN END

,,~I,,,,I,,,,I,,,~I~,,, 0 5 I0 15 2O 25

X ( c m )

Fig. 9. Pulse height yield as a function of distance of muon telescope from the photomultipfier face for the case where the crystal sides are wrapped in Teflon, and the photomuhiplier

was coupled with viscasil.

whether or not there is a wrapping on the far end. Thus, one can expect that the grease will increase the total light yield but will make the response curve steeper.

Actual measurements are shown in fig. 9. The light yield near the center of the crystal is increased by a factor of 1.6 for the wrapped-end case and 2.0 for the open-end case. The slope is indeed steeper in both cases. As expected from the above discussion, the ab- solute difference in pulse height yield between the re- sponse curves for the cases with or without reflector at the far end, which is dominated by the photons that are reflected by the Teflon at the far end, is quite similar for the air-gap coupling case (fig. 7) and the grease coupling case (fig. 9).

4.4. Monte Carlo f i t by a single set o f parameters

In fig. 10, the data in figs. 7 and 9 are simultaneously fit by the ray-tracing Monte Carlo using a single set of parameters. The agreement is in general quite good. The various features discussed above are all well simulated. The parameters used are 115 cm for the true absorption length, 7.5% optical coupling of teflon to BaF 2, and reflectivity of teflon 95%.

The true absorption length is reliably fixed by the fit to the bare crystal case. The overall difference between the response curves for the cases with or without the grease coupling is sensitive to the absorption length and thickness of the grease layer. The existence of optical coupling can be easily seen by holding the crystal under a lamp and observing a breaking of total internal reflec-

G. Graham, H. Yamamoto / Large BaF 2 crystals as an e.m. calorimeter 245

I02 z

L~

2

TEFLON SIDES

• FAR END WRAPPED IN TEFLON

o OPEN END

-- DATA

-- -- MDNTE CARLO

<?.

i

5 10 15 20 25

X crn)

Fig. 10. Pulse height yield as a function of distance of muon telescope from the photomultiplier face for the case where the crystal sides are wrapped in Teflon. Results of Monte Carlo

simulation are overlaid.

4. 5. Partially unwrapped crystal

The case with Teflon end and air-gap coupling (fig. 7) gives a response which is quite flat. It has been noted that it is possible to control the response curve by partial wrapping [9]. In order to make the response more uniform, the side of the crystal was unwrapped by 2.2 cm on the photomult ipl ier end and 0.7 cm on the far end. Fig. 11 shows the result together with the predict- ion by the Monte Carlo. The effect of the partial unwrapping is well simulated. Furthermore, the mea- surement is consistent with being flat within an error of 1.5% on each point.

Note that if the surfaces of the crystal are completely flat and there is no optical coupling between the Teflon and BaF z, then there should be no change by unwrap- ping the Teflon from the sides. Effects similar to those described above, however, can also be caused by imper- fections on the surfaces of the crystal which allow light to escape that would otherwise be totally internally reflected. This possibility was supported by the Monte Carlo, but so far we did not determine the relative importance of these two effects.

tion when pressing a piece of Teflon against the surface. The value 7.5% optical coupling was chosen to fit the partially unwrapped case which is discussed in the next section.

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FAR END WRAPPED IN TEFLON,

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cm) Fig. 11. Pulse height yield as a function of the distance of the muon telescope from the photomultiplier face for the case where the crystal sides and far end are wrapped in Teflon, and the photomultiplier was coupled with an air gap. Teflon on the sides was partially unwrapped to make the final curve flat with

a standard deviation of 1.5%.

5. Conclusion and discussion

We have shown that the light collection inside BaF 2 can be well understood and controlled and that it is possible to make the response curve flat within 1 or 2%. This is adequate to suppress the measurement error of shower energy caused by the coupling of longitudinal shower fluctuation to the non-uniformity of the re- sponse curve. Various features of the response curve such as the effect of an end reflector and grease cou- pling are well reproduced by the ray-tracing Monte Carlo. We have also measured the time resolution of a large crystal and obtained 161 _+ 8 ps. This is without pulse height correction and expected to be improved.

The optical coupling between Teflon and BaF 2 is sensitive to pressure, this means that there is a worry that the property may change when blocks are assem- bled into an array. In order to avoid this problem, other wrapping methods such as Teflon coating on a sanded surface are currently being studied. Also, a 3 x 3 x 2 array (10.8 x 10.8 x 50 cm 3) is under preparation, which should tell us how well such an array can actually perform as an electromagnetic calorimeter.

Acknowledgements

We would like to thank Richard Armstrong, Larry Fiscelli and Elizabeth Pod for valuable technical sup- port. We also have benefit ted from communications with Craig Woody and David Anderson. This work was

246 G. Graham, H. Yamamoto / Large B a F 2 crystals as an e.m. calorimeter

s u p p o r t e d b y N a t i o n a l Science F o u n d a t i o n u n d e r C o n - t ract No. P H Y 88-23033.

References

[1] M. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru and J. Vasher, Nucl. Instr. and Meth. 206 (1983) 169.

[2] P. Schotanus et al., Nucl. Instr. and Meth. A238 (1985) 564; also see ref. [4].

[3] S. Majewski and D.F. Anderson, Nucl. Instr. and Meth. A241 (1985) 76; A.J. Caffery et al., IEEE Trans. Nucl. Sci. NS-33 (1986) 230.

[4] H. Kobayashi et al., Nucl. Instr. and Meth. A270 (1988) 106.

[5] See, for example, R.J. Morrison et al., Nucl. Instr. and Meth. 143 (1977) 311.

[6] J.R. Patterson, Ph.D. Thesis, University of Chicago (1990). [7] Data supplied by Hamamatsu Photonics K.K., Hamamatsu,

Japan. [8] The values are at ~ = 220 nm. They were obtained from

values given for the sodium D line (589 nm) by Particle Data Group, Phys. Lett. B204 (1988), and extrapolating them to 220 nm by n2(c) = 1 + EdEo/(EZo - ~2) with c(eV) =1240 /~ (nm) and E 0 =13.8 eV for BaF 2 and 13.4 eV for quartz [S.H. Wemple, J. Chem. Phys. 67 (1977) 2151].

[9] C.L. Woody, a private communication.