*Corresponding author. Tel.: 612 626 8917; fax: 612 624 4578;e-mail: prisca@mnhep.hep.umn.edu.

Nuclear Instruments and Methods in Physics Research A 418 (1998) 300—305

Studies of hybrid photomultiplier tubesin magnetic fields up to 5 Tesla

P. Cushman!,*, A. Heering!, A. Ronzhin"

! Department of Physics and Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN 55455, USA" Fermi National Accelerator Laboratory, Batavia, IL, USA

Received 18 February 1998; received in revised form 8 May 1998

Abstract

Single and multi-channel hybrid photodiode tubes were operated inside a very high magnetic field of up to 5Tesla. Theeffects of focussing and image displacement on crosstalk and gain were examined as a function of angle in the B-field.Brightening of the response in the illuminated pixel, due to improved focussing of the backscatter was observed.Increased path length through the threshold layer of the photodiode decreased the response at large angles. ( 1998Elsevier Science B.V. All rights reserved.

PACS: 85.60.DW; 85.60.HA; 85.60.Gz; 29.40.Vj

Keywords: Calorimetry; HPD; Magnetic fields; Phototube

1. Introduction

The photodetector for the CMS hadron calori-meter (HCAL) must operate in a 4-Tesla field. Sincethis requirement eliminates most photomultiplierand electrostatically focussed devices, the photo-detector selected for the CMS HCAL is a hybridphotomultiplier tube with a multi-pixel photodiodeas the target [1], also called hybrid photodiodes(HPD). An HPD consists of a photocathode, fol-lowed by a small gap across which a high appliedelectric field accelerates the photoelectrons, and

finally a silicon PIN diode which converts thephotoelectrons to electron—hole pairs at the rate ofa pair per 3.6 eV. This structure is in principleunaffected by magnetic fields which run parallel tothe tube axis. Studies of similar tubes at moderatemagnetic fields confirm this [2,3]. However, it wasimportant to test HPD’s at the high fields expectedin CMS for the following reasons. Commerciallyavailable tubes do contain some amount of mag-netic material, e.g. kovar. At such high fields,Lorentz forces in magnetic material and eddy cur-rents in various elements could make the tubestructurally unsound after repeated cycling. It isalso important to understand any second ordereffects which could affect the resolution or calib-ration of the calorimeter read out by such devices.

0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 9 1 - 3

1Delft Electronische Producten, The Netherlands.

Fig. 1. The test rig inside the MRI magnet.

For multi-channel tubes, such as will be used byCMS, we will need to align the tubes parallel withthe B-field. When designing for mechanical toleran-ces, we need to know just how well the alignmentmust be made before image displacement begins tocreate crosstalk in the neighboring channel, andwhether the walls of the tube have a focussing effecton the edge pixels. The HPD’s investigated in thisstudy were supplied by DEP.1 We studied a singlechannel device, a 7-channel device in which theindividual pixels were wire-bonded to the outputpins, and a 25 channel device with bump-bondedpixels.

2. Experimental setup

The magnet used for the tests was a 21 cm boreMRI magnet with 5Tesla field, shimmed to a uni-formity of 0.3%. Our devices were inserted alongthe central axis of the 3.3m long magnet at a loca-tion 35 cm from the center of the magnet, and werethus subjected to a field of 4.8Tesla for most of thetests. The field lines were still parallel to 50mrad atthis point.

Blue light from an LED was piped to the tube viaa 4m long, 250lm diameter silicon fiber. The endof the fiber was mounted in a delrin cookie, fly-cutand polished, and the assembly was spring loadedagainst the window of the tube, which was a fiber-optic faceplate. There was therefore negligible op-tical crosstalk before the photocathode. The fiberassembly could be scanned horizontally and verti-cally across the tube in situ. The complete assemblywas mounted on a rotating arm which enabled usto change the angle of the tube axis with respect tothe axial B-field of the solenoid from 0° to 45° ineither direction. A diagram of the tube test setup isshown in Fig. 1.

The led was operated in both AC mode and DCmode. In the AC mode, a pulse height within a 1 lswide gate was recorded by a LRS 3001 multi-channel analyzer in V-mode with 1mV/channel.With the tube at 8 kV, the internal gain was 1850

and the preamplifier gain was 1250mV/pC, provid-ing a signal at the preamp output of 0.4mV perphotoelectron. The Ortec 450 research amplifierwas generally operated at a shaping time of 250 nsand an effective gain of 75. The rise and fall time ofthe AC pulse (without amplifier), as monitored ona scope, showed no change to within &1 ns be-tween field on and off conditions. In the DC mode,the current was measured directly by a Keithley617 picoammeter. A DC scan of the central 3.3mmwide square pixel (25 pixel tube) for three differentmagnetic field conditions is presented in Fig. 2. Theresolution is dominated by the size of the inputfiber.

The response, R, of the tube represents the con-version efficiency from photons to electrons (effec-tive quantum efficiency) multiplied by the gain, G.

R"N(c)*/]QE

%&&]G.

Since the target of the hybrid tube is a simple PINdiode with unity gain, the gain of the tube itself islinear with respect to the applied high voltage»

A11above some threshold voltage »

T)3which

P. Cushman et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 300—305 301

Fig. 2. Uniformity scan of central pixel using 250lm fiber un-der three different configurations: B-field parallel, at a 45° angle,and off.

Fig. 3. Confirmation that DC and AC methods yield the samegain.

depends on the surface treatment of the photo-diode:

G"

1

3.6 eV(»

A11!»

T)3) for »

A11'»

T)3.

The number of photoelectrons (N(c)*/]QE

%&&) can

therefore be isolated from the actual gain by divid-ing through by the calculated gain using the ex-trapolated x-intercept of the response curve for»

T)3. Once this is known, we can also plot absolute

gain curves. The effective quantum efficiency of theHPD can also be computed if the input number ofphotons, N(c)

*/, is known. A calibrated single

photon counting tube provided N(c)*/. The gain

curves for both AC and DC modes are identicaland are shown in Fig. 3.

3. Response as a function of B-field angle

3.1. Image displacement

The HPD had an applied electric field of 10—30 kV/cm when operated at the voltages and gapdistances used in these tests. The magnetic field is4.8Tesla. Comparing the strengths of the E andB fields with proper units we find cBAE. Thismeans that the photoelectrons basically follow heli-cal paths around the B-field lines as the E-fieldalong the tube axis is tilted with respect to themagnet axis. Since the average initial transverse

momentum of the photoelectron is very small, theradius of its helical path causes negligible blurring.There is a slight drift transverse to the E—B plane,but for the fields and accelerations in this applica-tion, the maximum transverse drift is calculated tobe about 50lm at a 45° angle between E and Bvectors.

In order to show that simple displacement of theimage is the main cause of changes in response andincreased crosstalk in multi-pixel tubes operated instrong magnetic fields, we performed the followingtest using the 7- and 25-channel tubes. The tubewas aligned with the magnetic field, the fiber wascentered on a middle pixel, and the DC responsewas measured. The angle between E and B fieldswas then increased in 5° intervals. Each time theangle was changed, the fiber was repositioned hori-zontally to find the new center of the responseplateau between the edges of the 3.3mm pixel. Thedisplacement from the original position was re-corded as x and is plotted with respect to thetangent of the E—B angle in Fig. 4. The slope of theresulting straight line therefore yields the accelerat-ing gap, d, between photocathode and diode assum-ing that tan h"x/d for the simple drift picturepresented above. The fitted value for the gap wasd"2.22$0.01mm for the 25-channel tube and5.27$0.08mm for the 7-channel tube. This is con-sistent with the manufacturer’s measured gapwidths. A vertical scan was also done, but revealedno change in the transverse position at any angleup to 45°, within the 150lm accuracy of the setup.

302 P. Cushman et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 300—305

Fig. 4. Simple image shift is predominant effect, even at 5Tesla,for both 7-pixel and 25-pixel HPDs. Slope gives the gap betweendiode and photocathode.

Fig. 5. Reduction in response as a function of the increase inpathlength through the inert surface layer.

3.2. Second order effects

There are slight differences in the central pixelresponse which are not accounted for by simpleimage displacement. The signal increases when theB-field is turned on and parallel to the E-field anddecreases as the angle between the two vectorswidens. This effect is obvious in the pixel scan ofFig. 1. One should also note that simple focussingand crosstalk arguments cannot account for thesechanges, since the edges of the pixel scan do notshow any significant broadening for zero B-fieldand high angle conditions.

The reduction of the signal at the repositionedcenter of the pixel as a function of angle can beattributed to an increased path length through theinert surface layer on the diode. Assuming that thereduction is simply given by the change in pathlength times a constant K, allows one to deducethat the change in response:

*R"K[»T)3(h)!»

T)3(0)]

"KC»T)3(0)A1

cos h!1BD

and, indeed, plotting *R against ((1/cos h)!1)gives a straight line in Fig. 5 whereK"(1/3.6 eV)(N(c)

*/]QE

%&&).

The response at each angle was measured fora range of accelerating voltages to obtain response

curves and gain curves for each angle. Note that theslopes of the response curves do not depend on theangle of the B-field (see Fig. 6a). Since the slope ofR is given by the effective quantum efficiency, iden-tical slopes indicate that the number of photo-electrons reaching the silicon is the same; and it isonly the change in »

T)3(corresponding to inert

material) that reduces the signal at high angles.The response and gain curves at 4.8T can also be

compared to the case with magnetic field off(Fig. 6b). In this case, the slope of R does changewhile the threshold remains the same, indicating anincrease in the number of photoelectrons reachingthe silicon. Following this lead, we did a careful setof runs at low light levels (averaging 3 photo-electrons) with the field off and on. The HPD canresolve individual photoelectron peaks up to 6with our electronics (see Fig. 7). There is a clearenhancement in the average number of photo-electrons reaching the centered pixel when the mag-netic field is applied. Although magnetic focussingof the primary photoelectron cloud is ruled out bypixel scans of nearby neighbors, we do observe adepletion in the rest of the pixels. The reduction inthe total current from all the other pixels is roughlyequal to the increase observed in the centered one.The increase in the center at 4.8 T was about 5%.This is most easily explained as a focussing ofbackscattered electrons from the silicon. Since theballistic path of the scattered electrons depends on

P. Cushman et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 300—305 303

Fig. 6. Gain and response curves: (a) for different B-field angles and (b) for magnet on and off.

Fig. 7. ADC pulse height spectrum from central pixel. Magneton produces more 4,5,6 pe events and less 1,2 pe events. Magneton improves resolution of peaks.

their initial energy and direction, and their max-imum range is approximately twice the accelerationgap [4], they will be sprayed across the whole faceof the tube when there is no applied magnetic field.

In addition, since backscatter does not come inunits of photoelectrons, the photoelectron peaks inFig. 7 widen when B"0, as expected for such anenergy loss mechanism [5,6]. Thus, the energy res-olution of the tube is improved by placing it ina high magnetic field, since most of the backscat-tered electrons are recaptured in the same pixel.

4. Conclusion

We have completed a series of tests to confirmthe operating behavior of commercially availableHPDs under magnetic fields up to 5Tesla. Despiteeddy currents and forces experienced by the tubesas they were repeatedly inserted into the solenoidand rotated, they continued to operate stably (al-though relaxation times on the order of a few min-utes were sometimes required for transient currentsto die out). The predominant effect was translation

304 P. Cushman et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 300—305

of the image when the tube’s E-field was not paral-lel to the applied B-field. Two second order effectswere also observed: (1) enhancement of the signalwhen the magnetic field is applied, due to focussingof backscattered electrons and (2) reduction of thesignal at large angles due to increased path lengththrough the silicon passivation layer. We repeatedthe tests on edge pixels as well as central pixels inthe 5]5 array, and their behavior was completelyconsistent. The actual shape of the AC pulse, asmeasured on a digital scope, showed no changefrom 0 to 5Tesla. There were no difference in gainwhether the signals were processed in DC mode bya picoammeter, or in AC mode with a 1ls gate andvoltage sampling.

Acknowledgements

Special thanks to Professor Bruce Hammer forassisting us with the MRI magnet in conjunction

with the Center for Interdisciplinary Applica-tions in Magnetic Resonance at the University ofMinnesota. This work supported in part by theDepartment of Energy under contract DOE-ER-40823.

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356.[5] T. Tabarelli di Fatis, Nucl. Instr. and Meth. A 385 (1997)

366.[6] E. Chesi et al., Nucl. Instr. and Meth. A 387 (1997) 122.

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