The calibration of a position-sensitive gamma-ray detector for 50–90 MeV photons

Nuclear Instruments and Methods 179 (1981) 39-44 © North-Holland Publishing Company

THE CALIBRATION OF A POSITION-SENSITIVE GAMMA-RAY DETECTOR FOR 5 0 - 9 0 MeV PHOTONS * D.D. LONG, O.P. GUPTA t , D.A. JENKINS Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. and P.T. DEBEVEC University o f Illinois, Urbana, Illinois 61801, U.S.A. Received 5 August 1980

This paper describes the calibration of a position-sensitive gamma-ray detector for use in the 50-90 MeV energy range. At 54 MeV, the energy resolution is 47% and the position resolution is 1.0 cm. The efficiency is independent of the point of photon interaction in the detector within a solid angle of 0.38 st. 1. Introduction

purpose o f this paper is to report on this calibration and its results, particularly with regard to the position sensitivity of the detector (section 5). In addition, we report on the basic detector construction (section 2), the energy calibration (section 3), and the measurement of efficiency and edge effects (section 4).

A lead-glass wire-chamber hodoscope has been constructed for a position sensitive gamma-ray detector in the energy range 5 0 - 9 0 MeV. The technique used is similar to that o f Heusch et al. [1]. Their system, for use in the energy range 0 . 1 5 - 1 GeV, had a position resolution o f the order o f 2 - 4 mm. Gilad et al. [2] have constructed a similar system with a 3.1 m m position resolution at 100 MeV for use in the range 5 0 - 5 0 0 MeV. The present system is capable of use at relatively large solid angles (typically 0.38 sr), and is designed for use in the energy range 5 0 - 9 0 MeV. Its fullwidth-at-half-maximum (fwhm) position resolution is 1.0 cm. The detector system, consisting of two identical arms, is being used to measure the differential cross section near threshold for the (p, n °) reaction on a range of nuclei. The angular distribution of the emitted pions is inferred from a measurement of the angular correlations o f the two decay gammas relative to the incoming beam. We have calibrated this system with a tagged-photon beam produced at the University o f Illinois 100%duty-factor electron accelerator MUSL-II. The main

2. Detector construction Fig. 1 is a scale diagram of one arm of the detector system. The 7-rays are converted in an active F-2 lead-glass [3] converter (AE) 15 cm × 15 cm × 2.5 cm (1 radiation length) thick. Two RCA 8575 photomultiplier (PM) tubes are mounted on one edge of the converter via 2.54 cm long UVT Plexiglas light pipes. The analog signals from the two tubes were added prior to their use for generating a logic signal. Wire chambers (Wxl,Wyl, W x 2 , Wy2) measure the trajectory of the converted electrons. Each wire chamber contains 128 wires spaced 2.54 mm apart, for a total active area 30 cm × 30 cm. Individual wire readout is via a CAMAC system [4] to a PDP-11 computer. The interaction point of a gamma is determined by the paths through the wire chambers of electrons produced in the converter. Four F-2 lead-glass counters (E) 15 cm × 15 cm × 30 cm, optically separated from each other, absorb the remaining shower energy. For one detector arm,

* Work supported in part by the National Science Foundation. t Present address: Singer, Kearfott Division, 150 Totowa Rd., Wayne, NJ 07470, U.S.A. 39

40

D.D. Long et al. / Calibration o f a y-ray detector wyl

3. Energy calibration

wy2

X~--POSITION

Wxl

Wx2

I0 c m

Fig. 1. Scale diagram of each arm of the detector system. C = electron veto counter; zXE = converter; Wxl,Wx2 = wire chambers for horizontal position; W y l , W y z = wire chambers for vertical position; S = scintillator; E = total-absorption Pb-glass counter.

an RCA 4525 Phi tube was glued directly to the rear surface of each quadrant of the E counter. The other arm used RCA 8055 PM tubes. A preamplifier, with gain that could be varied between 10 and 190, was mounted inside the bases for the 4525 and 8055 tubes. The amplifier design was that of Studebaker [5], but was constructed on a circular circuit board for mounting inside each base. Both the amplifier and voltage divider were mounted in a cylindrical aluminium container 6.4 cm in diameter by 14 cm long. The signals from the four E counters were added, and this signal was added to that from the AE counter for a total energy measurement. The 30 cm × 30 cm X 0.635 cm plastic scintillator (S) before the E counter forms a fast coincidence with the 2u~" in the logic. To minimize the count rate in the wire chambers and the scintillation counter, a graphite block in front of the gamma counter assembly stops elastically scattered protons from the target. An 18 cm × 18 cm × 2.54 cm Cherenkov counter [6] (C) between the graphite and the AE counter vetos electrons from gamma conversion in the graphite. One RCA 8575 PM tube was mounted on each S and C counter via UVT Plexiglas light pipes. As an aid in timing the system and in energy calibration, an LED pulser was mounted on each counter. The pulser uses Monsanto MV52 green LEDs, and is a multichannel version [7] o f the design of Ernwein and Gorodetzky [8]. The output, which is fed simultaneously to all LEDs, is digitally programmable for jointly varying all pulse heights under computer control.

The photon tagging system shown in fig. 2 determined the energy of the photons incident upon the detector. Bremsstrahlung was produced by a 10 -3 radiation length A1 radiator placed in the electron beam, and the post-bremsstrahlung electrons were momentum analyzed by a magnetic spectrometer. Photon triggers of the detector were accepted as valid only if they were in coincidence with a pulse in one counter of the electron counter array. The spectrometer field setting and incoming beam energy then determined the photon energy. The maximum attainable photon energy was 54.2 MeV. Pulse height spectra were collected for each segment of the E counters, and the centroid of each spectrum was determined as a function of photon energy between 11.7 and 54.2 MeV. The response was linear within about 2 MeV over this range. As a permanent calibration point, we determined, for each E-counter segment, the pulse-height centroid for a 2~°Po alpha source. An 18 Bq source [9] was sandwiched between a 6.8 mm diameter × 0.46 mm thick plastic scintillator and a thin aluminum disk (as in fig. 3). One such assembly was mounted near a corner o f the back face of each E-counter Pb-glass, adjacent to the PM tube. The fwhm of the polonium spectrum was typically about 40% of its pulse height. For the counters with 4525 tubes, the polonium pulse-height centroid corresponded to that of a 22 MeV photon. For the counters with 8055 tubes, the polonium pulse height corresponded to that of a 20 MeV photon. Similar polonium-source assemblies were mounted on the light pipes for the C and S counters, and on the edge of the AE counter opposite the PM tubes. The resolution of the individual segments of the E counter varied between 32% and 38% for 54 MeV photons. The average of 35% is slightly higher than the 32% obtained from scaling by E -~/2 the value /rMAGNETIC SPECTROMETER RESIDUAL ELECTRON COUNTER ARRAY

J MAINBEAMDUMP

BREMSSTRAHLUNG TARGET X

~[

ELECTRON= BEAM L J

Fig. 2. The photon tagging system.

Pb COLLIMATOR

D.D. Long et al. / Calibration o f a 7-ray detector

41

SIVE TRANSFER TAPE

~,AI RING AI DISK

ZI°Po SOURCE- ~

19 mm dia.

Fig. 3. Cross section of the 210po calibration source and scintillator assembly. The source is on a thin Ni backing.

found by Gilad et al. [2] at 100 MeV. When pulses from the individual quadrants of the E counter are added, and this signal added to that of the AE counter, the resolution is about 47% at 54.2 MeV. This value scales to 41% for the median-energy gammas expected in the (p, n °) experiments. The E and AE counters were balanced as follows before adding the signals. The relative voltages on the AE PM tubes were adjusted until each gave the same pulse height from the 21°po calibration source (fig. 3), which was placed symmetrically with respect to the tubes. A spectrum for the E counter was obtained with E alone in the photon beam. The S and AE counters were then placed in front of the E counter. The voltage on the AE PM tubes was adjusted until the centroid of the summed AE + E signal fell at the same pulse height as for the E-counter spectrum alone.

4. Efficiency and edge effects The absolute efficiency of the system including only the AE, S, and E counters is 26%, as determined by comparison with the count rate from a 25.4 cm diameter by 25.4 cm long NaI detector. This value includes the conversion efficiency of 26.3% and the E-counter efficiency of 99.1%. The discriminator for the E counter was set at its minimum level (30 mV), which corresponded to approximately 5 MeV in photon energy. The overall efficiency dropped by approximately 0.6% as the E threshold was increased from 30 mV to 50 mV. Increasing the AE discriminator setting from its normal setting of 30 mV to

50 mV dropped the efficiency by about 0.5%. The S-counter discriminator setting, up to several hundred millivolts, had no effect upon efficiency. The absorption coefficient of the graphite absorber was measured to be 0.0261 cm -1. Thus a 3.8 cm thick absorber attenuates the photon beam by 10%. The veto efficiency of the C counter was measured as 95% over a broad range of its discriminator settings. An important factor in measuring angular distributions is the variation of efficiency with position and 7-ray angle relative to the detector axis. With a 1.3 cm diameter collimator in the beam, we rotated the detector system about a point on the beam axis 15 cm in front of the AE counter (fig. 4a). [The 15 cm is approximately the target-to-AE distance being used in the (p, n °) experiments.] Fig. 4b shows the variation in efficiency with distance from the counter axis to the 7 interaction point. The efficiency drops significantly when the lnteraction point is within 2.5 cm of the edge of the AE, but is relatively constant over a solid angle of about 0.38 st.

5. Position resolution

The major purpose of the calibration was to measure the precision with which we c a n determine the interaction point of photons striking the converter. A collimated beam of photons was directed at the center of the AE counter. The chambers were read out for an event determined b y N e • AE. W- S. Here N e is a pulse from the tagging electron counter and ~t] ----W1 ' W2 " [4/3 " [4]4, where Wi is a fast pulse from

42

D.D. Long et al. / Calibration o f a 7-ray detector

(a)

\scm~

j.,,¢~

-

PHOTON BEAM

30

I

I

I

I

(b)

]

I

~

I

I

I

I

I

I

=

J

>- 20 o

z w 0

[

-.0

u.. I0 w

0

I

I

I

1

I

1

I

I

I

I

I

I

I

-7

-6

-5

-4

-3

-2

-I

0

I

3

4

5

6

Y (cm) 4. (a) Detector arrangement for determining edge effects. (b) Relative efficiency of the detector system at various off-axis distances. Fig.

the ith wire chamber. Electron tracks in the wire chambers were projected back to a plane within the converter (5/6)t from its front face, where t is the converter thickness. When two or more tracks oc-

curred nearly points sional

for a single event, we chose t h e t r a c k m o s t parallel to t h e p h o t o n d i r e c t i o n . Using t h e s e o f i n t e r s e c t i o n , we g e n e r a t e d a t w o - d i m e n h i s t o g r a m o f t h e i n t e r a c t i o n p o i n t s in t h e con-

Table 1 Wire chamber results. Run

Collimator diameter (cm)

Beam energy (MeV)

Measured diameter a fwhm of peak (cm)

Calculated diameter b (cm)

45 46 66 67

0.5 1.9 1.9 1.9

54.2 54.2 36.2 36.2

1.0

1.0

a Collimator size has not been removed from this diameter. b Assumes incident photons interact at a point in the converter.

1.9

1.0

1.9 2.0

0.94 0.94

43

D.D. Long et al. / Calibration o f a "r-ray detector

t t"L--

1"3

i'1

t"l

I-1

I-1

r'L.....t-I

t-I

I-1

~

I-1

t-I

t"l

r'l

I-1 t-t

R I-1

Fig. 5. Histogram of the number of events vs. x and y position on the converter. Photon energy = 54.2 MeV; collimator diameter = 0.5 cm; fwhm = 1.0 cm.

verter. Fig. 5 shows a typical spectrum for 54.2 MeV photons collimated to a beam 0.5 cm in diameter. Table 1 shows the resolution for various collimator sizes and energies. This table also compares the measured resolution with a calculation by Pickar [I0] that predicts the spread of the shower in a one-radiationlength converter. Each measurement is consistent with the collimator size, within the calculated resolution. The position resolution o f the system is, therefore, 1.0 cm.

6. Conclusion The detector system described has an energy resolution o f 47% at 54 MeV. The centroid o f the pulse height spectrum varies linearly with energy between

12 and 54 MeV. The absolute efficiency o f the system, excluding wire chambers, is 26%. This efficiency is nearly constant over the face o f the detector, within a solid angle o f 0.38 sr. The position resolution at 54 MeV is 1.0 cm (fwhm). This resolution compares favorably with the calculated resolution for a 1 radiation length converter. Since a 3 MeV n ° decays with 7-rays in the energy range 5 4 - 8 4 MeV, the highest photon energy available for this calibration corresponds approximately to the minimum 7-ray energy expected in the (p, ~0) experiment. It is a pleasure for the VPI collaborators to acknowledge the hospitality of Prof. P. Axel and the staff of the Nuclear Physics Research Laboratory. The construction and operation o f MUSL-II and its experimental areas were supported by grants from the

44

D.D. Long et al. / Calibration o f a 3,-ray detector

National Science Foundation. We appreciate the work of D. Schutt, L. Barnett, and their respective staffs in constructing electronic and mechanical components of the counter system. We thank M. Madden for his contributions to the computer analysis of the data, and M. Pickar for communicating to us the results of Iris calculations on the properties of showers within lead-glass counters.

References [1] C.A. Heusch, R.V. Kline and S.J. Yellin, Nucl. Instr. and Meth. 120 (1974) 237. [2] S. Gilad, J.D. Bowman, M.D. Cooper, R.H. Heffner,

C.M. Hoffman, M.A. Moinester, J.M. Potter, t..H. Cverna, H.W. Bae~, P.R. Bevington and M.W. McNaughton, Nucl. Instr. and Meth. 144 (1977) 103. [3] Schott Optical Glass, Inc., Duryea, PA 18642, U.S.A. [41 Scanning Medule WCS-200, MWPC Readout S-710, Nano Systems, 837 N. Cuyler Ave., Oak Park, IL 60302, U.S.A. [5] J.K. Studebaker, Los Alarnos Scientific Laboratory report LA-5749-MS (1974). [6] Pilot 425, Nuclear Enterprises, Inc., 931 Termin,,J Way, San Carlos, CA 94070, U.S.A. [7] D.W. Schutt, Virginia Polytechnic Institute and State University, to be published. [8] R. Ernwein and Ph. Gorodetzky, Nucl. Instr. and Meth. 138 (1976) 57. [9] Purchased from American Nuclear Products, 1232 E. Commercial, Springfield, MO 65803, U.S.A. [10] M. Pickar, Indiana University, private communication.