An alternative to polishing the surfaces of scintillation detectors

S . Siegel', M . Erik~so$*~, L. Erik~so$*~, M . Casey2, R. Nutt2 'Concorde Microsystems, Knoxville, TN, 37932

'CTI PET Systems, Knoxville, TN, 37932 3Karolinska Institute, Stockholm, Sweden

Abstract Experiments have been performed to evaluate the effect of

a thin layer of low index of refraction epoxy applied to Lutetium Oxyorthosilicate crystals. Where possible, the results have been compared to chemical polishing/etching. It has been found that there is a significant improvement in energy resolution when cut crystal elements are treated with the epoxy, while the mean light output is unaffected.

As the same effects were noted with scintillator blocks as with individual crystal elements, treatment with the low index of refraction epoxy may be of advantage in situations where etching is difficult to perform. These situationsinclude negative acid and temperature effects on adhesives and detector assemblies.

Once etched, applying the low index epoxy yields no light collecting benefit, but in all cases the epoxy provides a mechanically robust coating.

I. INTRODUCTION Improving the efficiency in collecting light from

scintillation crystals is a recurring theme in radiation detection physics and, in particular, in nuclear medicine device development Positron and single photon emission tomography devices generally depend on the collection of light from scintillabrs for energy resolutioq to discriminate against scattered gamma rays, and for the localization of the gamma-ray interactions, by the sharing of this same scintillation light.

Mechanical and chemical polishing enhance the performance of scintillation detectors by increasing the light transmission properties of the crystah. These polishing procedures may be expensive and labor intensive. In addition, the drive for detectors comprised of physically smaltr elements, for higher resolution and denser packing, often renders polishing impractical. Likewise, multicrystal assemblies (blocks) tend to be highly sensitive to mechanical and thermal stress. Often, the only way to realize some block designs is to incorporate dissimilar materials and bond joints, which render the blocks even more sensitive to such stresses.

It.has been demonstrated [l] that the performance of a scintillator may be improved by applying a SoLGel coating, with a low index of refraction, on its surface. This process involves multiple dipping to achieve a useful thicknessand produces a fragile coating.

We have found a durable, easy to apply, single component epoxy (OG-197 Epoxy Technology, Billerica, MA, USA) with an index of refraction of 1.380 that significantly improves the

scintillation performance of LSO. Though the mean light output is not affected, there is a significant improvement in the energy resolution.

11. MATERIALS AND METHODS

A. General Procedures All individual Lutetium Oxyorthosilicate (LSO) element

measurements were taken by wrapping the crystals in Polytetrafluoroethylene (Teflon) and coupling them to a 5 cm photomultiplier tube (PMT) The anode signal was amplified and fed to a multichannel analyzer. The block measurements were taken by filling the blocks with reflective powder, wrapping in Teflon and coupling, via a slotted light guide, to four PMTs. The light output and energy resolutiol of each individual detector element of the blocks were evaluated using block analyzer software and hardware previously described [2].

The blocks used in this project were made from 19x19~10 mm3 LSO blanks cut into an 8x8 element matrix by introducing 9 mn deep groves into the blanks. Each array element was 2 x 2 ~ 9 mm3 on a 1 mm thick substrate. All measurements were done by exposure to a 5 11 keV gamma ray source.

B. Experiments Performed In the first experiment, thirteen 4 x 4 ~ 1 0 mrd LSO

elements with polished ends were used. A baseline study was performed to determine each elementk light output and energy resolution. The elements were then treated with the epoxy, re-wrapped with Teflon and reevaluated for light output and energy resolution.

In the second experiment, nine LSO elements were used without any surface finish, just cut into 4 x 4 ~ 1 0 m d elements. A baseline study was performed to measure their light output and energy resolution. The elements were then etched in phosphoric acid at 200 OC and evaluated again with respect to light output and energy resolution. As a last step in this experiment, the etched elements were treated with the low index of refraction epoxy and reevaluated.

In the third experiment, two LSO blocks were first filled with reflector material and then evaluated, on an element per element basis, for light output and energy resolution. The blocks were then emptied of reflector material, treated with the low index of refraction epoxy, refilled with reflector and re-evaluated. Subsequently, the blocks were once again emptied, etched for 15 min in 200 OC phosphoric acid, re- filled with reflector and reevaluated. In a final step to this

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experiment, the etched blocks were emptied, epoxy treated, refilled with reflector and reevaluated.

coating, showed that there was no significant difference between them.

1 3 5 7 9 1 1 1 3 crystal #

Figure 1. Energy resolutionof the thirteen LSO elements before and after epoxy coating their cut surfaces. The ratio of the energy

Mean energy Mean energy resolution (arbitrary units) (YoFWHM)

Base line 199*4 12.7~k0.2

EPOXY 196*2 12.2k0.4 Etched 247*5 0.4*0.1

24 1 *3 10.6*0.1 Etched&epoxy

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Pulse height of 8x8 block of 2 x 2 ~ 1 0 rnd LSO Rough cut versus epoxy coated

80

70

60

50

40

30 0 8 16 24 32 40 48 56 64

crystal#

Block# 1

Energy Energy (arb, units) resolution

(%F W HM)

Base 56.1 18.6 line

Epoxy 61.1 16.5 Etched 68.8 12.8

Etched 65.0 13.2 & epoxy

Energy resolution of 8x8 block of 2 x 2 ~ 1 0 rnd LSO .. Rough cut versus epoxy coated versus etched

Block# 2

Energy Energy (arb.units) resolution

(%FWHM)

53.0 20.4

53.9 17.8 65.8 12.5 64.1 13.0

Energy resolution as cut Energy resolution with epoxy 5 131, ;, , Energy , , resolution , , , , post , , etch , , , l ~

n ., 1 10 19 28 37 46 55 64

crystal#

Figure 3. Pulse height (upper) and energy resolution (lower) for each element of an LSO block with sixtyfour 2x2~10 mm3 elements.

Energy resolution of 8x8 block of 2 x 2 ~ 1 0 rnd LSO Etched versus etched+epoxy coated

20

2 18 2e c 16 9 ?3

14 E

2) 12

10

I x."

- %

5 0 8 16 24 32 40 48 56 64

crystal#

Figure 4. Energy resolution of the same LSO block as in Figure 3, denoting the effect of epoxy coating posktching.

IV. CONCLUSIONS Proper surface treatment of a scintillator increases the

scintillation light transmission with a concomitant improvement in energy Esolution. Mechanical and chemical polishing (etching) yields the best surface treatment. These procedures may be problematic for detector blocks with small elements or multiple bonded elements. We propose an alternative approach which, while producing novhere near as dramatic an effect on the scintillatork performance, is easy to implement reproducibly, is mechanically robust, is cost effective and produces a significant improvement in the light transmission of the scintillator.

V. ACKNOWLEDGMENTS The authors wish to acknowledge the aid given them by

the following people: Chris Barton, Joyce Plemmons, andA. A. (Drew) Carey CTI, Knoxville, TN, Rhonda N. Goble and Robert E. Nutt, Concorde Microsystems, Knoxville, TN; Simon R. Cherry, Arion Chatziioannou and Randal Slates, UCLA, Los Angeles, CA.

VI. REFERENCES

[ l ] R. Chipeaux, M. Geleoc, P. Belleville and B. Lambert, "Sol-Gel coating of scintillating crystals', presented at the IEEE Nuclear Science Symposium and Medical Imaging Conference, Toronto, CA, 1998.

[2] M. Schmand, L. Eriksson, M.E. Casey, M.S. Andreaco, C. Melcher, K. Wienhard, G. Flugge and R. Nutt, "Performance results of a new DO1 detector block for a High resolution PET - LSO Research Tomograph HRRT", IEEE Trans Nucl Sci. Vol. 45 no. 6, pp 3000- 3006,1998.

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