Chemical Polishing of LSO Crystals to Increase Light Output

Randal Slates, Arion Chatziioannou, *Brianna Fehlberg, "Taekyeung Lee, Simon Cherry Crump Institute for Biological Imaging, Department of Molecular and Medical Pharmacology,

*Department of Materials Science and Engineering, UCLA UCLA School of Medicine, Los Angeles, CA 90095-1770, USA

Abstract Surface treatment of scintillators is critical for light

collection from narrow rectangular crystals. We investigated chemical polishing which is less labor intensive and costly than hand and machine polishing. We used phosphoric acid to chemically polish LSO crystals at llO"C, 150°C and 190"C, and compared these with unpolished and mechanically polished crystals. Groups of five 2 x 2 ~ 1 0 mm crystals were etched for different times at each temperature. Weight loss, light output and energy resolution were measured as a function of treatment time and temperature. We found that chemical polishing can increase light output by 250% relative to unpolished crystals and by 16% relative to mechanically polished crystals. The energy resolution was relatively independent of surface treatment, with values of between 14 and 16%. The rate of loss of LSO was 0.03%/min at llO"C, O.l%/min at 150°C and 0.36%/min at 190°C. Maximum light output occurred when 5-10% of the LSO was removed. The crystals were also imaged using a scanning electron microscopy and the surface roughness quantitatively assessed by using a profilometer. These measurements helped to clarify the effect of acid polishing on the surface of the scintillator. In summary, chemical polishing appears to be a convenient and effective method for improving light output from small LSO crystals.

I. INTRODUCTION

Maximizing light collection efficiency from long, narrow scintillation crystals is important in high resolution gamma ray detectors, particularly those used for positron emission tomography (PET) [ 1-31. Polishing of the scintillator crystals allows for increased light collection (by improving internal reflection along the sides of the crystal) which in turn improves energy and timing resolution. Up until recently, crystals have largely been polished by hand or mechanically, however these methods are both time-consuming and expensive. F8r this reason, we are investigating an alternative method of chemical polishing [4,5] using concentrated phosphoric acid.

11. MATERIALS AND METHODS

A. Chemical Polishing 120 LSO crystals ( 2 x 2 ~ 1 0 mm) were split up into three

groups of 40 and each group was exposed to a different temperature of phosphoric acid (llO°C, 150"C, 190°C). For each acid temperature, the 40 crystals were split up again into groups of five crystals, each being exposed to the acid for

different periods of time. All crystals were weighed prior to chemical polishing. The phosphoric acid (85% by volume) was first heated up in an oil bath above 150" and left there for over an hour to boil off the water, forming higher order acids including pyro-phosphoric acid. This process was judged complete when the acid volume r e d u d by 15%. The temperature was monitored with a thermometer placed in the acid itself. Once the water has evaporated, the acid was then either heated up or cooled down to the desired temperature. The setup is shown in figure 1.

Before being dipped into the pyrophosphoric acid, the crystals were placed into room temperature hydrochloric acid for cleaning. After being chemically polished for the appropriate time and temperature, the crystals were cleaned once again and then weighed. Examples of crystals with different surface treatments are shown in figure 2

Figure 1. Picture of the chemical polishing setup. The inner beaker contains the phosphoric acid along with the teflon holder that holds the LSO crystals. The outer glassware contains silicon oil for heating the acid.

Figure 2. Crystals with different surface treatments: a) mechanically polished, b) unpolished, c-e) chemically polished at 190°C for 1 minute, 16 minutes and 64 minutes respectively.

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B. Light Collection Experiments The treated crystals were wrapped in 3-4 layers of PTFE

tape and coupled to a single channel PMT with a silicone based optical grease. A Na-22 source was used to obtain an energy spectrum from which light output and energy resolution was measured. Measurements were compared to control groups consisting of 11 crystals that were left unpolished and 8 crystals that were mechanically polished (3D Precision Optics, Ravenna, OH).

C. Scanning Electron Microscopy After being chemically polished, representative samples

from different groups of crystals were coated with a very thin layer of gold (60 - 70 nm) and scanned using a Scanning Electron Microscope (Amray SEM 1830). Images of the surfaces at submicron resolution were obtained.

D. ProJilometry Surface profiles were taken of representative samples from

the different groups of crystals using a Tencor Alpha-step 200 Surface Profilometer that has a stylus diameter of 10 microns. The stylus was passed across the surface of each crystal at 10 different random starting positions [4,5]. For each starting position a total of 2006 data points were collected covering a distance of 80 microns across the face of the crystal.

110 C Acid TemDerature

111. RESULTS

A . Light Collection Experiments As shown in figure 3, chemical polishing of the LSO

crystals yields an increase in relative light output of up to 250% compared to unpolished crystals. Mechanically polished crystals yielded an increase in relative light output of 207%. At all temperatures, chemical polishing produced higher light output than mechanical polishing for a range of treatment times. The energy resolution however stayed relatively constant in the range of 14 to 16% for all crystals independent of acid temperature and time spent in the acid. This suggests that energy resolution in these narrow crystals is dominated by depth dependent effects rather than the amount of scintillation light. The rate of LSO loss at 110" C, 150" C, and 190" C was 0.03%, 0.10%, and 0.36% per minute respectively. Optimal light output is generally obtained when 5 1 0 % of the LSO has been removed.

For the 1 10°C temperature, two of the data points (80 and 600 minutes) appear to be slightly lower than expected. This may have been caused by the use of a different thermometer with questionable accuracy for these particular measurements.

B. Scanning Electron Microscopy The SEM images (figure 4) show the surfaces of

unpolished, chemically polished and mechanically polished crystals. The surfaces of the mechanically polished crystals

- 1 250 c 3 g 200 0

6 150 z a 100

50

0

c

> .- - - 2

U 5 10 20 40 EO 200 6001200 M -Tihe in Acid (min)

a

I .. 150 C Acid Temperature

h

$. 2501 a 1

U 1 2 4 8 16 32 100 200 M Time in Acid (min)

b

190 C Acid Temperature

U 0.5 1 2 4 8 16 32 64 M Time in Acid (min)

C Figure 3. Relative light output for 3 different acid&mperatures: (a) 110" C , (b) 150" C , and (c) 190" C for different chemical polishing times. (U=unpolished, M=mechanically polished).

are very smooth compared to those of the unpolished crystal as expected. Note the effect of the acid on the chemically polished crystal in removing much of the surface roughness, although large scale ripples remain. This structure may relate to boundaries and defects in the crystal lattice of LSO. Why the chemically treated crystals give superior light output to mechanically polished crystals is not entirely clear, although it could be related to internal trapping of light in the almost perfect plane surfaces of the mechanically polished crystals.

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a

b

C

Figure 4. SEM images of (a) an unpolished crystal, (b) a chemically polished crystal (16 minutes in 190" acid) and (c) a mechanically polished crystal.

C. Profdometry Representative profilometer data from three crystals, each

with a different surface treatment, are shown in figure 5. The same three groups of crystals are represented as in the scanning electron microscopy images: an unpolished crystal, a crystal chemically polished for 16 minutes, and a mechanically polished crystal. The profiles from the three crystals give a quantitative idea of the surface roughness. The first derivative is plotted in figure 6 by taking the difference between adjacent

points. This illustrates the locally smooth nature of the acid etched crystal, however large discontinuities are still present compared with the mechanically polished crystal. These graphs match well with the topography seen in the SEM images.

'

h Unpolished 2 I , ' , , - " " " "

2 E 0 .- U

C 0

m > Q 0

.- CI .-

8 - 4 1 , , , , , I Q

3 0 10 20 30 40 50 60 70 80 't -5

Profilometer Distance (microns)

a

19OC 16 minutes in Acid h U) 2 , C

0 2 E .- Y

C 0

m > Q)

Q 0 m 3 v)

.- U .- n

rc L

0 10 20 30 40 50 60 70 80 Profilometer Distance (microns)

b

Polished

.- U .z -2

g > I -3

r -5 -4L .L 0 10 20 30 40 50 60 70 80 V I

Profilometer Distance (microns)

C

Figure 5 . Profile data are shown of a) an unpolished crystal, b) a crystal polished in acid for 16 minutes, and c) a mechanically polished crystal.

0-7803-5696-9/00/$10.00 (c) 2000 EEE 94 1

h v) Unpolished r 0.08 ,

.- E

0.04 C 0

m > Q

.- CI

.- n Q 0 C

Q

’c

2

n c

0.02

0

-0.04 -0.02 ! -0.06

-0.08 I ’ ’ I 0 10 20 30 40 50 60 70 80

Profilometer Distance (microns)

h

190 C 16 minutes in Acid

5 0.04 C 0 0.02

.- 0 0

.- c.

> 0

al 0 -0.04 C

-0.02

2 -0.06 al ’c -0.08 n 0 10 20 30 40 50 60 70 80

c

Profilometer Distance (microns)

A

v) C

0 2 .- E v

C 0

m > al

0 0 C

al

.- CI .- n

2 c U- .- 0

b

Polished

0.06 t 0.04 . 0.02 .

0

-0.02 1 1

t -0.04

-0.06

-0.08I . . . ’ ’ . . ’ I 0 10 20 30 40 50 60 70 80

Profilometer Distance (microns)

C

V. DISCUS SION/CONCLUSION

Chemical polishing is an attractive alternative method for improving light output from long, narrow scintillation crystals. It is less time consuming and more cost effective, and results in an increase in light output compared to mechanical polishing methods for the particular geometry studied.

Both chemical and mechanical polishing result in substantial increases in light output (up to 250%) compared with unpolished crystals. In chemically polished crystals, i t appears that local deviations are smoothed out, improving internal reflection of light relative to unpolished crystals. Remaining discontinuities seen in the surfaces of chemically polished crystals may help reduce trapping of light that occurs in mechanically polished crystals. This may explain why the light output improves by 10-20% in the chemical polished crystals compared to the mechanically polished crystals. However, the higher light output does not translate into significantly better energy resolution, presumably because the energy spectra from long narrow crystals is dominated by depth dependent light collection.

VI. ACKNOWLEDGMENTS We would like to thank Bill Moses and Jenny Huber

(LBNL), Lars Eriksson and Matthias Schmand (CTI), and Stefan Siege1 (Concorde) for their expert input and suggestions. We would also like to thank Tatsushi Toyokuni, Kauser Akhoon and Joe Walsh for the use of their laboratory and assistance. This work was supported by the National Institutes of Health (R01 CA74036, R 0 1 CA69370) and the Department of Energy (DE-FC03-87-ER606 15).

VII. REFERENCES T. Smith and B.M. Jasani, “Assessment of various reflectors used with rectangular scintillation detectors”, Journal of Physics E: Scientific Instruments. Vol. 5, pp. 1083-8, 1972. A.J. Bird, T. Carter, A.J. Dean, D. Ramsden, B.M. Swinyard, “The Optimisation of Small CsI(T1) Gamma-ray Detectors”, IEEE Transactions on Nuclear Science. Vol. 40, No. 4, pp . 395-9, 1993. C. Carrier and R. Lecomte, “Effect of Geometrical Modifications and Crystal Defects on Light Collection in Ideal Rectangular Parallelepipedic BGO Scintillators ”, Nucl. Inst. Meth. Vol. A294, pp. 355-64, 1990. J. S. Huber, W. W. Moses, M. S. Andreaco, M. Loope, C. L. Melcher, and R. Nutt, “Geometry and Surface Treatment Dependence of the Light Collection from LSO Crystals”. Submitted to Nucl. Inst. Meth.

K. Kurashige, Y. Kurata, H. Ishibashi, and K. Susa, “Surface Polishing of GSO Scintillator Using Chemical Process”, IEEE Transactions on Nuclear Science. Vol. 45, No. 3: 522-4, 1998.

E. Marx and T. V. Vorburger, “Direct and Inverse problems for light scattered by rough surfaces”, Applied Optics. Vol. 29. NO. 25, ~1~.3613-26, 1990.

Figure 6. The point-to-point difference (first derivative) of the [6] data in figure 5 showing a) unpolished crystal, b) chemically polished crystal and c) mechanically polished crystal. The demonstrates how the chemically polished crystals exhibit [7] C. Moisan, A. Levin, and H. Laman, “Testing Scintillation regions with very smooth surfaces interspersed with Transport Models with Photoelectron Yields Measured under discontinuities that may be related to boundaries or defects in the Different Surface Finishes”, IEEE Nuclear Science Symposium crystalline structure of LSO. and Medical Imaging Conference Record, 1997.

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