2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS/MIC) M13-5

X'tal Cube Detector Composed of a Stack of Scintillator Plates Segmented by Laser Processing

Naoko Inadama, Takahiro Moriya, Yoshiyuki Hirano, Fumihiko Nishikido, Hideo Murayama, Eiji Yoshida, Hideaki Tashima, Munetaka Nitta, Hiroshi Ito, and Taiga Yamaya, Member IEEE

Abstract- We have developed a 3-dimasional (3D) position

sensitive radiation detector named "X'tal cube". The X'tal

cube is composed of a scintillation crystal block and multi­

pixel photon counters (MPPCs). No reflector is inserted in the

crystal block. It is segmented 3 dimensionally into small cubes

(3D segmentation) by only optical discontinuity and

scintillation light from a segment is detected by all MPPCs

which are coupled on all six surfaces of the crystal block. We

have already succeeded to make the 3D segmentation inside of

a monolithic scintillator block by laser processing technique

instead of arranging small cubic scintillator elements into a

3D array as a general way. We obtained the result that

utilizing the laser processing technique not only eliminates the

difficulty of fabrication with the small scintillator elements

but also improves detector performance.

As a new trial, we fabricated the crystal block by stacking

the scintillator plates which are segmented 2 dimensionally by

the laser processing technique (2D segmentation). Plates are

also easy to handle compared to the small scintillator elements

and for the laser processing, 2D segmentation is much simpler

than the 3D segmentation. In this study, we evaluated

performance of the X'tal cube with the scintillator plates

(Plate-XC) to confirm its feasibility. Performance was

measured by irradiating 662 keY gamma-rays. Used

scintillator were LYSO plates of 2.0 mm in thick. They had 2D

segmentation by laser to make 2.0 mm x 2.0 mm x 2.0 mm

segments. Between the plates was chosen as air gap. We

coupled a 4 x 4 array of the MPPC to six surfaces of the

crystal block. Results showed sufficient performance in crystal

identification performance and energy resolution. As a further

investigation to understand character of the Plate-XC,

scintillation light distribution in the crystal block was also

measured. Results indicate that light spread from outer

segments is influenced by the segment boundary conditions,

air gap or laser processed gap, while the spread from the

center segment does not seem to get the influence. In energy

performance, around 10% energy resolution was obtained for

the outer segments as well as for the center segment.

1. INTRODUCTION

On the purpose of obtaining a PET image of high spatial

resolution, PET detectors using fine scintillation crystal

elements have been developed. We have also developed the

detector named X'tal cube, which has high spatial

resolution in all 3 dimensions [I]. Fig. 1 illustrates structure

of the X'tal cube. It is composed of a scintillation crystal

block and a number of multi-pixel photon counters

(MPPCs), thin and lightweight photo-detectors, optically

coupled on all six surfaces of the crystal block. The region

between active areas of the MPPCs is covered with

reflectors so as not to lose scintillation light. The crystal

block is segmented into small cubes. There is no reflector

between the crystal segments and they are segmented by

only optical discontinuity. Scintillation light then spreads in

3 dimensions, x, y, and z, from an originating segment and

detected by the all MPPCs on the crystal block surfaces.

At the first stage, we fabricated the crystal block by

arranging small scintillation crystal elements into a 3-

978-1-4673-2030-6112/$3l.00 ©2012 IEEE 2848

dimensional (3�) array, for instance, 729 of 2.0 mm x 2.0

mm x 2.0 mm elements were arranged into a 9 x 9 x 9

array for a prototype X'tal cube. The difficulty to treat

many small crystal elements was eliminated by the laser

processing technique later we developed [2]. Because the

laser processes optical discontinuity planes only inside of a

monolithic crystal block, the block never falls apart in

pieces. The block is then easy to handle and furthermore

we found that the crystal block processed by the laser

technique showed better detector performance compared to

the block composed of crystal elements.

However, the 3D segmentation by laser needs skill to

deal with the limitation in accuracy for the processing in

depth direction. To generalize fabrication of the crystal

block, as a new trial, we composed the block by stacking

the scintillator plates which are segmented 2 dimensionally

by the laser processing technique instead applying the 3D

segmentation to a monolithic crystal block. The limitation

in the laser processing will not pose any problem to the 20

segmentation in the thin plate scintillator. Also, we can

process several plates at one time and that is favorable for

mass production to consist PET detector rings.

In this study, we prepared the X'tal cube composed of

the scintillator plates (Plate-XC) and evaluated its

performance to see if the use of the plates is feasible or not.

We also analyzed scintillation light spread inside of the

crystal block to understand character of the Plate-XC.

Reflector I� .;;stal block

� x �"'" Optical Y discontinuity

Fig. l. Structure of the X'tal cube. MPPCs are coupled on all six surfaces of the crystal block which segmented 3 dimensionally into small cubes.

II. MATERIALS AND METHODS

Used scintillator was LU2(!.x)Y2xSiOs (LYSO, Crystal

Photonics Inc. (CPI); USA) and surface finish of the LYSO

plates was mechanically polished to apply laser processing.

For the Plate-XC, we stacked nine LYSO plates of 2.0 mm

thickness in y direction as shown in Fig. 2. Each plate was

segmented into a 9 x 9 array of 2.0 mm x 2.0 mm segments

by the laser processing. Between the plates was chosen as

air gap. We coupled a 4 x 4 array of the MPPC to six

surfaces of the crystal block (Fig. 1) with RTV rubber

(KE420, Shin-Etsu Chemical Co., Ltd., Japan, 1.45

refractive index). Used MPPCs were S 1 0931-050P, the

product of Hamamatsu Photonics K.K., Japan (3600 of 50

flm x 50 flill micro-pixels, a 61.5 % fill factor, 3 mm x 3

mm active area). As reflector, we used multilayer polymer

mirrors (MPM) of 98% reflectivity and 0.065 mm thickness

(Sumitomo 3M, Ltd., Japan). Fig. 3 illustrates location of

the 4 x 4 MPPC active areas on each crystal block surface.

The middle lines of a 9 x 9 segment array were completely

covered with reflector. We used no light guide for the

Plate-XC.

662 keY gamma-rays from 137Cs point sources were

irradiated uniformly from three directions. MPPC signals

were collected by NIM and CAMAC system. Results of a

simple Anger-type calculation with all MPPC signals on

each x, y, z direction were expressed in a 3D position

histogram so that responses for all crystal segments were

formed in the histogram. Then segment identification

performance was evaluated in the histogram as segment

response discrimination.

We analyzed scintillation light spread inside of the

crystal block by pulse height difference among MPPCs.

Pulse height of a MPPC was determined as the peak

position of the pulse height distribution obtained by the

single MPPC (Fig. 4(a)). The pulse heights of the 4 x 4

MPPCs on a crystal block surface were expressed in the

vertical axis of a graph at the location of the corresponding

MPPC (Fig. 4(b)). Then we know obviously the difference

of amount of scintillation light obtained at each MPPC

location by the graph.

Fig. 2. Crystal block for the Plate-XC.

mm. 0.5 1.5 2.0 1.5

lUll.

mff

mrr

2.0mm U ri n 1.5 mm

l' l' l' Reflector

M

7(3 PPC active area mmx3 mm)

-. --IF'JPO.5 mm

�1.0mm 0.5 MPPC

'9 x 9 array of crystal segments

Fig. 3. Location of the 4 x 4 MPPC active areas on each crystal block surface. Dimensions of the MPPC active area were 3 mm x 3 mm and that of each crystal segment were 2 mm x 2 mm x 2 mm.

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(a)

300L '" 1:;200 81:

o �.:OOf Pulse hel�a.u.)

Pulse height of thisMPPC

I

(b)

Fig. 4. (a) Illustration of the pulse height for a MPPC. The peak position of the pulse height distribution obtained by the single MPPC is defined as the pulse height of the MPPC. (b) Pulse height difference among 4 x 4 MPPCs on a crystal block surface, which indicates amount of light obtained at each MPPC location.

III. RESULTS

Fig. 5 shows the obtained 3D position histogram for the

Plate-XC. Crystal responses are clear and separated. That is

the proof of sufficient segment identification performance

of the Plate-XC.

Figs. 6(a) and (b) are the 2D position histograms for the

3rd layers in the z and y directions, respectively. Responses

of the 9 x 9 segments in the layers are obviously

discriminated, however, it is observed that outer responses

in the y direction, adjacent through air gap each other,

make zigzag lines. In the x and z directions, the same

tendency is slightly shown in the alignment of outer

responses that were segmented by laser.

Figs. 7(a) and (b) show the distributions of scintillation

light from two outer segments. We name here these

segments as segment-Oa and segment-Ob, respectively.

The corresponding response of each segment are indicated

by an arrow in the 2D position histograms of the 5th-layer

in Z direction and their locations are also done in each

graph. The graph shows pulse heights of the 4 x 4 MPPCs

on each crystal block surface and we can figure out

tendency of scintillation light spread by the graphs. For

both segment-Oa and segment-Ob, the surface which

constitutes of the crystal block surface was all covered with

reflector and scintillation light from the segment-Oa

spreads longest distance through optical discontinuities

processed by laser, while the light from the segment-Ob

does through air gaps. It is observed that most light from

the segment-Ob spreads only in the scintillator plate

containing the segment-Ob. The light from the segment-Oa

seems to spread more widely through other scintillator

plates. Fig. 7( c) shows the distribution of scintillation light

from the center segment, segment-c. We cannot see

obvious effect on the distribution, which comes from the

difference of segment boundary conditions, air gap or laser

processed gap.

Pulse height distributions for all segments, the segment­

Oa, Ob, and C are shown in Fig. 8. Energy resolution

(FWHM) and relative light output, that is indicated by the

peak position of the pulse height distribution, are

summarized in Table I.

Fig. 5. 3D position histogram for the Plate-XC.

3rd layer(y)

z

i Au

Fig. 6. Position histograms for the 3rd layers in (a) the z and (b) y directions for Plate-XC. Each contains responses of the 9 x 9 segments.

I I \

I § � §

r T T T

I I I \

(a)

Laser �Y

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Fig. 7. Distributions of scintillation light originating in (a), (b) two outer segments (segment-Oa, segment-Ob) and (c) the center segment (segment-C) of the Plate-XC. Each graph shows measured pulse heights of the 4 x 4 MPPCs on each surface of the crystal block.

i Segment-Oa U 1 x 105

All segments

u .... o§�: bL 0 -!l---����----:;===r=..--,-� o 200 400 600 800 1000

Pulse height (a. u.) o 0 200 400 600 800

Pulse height (a.�u�.)���?b. Segment-C

200

o 0 200 400 600 800 Pulse height (a.u.)

Fig. 8. Pulse height distributions for all segments, the segment-Oa, Ob, and C.

TABLE I RELATIVE LIGHT OUTPUTS AND ENERGY RESOLUTIONS

Segment Oa Ob C All

Light output [a.u.] 0.95 1.03 0.99 Energy resolution [%] 10.3 9.4 9.6 15.4

IV. DISCUSSION AND CONCLUSION

In this study, as a new trial, we fabricated the crystal

block of the X'tal cube by stacking the scintillator plates

which are segmented 2-dimensionally by the laser

processing technique and evaluated detector performance

of the Plate-XC to examine its feasibility.

The obtained 3D position histogram in Fig. 5 and 2D

position histograms in Fig. 6 indicate sufficient segment

identification performance of the Plate-XC which consisted

of a 9 x 9 x 9 array of 2.0 mm x 2.0 mm x 2.0 mm crystal

segments as the crystal block and 4 x 4 MPPCs on each

side of the crystal block. However, in these position

histograms, we see tendencies in response alignment. One

is clear distinction between responses corresponding to

different scintillator plates as shown in Fig. 5. Another is

zigzag alignment of the outer responses only in the y­

direction in which crystal segments are adjacent through air

gap each other. That is shown in Fig. 6(a) and causes

inferior segment identification performance particularly at

outer two segments of the 5th-layer in the y-direction,

which corresponds to the scintillator plate covered with

reflector on all side faces (Fig. 3). The zigzag alignment of

outer responses was also observed previously when we

used crystal elements to fabricate the crystal block and

chose air gap for the condition between crystal elements.

For outer two segments in the 5th-layers in the x and z­

directions, the same tendency is slightly shown. The

segments are adjacent through optical discontinuity

processed by laser and similar position histogram was

obtained when we used the crystal block having 3D laser

processing.

The difference of scintillation light spread shown in Figs.

7(a) and (b) will be the cause of the tendency regarding

outer segments in the 5th-layer. Segment-Oa and Ob

correspond to the outer responses of the 5th-layers in the y­

direction and x or z-direction, respectively. Because most

amount of scintillation light from segment-Ob is detected

by the MPPCs located at upper part in Fig. 7(b), the

response, that is the results of Anger-type calculation with

all MPPC signals, is kept peripheral position in the position

histogram. The light from segment-Oa is, on the other hand,

detected also by the MPPCs located at middle part in Fig.

7(a), which make the move of response position toward the

center and closer to the next response. Although this

tendency does not affect detector performance of the Plate­

XC consists of 2.0 mm x 2.0 mm x 2.0 mm crystal

segments, it will be a problem when the segmentation

becomes finer. Fig. 7( c) shows the result that distribution of

scintillation light from segment-C is similar at all surfaces

of the crystal block, independently on the segment

boundary condition.

As indicated in the Fig. 8 and Table I, all segment-Oa,

Ob and C showed sufficient energy performance. Light

output of the center segment, segment-C, was comparable

with outer segments, segment-Oa and Ob. With the

observation that the peak of the pulse height distribution for

all segments is not broaden, we can expect light output is

comparable for all segments in the Plate-XC. For energy

resolution, we obtained around 10.0% for both center and

outer segments.

ACKNOWLEDGEMENT

This study was conducted as a part of the project, a

Grant-in-Aid for Scientific Research (no. 23602017) from

the Japan Society for the Promotion of Science.

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REFERENCES

[I] Y. Yazaki, N. Inadama, F. Nishikido, T. Mitsuhashi, M. Suga, K. Shibuya, M. Watanabe, T. Yamashita, E. Yoshida, H. Murayama, T. Yamaya., "Development of the X'tal Cube: A 3D position-sensitive radiation detector with all-surface MPPC readout," IEEE Trans. Nuc!. Sci., vol. 59, pp. 462-468, 2012.

[2] T. Moriya, K. Fukumitsu, T. Sakai, S. Ohsuka, T. Okamoto, H. Takahashi, M. Watanabe, and T. Yamashita, "Development of PET detectors using monolithic scintillation crystals processed with sub-surface laser engraving technique," IEEE Trans. Nuc!. SCi., Vol. 57,pp. 2455-2459,2010.