22.10.02
Gamma Imager prototypes with LSO and YAP crystals
The idea of this study was to obtain the highest light yield for the best
combination of the crystal and WLS fibers. We got several 1mm thick LSO crystals
(2020 mm2) from Proteus.
First of all we measured the crystal light yield with an ordinary (XP2262) tube,
which was calibrated with LED. The samples were irradiated with uncollimated 57Co
r/active source (122 keV gamma). The crystals were placed directly onto photocathode
window without optical grease and any reflector. All edges of the samples were opened.
The results are listed in Table 1.
Table 1. Light output.
Type of the
Emission
crystal
spectrum,
Detection
Dimensions
Light yield,
Efficiency,
p.e.
%
887.2
mm3
886 mm3
42
19
39
5
Czekh.
sample
Ohoyokoken
1 mm
64
11
Pixel array
1.5 mm
75
nm
YAP:Ce
2 mm pixel
YAP:Ce
1 mm pixel
YAP
370
CsI
Comments
LSO #1
420
1 mm
125, 130
Proteus
LSO #2
420
1 mm
110
Proteus
LSO #3
420
1 mm
123
Proteus
LSO #1
420
1 mm
50
44
Black edges
LSO #1+2
2 mm
73
Black edges
LSO #1+2+3
3 mm
85
Black edges
The significant part of the emitted light is concentrated at the lateral edges of the
crystal and cannot be used in a coordinate detector; moreover it creates a non-negligible
additional background at the extremities of the detector. The situation may be improved
with an optical absorber (black painting) at the lateral surfaces of the crystal, which is
shown in the last 3 lines of the Table 1. In this case an LSO light yield is quite
comparable with a YAP one.
Using the data obtained for the different LSO thickness (57Co source, 3 mm hole
in 10 mm lead plate, XP2262) we estimated attenuation of 122 keV gamma rays in the
crystal =1.55 mm, and detection efficiency, which is ~96% for 5 mm and ~92% for 4
mm crystal. Efficiencies for smaller thickness are shown in the 3 last lines Table 1. Poor
efficiency results for thick YAP arrays may be due to the bad geometry of measurements
(gamma ray divergence).
The main drawback of the LSO crystal (1 mm thick) is a large natural background
of order of 50 pulses per second per cm2 with a wide amplitude distribution with a peak
corresponding to ~600 keV gamma ray; the spectrum is shown in Fig.1. This fact
prevents the use of the crystal at low gamma source intensities. LSO crystal has also
phosphorescence and it is necessary to wait couple of hours to start the measurements.
Fig.1 LSO crystal (1 mm thick) background spectrum (XP2262)
Gamma Imager structure
1 mm LSO crystal was fixed with an optical contact between 2 layers of Y11
single cladding WLS fibers of 1-mm square cross-section. Upper layer (X) has light
separators between fibers and bottom one (Y) – has not. Both edges of the fiber layers
were polished and far edge was covered with Mylar mirror. Additional mirrors were
mounted at the top and bottom of the detector (above the WLS fibers).
DIRAC PSC discriminators were used as a front-end electronics. The outputs
were connected to the multihit 3377 TDC with 0.5 ns bin scale. Coincidence (200 ns) of
the last dynodes of the PSPM (after discriminators with ~100 mV threshold and 100 ns
pulse) was used as a trigger signal. Operation voltage of the PSPM was tuned to get equal
last dynodes amplitudes for both tubes: 850 V – X-plane and 920 – Y-plane. The last
dynode (after FAN IN/OUT) signals were recorded with 2249A ADC.
Two optical readout configurations with optical grease between WLS and crystal
were tested: LSO with polished and unpolished surfaces. The maximal light output with
57
Co r/active source (last dynode amplitudes: X  2.62 and Y  2.48 p.e., Run 311) was
obtained in the last configuration which is ~1.5 higher than for polished surfaces.
As one can see from Fig.2, the spatial resolution obtained for 0.6 mm slit (10 mm
lead collimator) is 1.15 mm in  (2.7 mm FWHM). For this test we got multiplicity of
1.46 (X) and 1.47 (Y); cross talk range – 2.9 mm (X) and 3.4 mm (Y).
Fig.2.Typical results obtained with LSO crystal and 57Co r/active source
(unpolished crystal, run 317).
Other results, shown below, were obtained for YAP imager prototype with ~1 mm
thick crystal array composed from 1120 mm3 pillars and B1(400) WLS readout. The
Y-layer WLS fiber array was aligned along the length of the pixel and the optical cross
talk attenuation is smaller in this direction (1.95 mm) than in X (2.34 mm).
To find optimal conditions for the image (0.6 mm wide slit along X8) quality,
scan of the anode discriminators thresholds has been performed in a wide discriminator
range. The following quantities were used for the data handling and analyses:
 Detection Efficiency (DE) – ratio: Number of events with nonzero
multiplicity to the total number of triggers;
 Spatial resolution (SR) – sigma of the normal distribution fit with constant
background;
 Reconstruction efficiency: ratio to the total number of triggers of:
o Number of single hit events (Single);
o Number of events with reconstructed projection (Reconstr. Proj.);
o Number of events with reconstructed projection with a hit closest
to the peak of timing distribution (Reconstr. First).
The SR dependence together with DE is shown in Fig.3 (left) as a function of the
threshold. One can see that SR may be improved by factor of ~2 with a high threshold,
but for the expense of the detection efficiency. Reconstruction dependencies are shown in
Fig.3 (right) and several conclusions are evident:
Single is not optimal due to the poor (~50% at maximum) efficiency;
Reconstr. Proj. is quite efficient at high thresholds when multiplicity becomes
lower;
Reconstr. First may be reasonably used at low thresholds with high
reconstruction efficiency.
1.5
0.9
0.8
0.7
Det.Eff.
SR
0.6
0.5
Reconstruction Efficiency
Detection Efficiency and Spatial Resolution , mm
1.0
Single
1.0
Reconstr.First
Reconstr.Proj
0.5
0.4
0.3
0.2
0.0
100
120
140
160
180
200
220
240
100
120
Discriminator Threshold, arb.units
140
160
180
200
220
240
Discrinator threshold, arb.units
Fig.3. Threshold dependences
The examples of the distributions at different thresholds are shown below:
Run 326 – low threshold (105 units), high efficiency; Run 333 – medium threshold (165
units); Run 337 – high threshold (225 units). The S/N ratio, expressed in a number of
events in a peak/mean background, becomes better at a high threshold (up to 30),
comparing to ~5 at low threshold. We tried to improve this ratio at nominal low threshold
with software cut applied to the last dynode ADC spectra but did not find any significant
improvement of S/N, the only detection efficiency became smaller.
The light yield of the last dynode is estimated to be 3.5 p.e. for the X-plane (upper
layer) and 2.7 p.e. for Y-plane. Direct measurement of the analogue anode spectrum in Xlayer gave light output estimation of 1.25 photoelectrons. This value seems to be
underestimated due to the contamination in signal spectrum unresolved cross talk events,
whose amount is not negligible.
Fig.4. Low Threshold, Run 326
Fi.5. Medium Threshold, Run 333
Fi.6. High Threshold, Run 337
In addition we have a lot of spectra with Y-direction slit, inclined slits, wide area
irradiation, which was not mentioned above.