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Identification of charged particles using plastic phoswich detectors
Nuclear Instruments and Methods in Physics Research A253 (1987) 305-306 North-Holland, Amsterdam
305
Letter to the Editor
IDENTIFICATION
OF CHARGED PARTICLES USING PLASTIC PHOSWICH
DETECTORS
F r e d r i k LIDI~N, J o h a n N Y B E R G a n d A r n e J O H N S O N Research Institute of Physics and Royal Institute of Technology, Phys. Dept. I, Stockholm, Sweden
Andras KEREK Research Institute of Physics, Stockholm, Sweden
Received 29 August 1986
A plastic phoswich * detector consisting of a 0.1 mm thick (NE102A) fast scintillator and a 10 mm thick slow scintillator (NEll5 or BC-444), heat pressed together, has been tested and found to give a satisfactory charge separation of light particles. A multidetector system for light charged particles is being developed. The system is designed to give a high detection efficiency for light charged particles following compound nucleus reactions, in order to enhance weak reaction channels. In the first version the system consists of 16 plastic A E - E phoswich detectors [1,2] covering about 25% of the solid angle. Each phoswich detector is composed of a thin (0.1 mm) A E element of a fast plastic scintillator (NE102A) and a thick (10 mm) E element giving a slow response signal ( N E l l 5 or BC-444) [3,7]. In order to get a perfect coupling without a dead layer of glue or grease, the two scintillators were heat pressed together at a temperature of about 100 o C. The heat pressed plastic phoswich detector, having a diameter of 20 ram, was directly coupled to a photomultiplier tube (RTC XP 1911) using optical cement. A 6 /~m aluminized mylar strip was wrapped around the sides to increase the reflecting power and to shield the detector from undesired light [4]. The signal from the phoswich detector generally consists of a large amplitude pulse of short duration ( A E ) superimposed on a lower amplitude pulse with a longer decay time (E). The anode signal was amplified and split into two charge-to-digital converters (QDC LeCroy 2249W). Separate gate signals were also generated from the anode signal, The main decay time of the NE102A is approximately 2.4 ns, which means that 99% of the charge would be collected within a gate of 15 ns. However, the pulse was broadened, mainly due to the charge collection circuits. Therefore a time window of about 25 ns was used for the A E signal,
• Phoswich = phosphor sandwich [1].
The integration time for the E signal (having a decay time of about 225 ns) should in principle be as long as possible for the best efficiency. In our case the E signal was integrated during 200 ns, starting about 150 ns after the rise of the A E signal in order to [5,6]: 1) minimize the intrinsic pedestal, which is proportional to the width of the gate, 2) avoid any reflection of the fast signal, 3) avoid the nonexponential delayed component (---100 ns) of the fast A E signal [3]. Tests with various time windows, however, did not affect the resolution in any singificant way. Charged particles emitted or scattered when bombarding targets of 27A1, 122Sn and 197Au with 54 MeV a-particles, 53 MeV 6Li ions and 106 MeV 12C ions from the 225 cm Stockholm cyclotron have been detected with the plastic phoswich detector. The detector was situated at a distance of 15 cm from the target and at an angle of 20 ° relative to the beam direction, implying that the cross section for elastic scattering was rather high (fig. 1). For all cases studied a good separation of the different charges was obtained when a 14 mm diameter ring collimator was applied to shield the outermost part of the detector. Tests with lightguides are being prepared. We intend to eliminate the collimator while retaining the same resolution. Thus heat pressed plastic phoswich detectors have proven to give a good charge separation of light ions at moderate energies. Therefore, although no reliable mass separation was obtained, this type of detector may provide a powerful tool to enhance weak reaction channels in heavy ion induced reactions. They are especially suitable in multidetector (49r) systems as they are relatively cheap, compact, easily tooled to any desired shape, do not require too complicated electronics and, as opposed to CsI(TI) [8], have short decay times.
306
F. Lid~n et al. / I d e n t i f i c a t i o n o f c h a r g e d p a r t i cl es I 500
I :i"-':" ':'": . . . .
.Li-.. ,. t +~: ,.:" . . . . , : . , :
f. 400
t
I
:.
•
:3:",.~: '.... . +:... . ... : . , . f~¢.' . . , . . ~ . . :... ¢. .
.
~
I
t
~
I
12C-ions
~
.: ..
~
;.
.
. . ..
.
...
.
', .~ .:.
:
elastic
scattering
!: ~ 200
. ~ . . ..~," . . . . .
it+Y
. ........
: .......
,,..,
.-",',
',.
, .....
::':.
.
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.,
~ . . ,....
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..
100
I
.
.
.
.
.
.
~
-
"
'
300
'
@
¢ _ ;_.:: , . + . ~ , ~ , , . . ,
' ~ ~ , ~ . ~ J ' . ~
... . . . . •. ". ,
I
200
D
. ...
+
,....
, ~ . + . ' ~ . . .. . . .
~ :.r'~.., ...
~
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.
.,... ~ ........ . .
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. . . . ..... . . ,-"f"~.%,~:. ~'. : , , , r ~ •., ,. ~.,.. ' . • ..~,..
0
I
o n 27A1
..
..... '" - '" ...... ' ". ." "+,. . . ... .. ...'... . .. . •~ :i.~i:'."~ I• . . ". ,.. : .. . .. ' ' . - . , ' : . . *.:
'(~
~
106 MeV
I
400
.... • "
,.,
.I
~ . ~ 1 ~ .'~: .".
:.
'
I
500
Channel number Fig. 1. Results from a b o m b a r d m e n t of 106 MeV ~ C ions on a target of :TAI (1.25 m g / c m 2) are illustrated. Particles with charges from 1 to 8 as well as pulses from "t-rays a n d / o r neutrons are identified in this experiment• The thickness of the A E detector determines the low energy boundary of the dynamic range of 2.3 MeV for protons and 8.9 MeV for a-particles.
Acknowledgements
[3] F.D. Brooks, Nucl. Instr. and Meth. 162 (1979) 477. [4] H.H+ Gutbrod et al., Nucl. Instr. and Meth. 203 (1982) 189.
W e w o u l d like to t h a n k D r . U . G e d d e a n d S. Lundmark at the Royal Institute of Technology, Dep a r t m e n t o f P o l y m e r T e c h n o l o g y , for u s e f u l d i s c u s s i o n s
[5] C. Pastor et al., Nucl. Instr• and Meth. 212 (1983) 209. [6] M. Bantel et al., Nucl. Instr. and Meth. 226 (1984) 394. [7] P.V. Degtyarenko et al., Nucl. Instr. and Methl A239 (1985) 527. [8] J. Alarja et al., Nucl. Instr. and Meth. A242 (1986) 352+
and valuable advice on the heat pressing procedure.
Re|erences
[1] D.H. Wilkinson, Rev. Sci. Instr. 23 (1952) 414, [2] D. Bodansky and S.F. Eccles, Rev. Sci. Instr. 28 (1957) 464.
305
Letter to the Editor
IDENTIFICATION
OF CHARGED PARTICLES USING PLASTIC PHOSWICH
DETECTORS
F r e d r i k LIDI~N, J o h a n N Y B E R G a n d A r n e J O H N S O N Research Institute of Physics and Royal Institute of Technology, Phys. Dept. I, Stockholm, Sweden
Andras KEREK Research Institute of Physics, Stockholm, Sweden
Received 29 August 1986
A plastic phoswich * detector consisting of a 0.1 mm thick (NE102A) fast scintillator and a 10 mm thick slow scintillator (NEll5 or BC-444), heat pressed together, has been tested and found to give a satisfactory charge separation of light particles. A multidetector system for light charged particles is being developed. The system is designed to give a high detection efficiency for light charged particles following compound nucleus reactions, in order to enhance weak reaction channels. In the first version the system consists of 16 plastic A E - E phoswich detectors [1,2] covering about 25% of the solid angle. Each phoswich detector is composed of a thin (0.1 mm) A E element of a fast plastic scintillator (NE102A) and a thick (10 mm) E element giving a slow response signal ( N E l l 5 or BC-444) [3,7]. In order to get a perfect coupling without a dead layer of glue or grease, the two scintillators were heat pressed together at a temperature of about 100 o C. The heat pressed plastic phoswich detector, having a diameter of 20 ram, was directly coupled to a photomultiplier tube (RTC XP 1911) using optical cement. A 6 /~m aluminized mylar strip was wrapped around the sides to increase the reflecting power and to shield the detector from undesired light [4]. The signal from the phoswich detector generally consists of a large amplitude pulse of short duration ( A E ) superimposed on a lower amplitude pulse with a longer decay time (E). The anode signal was amplified and split into two charge-to-digital converters (QDC LeCroy 2249W). Separate gate signals were also generated from the anode signal, The main decay time of the NE102A is approximately 2.4 ns, which means that 99% of the charge would be collected within a gate of 15 ns. However, the pulse was broadened, mainly due to the charge collection circuits. Therefore a time window of about 25 ns was used for the A E signal,
• Phoswich = phosphor sandwich [1].
The integration time for the E signal (having a decay time of about 225 ns) should in principle be as long as possible for the best efficiency. In our case the E signal was integrated during 200 ns, starting about 150 ns after the rise of the A E signal in order to [5,6]: 1) minimize the intrinsic pedestal, which is proportional to the width of the gate, 2) avoid any reflection of the fast signal, 3) avoid the nonexponential delayed component (---100 ns) of the fast A E signal [3]. Tests with various time windows, however, did not affect the resolution in any singificant way. Charged particles emitted or scattered when bombarding targets of 27A1, 122Sn and 197Au with 54 MeV a-particles, 53 MeV 6Li ions and 106 MeV 12C ions from the 225 cm Stockholm cyclotron have been detected with the plastic phoswich detector. The detector was situated at a distance of 15 cm from the target and at an angle of 20 ° relative to the beam direction, implying that the cross section for elastic scattering was rather high (fig. 1). For all cases studied a good separation of the different charges was obtained when a 14 mm diameter ring collimator was applied to shield the outermost part of the detector. Tests with lightguides are being prepared. We intend to eliminate the collimator while retaining the same resolution. Thus heat pressed plastic phoswich detectors have proven to give a good charge separation of light ions at moderate energies. Therefore, although no reliable mass separation was obtained, this type of detector may provide a powerful tool to enhance weak reaction channels in heavy ion induced reactions. They are especially suitable in multidetector (49r) systems as they are relatively cheap, compact, easily tooled to any desired shape, do not require too complicated electronics and, as opposed to CsI(TI) [8], have short decay times.
306
F. Lid~n et al. / I d e n t i f i c a t i o n o f c h a r g e d p a r t i cl es I 500
I :i"-':" ':'": . . . .
.Li-.. ,. t +~: ,.:" . . . . , : . , :
f. 400
t
I
:.
•
:3:",.~: '.... . +:... . ... : . , . f~¢.' . . , . . ~ . . :... ¢. .
.
~
I
t
~
I
12C-ions
~
.: ..
~
;.
.
. . ..
.
...
.
', .~ .:.
:
elastic
scattering
!: ~ 200
. ~ . . ..~," . . . . .
it+Y
. ........
: .......
,,..,
.-",',
',.
, .....
::':.
.
' ......
'. ~
.,
~ . . ,....
,+'
..
100
I
.
.
.
.
.
.
~
-
"
'
300
'
@
¢ _ ;_.:: , . + . ~ , ~ , , . . ,
' ~ ~ , ~ . ~ J ' . ~
... . . . . •. ". ,
I
200
D
. ...
+
,....
, ~ . + . ' ~ . . .. . . .
~ :.r'~.., ...
~
0
.
.,... ~ ........ . .
r+ ~. . . , , , . , ~ , :, . . . .
. . . . ..... . . ,-"f"~.%,~:. ~'. : , , , r ~ •., ,. ~.,.. ' . • ..~,..
0
I
o n 27A1
..
..... '" - '" ...... ' ". ." "+,. . . ... .. ...'... . .. . •~ :i.~i:'."~ I• . . ". ,.. : .. . .. ' ' . - . , ' : . . *.:
'(~
~
106 MeV
I
400
.... • "
,.,
.I
~ . ~ 1 ~ .'~: .".
:.
'
I
500
Channel number Fig. 1. Results from a b o m b a r d m e n t of 106 MeV ~ C ions on a target of :TAI (1.25 m g / c m 2) are illustrated. Particles with charges from 1 to 8 as well as pulses from "t-rays a n d / o r neutrons are identified in this experiment• The thickness of the A E detector determines the low energy boundary of the dynamic range of 2.3 MeV for protons and 8.9 MeV for a-particles.
Acknowledgements
[3] F.D. Brooks, Nucl. Instr. and Meth. 162 (1979) 477. [4] H.H+ Gutbrod et al., Nucl. Instr. and Meth. 203 (1982) 189.
W e w o u l d like to t h a n k D r . U . G e d d e a n d S. Lundmark at the Royal Institute of Technology, Dep a r t m e n t o f P o l y m e r T e c h n o l o g y , for u s e f u l d i s c u s s i o n s
[5] C. Pastor et al., Nucl. Instr• and Meth. 212 (1983) 209. [6] M. Bantel et al., Nucl. Instr. and Meth. 226 (1984) 394. [7] P.V. Degtyarenko et al., Nucl. Instr. and Methl A239 (1985) 527. [8] J. Alarja et al., Nucl. Instr. and Meth. A242 (1986) 352+
and valuable advice on the heat pressing procedure.
Re|erences
[1] D.H. Wilkinson, Rev. Sci. Instr. 23 (1952) 414, [2] D. Bodansky and S.F. Eccles, Rev. Sci. Instr. 28 (1957) 464.