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Achieving uniform spatial response from large area liquid scintillation detectors
NUCLEAR INSTRUMENTS
AND
METHODS
89
(I97O) 2 9 1 - 2 9 3 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
ACHIEVING U N I F O R M SPATIAL R E S P O N S E F R O M LARGE AREA L I Q U I D S C I N T I L L A T I O N D E T E C T O R S * M. H. La POINTE, B. G. R E N N E X , F. S I O H A N and J. R. W A Y L A N D
Department of Physics and Astronomy, University of Maryland, College Park, Maryland, U.S.A. Received 17 August 1970 and in revised f o r m 7 October 1970 This note describes a method of obtaining uniform spatial response from large, thin (1 m x 2 m x 8 cm), liquid scintillator detectors. Lucite steps are used to vary the depth of the scintillant and compensate for geometrical non-uniformities.
Three photomultipliers (RCA 6655A) are used at each end of the tanks to insure lateral uniformity. Each tube voltage is adjusted for a standard gain. The outer tubes are canted slightly to point towards the side wall at the center of the tank. With this arrangement no lateral nonuniformity was observed with the tube-to-window distances used ( l l > 12"). The anodes of the three tubes are connected together and the signals from the two ends of each tank are passively coupled with matched lengths of cables. The integrated signal from each tank is used (rather than the pulse height) as a measure of the light output. For the tests a gated pulse stretcher provided standard signals for a multichannel analyzer. The gate was supplied by a coincidence between two small scintillator paddles selecting near-vertical muons which traversed localized areas of the tank. For convenience the mode of the pulse height distribution was used for comparing response since we found a constant separation between the mode and the mean. A number of tests were carried out to determine what
In many experiments which use cosmic rays as a source of energetic particles, thin, large-area detectors are required. The usual requirement of such a detector is that the average signal amplitude produced by single relativistic particles be position independent. In this note a method is described of achieving a uniform ( t o + 5 % ) response from 1 m x 2 m liquid scintillation detectors. Fourteen of these detectors are used in a calorimeter experiment at the University of Maryland. Fig. 1 shows the design of the detector. The tanks are constructed of galvanized sheet steel. All inner surfaces are coated with a white, epoxy-based paint. Lucite windows separate the scintillant compartment from the end sections housing the photomultiplier tubes. The windows are bolted to ½" wide frames against gaskets which were formed in place from an RTV rubber compound. Covers over each compartment slide into place in deep channels designed to minimize light leakage problems. * This work was supported in part by the A F O S R under contract # F-44620-69-C-0019.
~
V =•1
COINCIDENCE TRIGGER
] LIQUID SCINTILLATOR
40"
i
Lo,,_L
/
~ ~
L
TCHER
.
n
TRIGGER COUNTER L U C I T EWINDOW
I
-l_
_
I-
144" Fig. 1. Basic design of detector.
291
Iv
292
M. H. La POINTE ct al.
measUres would be required to achieve longitudinal uniformity. A tank was first filled to uniform depth (d = 2½") and the photomultipliers moved to the back of their compartments (11 = 2Y). The signal at a distance, x = 10" from a window was 40% greater than that at the center of the tank. The mineral-oil-based scintillant used (Pilot Chemical P.S. 007) has a particularly long attenuation length ( 3 - 4 m) which seems to indicate that this non-uniformity is due to geometrical factors. The depth of the scintillant was next varied by inserting white wedge shaped reflectors (fig. 2a) which reduced the effective depth at the windows to only ½". This extreme depth compensation gave almost no improvement in uniformity, a result which was ascribed to the fact that the opaque reflectors blocked the direct light path from much of the scintillator volume. This indicates that diffuse reflections from the white paint contribute little to the collected light. Transparent wedges seem the obvious solution.
w, oo
~
Wedges machined from solid lucite were briefly considered. This is probably a reasonable possibility since the wetting action of the scintillant makes polished surfaces unnecessary. For tests however a set of wedges were made of ~" lucite and filled with pure mineral oil. Heavy mylar was glued to the bottom of the wedges to form an air gap which provided total internal reflection on the bottom surface of the tank. With these wedges the signals from particles traversing near the windows matched signals from the center of the tank. The linear depth variation caused some overcompensation at points intermediate to these two positions. Wedges might be constructed with concave tops for a correction but this would have added to the difficulty of fabrication. Also there is danger that the oil filled wedges would develop leaks. The most satisfactory compromise for our application was to replace the wedges with lucite pieces (fig. 2b) for stepwise compensation. Bottom pieces (]" thick), covering the tank area, have mylar glued
~ SClNTILLANT
~
TOTAL INTERNAL REFLECTOR o) WEDGES
b) LUCITE STEPS
Fig. 2. Two different approaches taken to obtain spatial linearity. PULSE HEIGHT '
~--- IDEALIZEDCURVE
MAXIMUM o l +5% F . . . . / , . . . . . -. } 7 OVER DETECTOR}'100% I- . ~ T - ~ L ~ , , - - - . ] - ~ ' ~ - . " " L--~,L,.~z/ : AVERAGE MAX AREA
"
_5 °/o I____~___ . . . . . . . . . . . . . . . . . . ~--'ERROR IN X TAKEN TO BE WIDTH OF TRIGGER COUNTER
d - 2 ~I "
INDOW--------~
LEVEL
x--4o" ( LONGITUDINAL CENTER )
go"
2b"\
~b" ~AIR GAP
Fig. 3. Detail and response of final detector.
_ 2 II
o"
R E S P O N S E FROM LARGE AREA L I Q U I D
to the bottom to produce the air gap providing total internal reflection. The outer ends of these pieces rest on the window ledges to produce some depth compensation. Small spacers are used under each step so that air bubbles can escape during filling. The number, lengths and thicknesses of the steps can be chosen in first approximation from the response of the tank with only the total reflecting surfaces installed. Some modifications are then necessary, possibly because lucite has a relatively short light attenuation length. We used five steps (all full width) for compensation with final lengths and thicknesses of: 27"x ¼", 19", 12", 6½" and 4" (allx~"). With ll =15", /2---25"
SCINTILLATION
DETECTORS
293
and d = 2½", the longitudinal distribution obtained from center-to-window is shown in fig. 3 along with an idealized representation of the response. The lateral response with this arrangement was checked and found to be linear to within our errors. This system o f compensation has been adopted for our scintillator tanks. The steps were easy to assemble and have been found to be quite durable. The tanks must be leveled carefully since the scintillant depth is so shallow especially near the windows. The authors are indebted to T. Bowen, R. Ellsworth and G. Yodh for valuable discussions.
AND
METHODS
89
(I97O) 2 9 1 - 2 9 3 ;
©
NORTH-HOLLAND
PUBLISHING
CO.
ACHIEVING U N I F O R M SPATIAL R E S P O N S E F R O M LARGE AREA L I Q U I D S C I N T I L L A T I O N D E T E C T O R S * M. H. La POINTE, B. G. R E N N E X , F. S I O H A N and J. R. W A Y L A N D
Department of Physics and Astronomy, University of Maryland, College Park, Maryland, U.S.A. Received 17 August 1970 and in revised f o r m 7 October 1970 This note describes a method of obtaining uniform spatial response from large, thin (1 m x 2 m x 8 cm), liquid scintillator detectors. Lucite steps are used to vary the depth of the scintillant and compensate for geometrical non-uniformities.
Three photomultipliers (RCA 6655A) are used at each end of the tanks to insure lateral uniformity. Each tube voltage is adjusted for a standard gain. The outer tubes are canted slightly to point towards the side wall at the center of the tank. With this arrangement no lateral nonuniformity was observed with the tube-to-window distances used ( l l > 12"). The anodes of the three tubes are connected together and the signals from the two ends of each tank are passively coupled with matched lengths of cables. The integrated signal from each tank is used (rather than the pulse height) as a measure of the light output. For the tests a gated pulse stretcher provided standard signals for a multichannel analyzer. The gate was supplied by a coincidence between two small scintillator paddles selecting near-vertical muons which traversed localized areas of the tank. For convenience the mode of the pulse height distribution was used for comparing response since we found a constant separation between the mode and the mean. A number of tests were carried out to determine what
In many experiments which use cosmic rays as a source of energetic particles, thin, large-area detectors are required. The usual requirement of such a detector is that the average signal amplitude produced by single relativistic particles be position independent. In this note a method is described of achieving a uniform ( t o + 5 % ) response from 1 m x 2 m liquid scintillation detectors. Fourteen of these detectors are used in a calorimeter experiment at the University of Maryland. Fig. 1 shows the design of the detector. The tanks are constructed of galvanized sheet steel. All inner surfaces are coated with a white, epoxy-based paint. Lucite windows separate the scintillant compartment from the end sections housing the photomultiplier tubes. The windows are bolted to ½" wide frames against gaskets which were formed in place from an RTV rubber compound. Covers over each compartment slide into place in deep channels designed to minimize light leakage problems. * This work was supported in part by the A F O S R under contract # F-44620-69-C-0019.
~
V =•1
COINCIDENCE TRIGGER
] LIQUID SCINTILLATOR
40"
i
Lo,,_L
/
~ ~
L
TCHER
.
n
TRIGGER COUNTER L U C I T EWINDOW
I
-l_
_
I-
144" Fig. 1. Basic design of detector.
291
Iv
292
M. H. La POINTE ct al.
measUres would be required to achieve longitudinal uniformity. A tank was first filled to uniform depth (d = 2½") and the photomultipliers moved to the back of their compartments (11 = 2Y). The signal at a distance, x = 10" from a window was 40% greater than that at the center of the tank. The mineral-oil-based scintillant used (Pilot Chemical P.S. 007) has a particularly long attenuation length ( 3 - 4 m) which seems to indicate that this non-uniformity is due to geometrical factors. The depth of the scintillant was next varied by inserting white wedge shaped reflectors (fig. 2a) which reduced the effective depth at the windows to only ½". This extreme depth compensation gave almost no improvement in uniformity, a result which was ascribed to the fact that the opaque reflectors blocked the direct light path from much of the scintillator volume. This indicates that diffuse reflections from the white paint contribute little to the collected light. Transparent wedges seem the obvious solution.
w, oo
~
Wedges machined from solid lucite were briefly considered. This is probably a reasonable possibility since the wetting action of the scintillant makes polished surfaces unnecessary. For tests however a set of wedges were made of ~" lucite and filled with pure mineral oil. Heavy mylar was glued to the bottom of the wedges to form an air gap which provided total internal reflection on the bottom surface of the tank. With these wedges the signals from particles traversing near the windows matched signals from the center of the tank. The linear depth variation caused some overcompensation at points intermediate to these two positions. Wedges might be constructed with concave tops for a correction but this would have added to the difficulty of fabrication. Also there is danger that the oil filled wedges would develop leaks. The most satisfactory compromise for our application was to replace the wedges with lucite pieces (fig. 2b) for stepwise compensation. Bottom pieces (]" thick), covering the tank area, have mylar glued
~ SClNTILLANT
~
TOTAL INTERNAL REFLECTOR o) WEDGES
b) LUCITE STEPS
Fig. 2. Two different approaches taken to obtain spatial linearity. PULSE HEIGHT '
~--- IDEALIZEDCURVE
MAXIMUM o l +5% F . . . . / , . . . . . -. } 7 OVER DETECTOR}'100% I- . ~ T - ~ L ~ , , - - - . ] - ~ ' ~ - . " " L--~,L,.~z/ : AVERAGE MAX AREA
"
_5 °/o I____~___ . . . . . . . . . . . . . . . . . . ~--'ERROR IN X TAKEN TO BE WIDTH OF TRIGGER COUNTER
d - 2 ~I "
INDOW--------~
LEVEL
x--4o" ( LONGITUDINAL CENTER )
go"
2b"\
~b" ~AIR GAP
Fig. 3. Detail and response of final detector.
_ 2 II
o"
R E S P O N S E FROM LARGE AREA L I Q U I D
to the bottom to produce the air gap providing total internal reflection. The outer ends of these pieces rest on the window ledges to produce some depth compensation. Small spacers are used under each step so that air bubbles can escape during filling. The number, lengths and thicknesses of the steps can be chosen in first approximation from the response of the tank with only the total reflecting surfaces installed. Some modifications are then necessary, possibly because lucite has a relatively short light attenuation length. We used five steps (all full width) for compensation with final lengths and thicknesses of: 27"x ¼", 19", 12", 6½" and 4" (allx~"). With ll =15", /2---25"
SCINTILLATION
DETECTORS
293
and d = 2½", the longitudinal distribution obtained from center-to-window is shown in fig. 3 along with an idealized representation of the response. The lateral response with this arrangement was checked and found to be linear to within our errors. This system o f compensation has been adopted for our scintillator tanks. The steps were easy to assemble and have been found to be quite durable. The tanks must be leveled carefully since the scintillant depth is so shallow especially near the windows. The authors are indebted to T. Bowen, R. Ellsworth and G. Yodh for valuable discussions.