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A scintillation counter hodoscope for 10 MHz beams
Nuclear Instruments and Methods 200 (1982) 245-247 North-Holland Publishing Company
A SCINTILLATION
COUNTER
HODOSCOPE
245
FOR I0 MHz BEAMS *
C. B R O M B E R G , S . R . W . C O O P E R a n d R . A . L E W I S Department of Physics. Michtgan State University, E. Lansing. MI 48824, U.S.A. Received 21 January 1982
A scintillation counter hodoscope with greater than 10 MHz capability is described. Each hodoscope plane has complete (100%) coverage for 2 cm across the beam,,contains elements as small as I mm, and interposes only 2 mm of scintillating material into the beam.
1. Introduction
2. Hodoscope construction
A study of large PT direct-photon production in hadron interactions at high energies, Fermilab experiment E629, required a beam intensity above 107 charged particles per second and a beam size of approximately I cm (fwhm). In such a beam the counting rate would exceed the limits of conventional multiwire proportional chambers and would approach or exceed the limits of a single element scintillation counter. For these reasons we chose to build a scintillation counter hodoscope to provide (I) a monitor of the beam profile and beam intensity; (2) latched information on the beam position and multiplicity in each triggered event; and (3) fast signals to veto the presence of two or more beam particles within a single rf bucket ( < 1 ns bursts, separated by 19 ns). Existing counter designs [1,2] for this purpose fell short of our goals for rate, efficiency or ease of repair. The counters described here feature two staggered rows of scintillator elements, set in a precision mounting frame, which allows 100% coverage of the beam while limiting the overlap of adjacent elements to 5% of the covered area. The poor quality control on machining and polishing of small commercially supplied scintillator elements has been a major difficulty in the construction of such hodoscopes. In addition, the amount of scintillating material in each element is quite small. Thus the machining dominates the cost of their preparation. We will discuss an inexpensive technique, developed by us, for the preparation of scintillator elements as small as 1 × 2 mm 2 in cross section.
A major component in the design of this hodoscope resulted from a desire to limit the counting rate on each element to a few X 106/s in normal operation. This rate could be easily handled by a straightforward phototube and base design. To achieve the rate goal, under the constraints of the beam size and intensity, 1 mm hodoscope elements were required in the central region of the hodoscope. Other design considerations were to avoid inefficient regions (holes) and to minimize the overlap
a)
b)
--.4 P ~ Imm
2ram j-_
~
OOOO0
000017
T
T
BEAM
* Research supported by the National Science Foundation. 0167-5087/82/0000-0000/$02.75 ~3 1982 North-Holland
Fig. 1. (a) A plastic window frame scintillator support showing the machined slots. (b) A sketch showing the orientation of the scintillator elements after bolting two frames together.
246
C. Bromberg et a L / Scinttllation counter hodoscope
between adjacent elements for veto purposes. Finally. in the event of a failure, we wished to be able to replace a single element without unduly disturbing the remainder. These properties are incorporated in the design described below. Precision slots were machined in a lucite window frame (shown in fig. la) which positioned the scintillator elements accurately to ~-0.01 mm. Two frames, when bolted face to face, formed two rows of staggered elements as indicated in fig. lb. The spacing of the slots was set such that adjacent elements would overlap by 0.01 ram. Eight 1 mm elements span the central portion of the hodoscope flanked on either side by one 2 mm and one 5 mm element. The slots are machined 2 mm in depth (the beam direction) to accept scintillators of this thickness. The scintillators were ordered machined and polished commercially [3]. The light output from the delivered scintillators was acceptable, however, the machining of the elements did not meet the ±0.01 mm tolerances; many of the elements were 0.05 mm outside of specifications. The surface polish was also clearly below standards. We therefore developed a fast and uncomplicated procedure by which additional rough cut blanks or oversized polished pieces from the original shipment could be brought within tolerances and polished. We have found that mylar backed abrasives available commercially [4] in various grit sizes down to 0.5 ~ m are ideal for the sanding and polishing of small scintillators. Starting with a slightly oversize scintillator blank and working by hand, a series of wet sandings followed by micrometer measurements produced a flat finished scintillator element 1 m m × 2 m m x 35 mm within the ±0.01 mm tolerance in about 15 min. After using the 0.5 p,m grit, a final polishing with ~ 2 aluminum silicate was almost unnecessary. The finished scintillators were wrapped with two layers of 0.008 mm aluminized mylar and sealed with a contact cement at one end and along the edge facing out from the slots. The aluminized mylar wrapping allowed the scintillators to be fit and held in the slots but not tightly enough to prevent easy removal. Approximately 3 mm of scintillator extended beyond the support frame allowing the light guides to be glued on. The light guides, of rectangular cross section, were tapered to approximately match the scintillator size on one end and the active area of the 1 / 2 " diameter phototube [5] on the other end. Before wrapping with aluminum foil and black tape, the light guides were heated and bent to shape in a jig in order to space the phototubes and bases as shown in fig. 2. Space limitations require that the six elements in one row be viewed from one side while the remaining six elements be viewed from the other side. The scintillator support frame, phototubes, and bases were mounted on a I / 4 " aluminum plate with a 6"
BASE
PH01
~TILLATOR ~ME
--
IOcm - - - ~ I
Fig. 2. Mounted locations of the scintillator frame, light guides. phototubes and bases.
square cut-out for the beam and halo. A light-tight box, through which the light guides penetrated, was also mounted on the aluminum plate. The penetrations were sealed with black RTV. In the event of a failure, an entire row of scintillators, light guides, and phototubes could be removed as a unit for repair. No such failures in fact occurred during approximately four months of continuous operation.
3. Operational performance Three hodoscope planes were built; two were placed just before the target ( x , . v ) and a third (x), placed 10m downstream of the target, was used as a tag for a noninteracting beam track. A minimum ionizing particle passing through a 1 mm element produced an output pulse of ~ 200 mV into 50 f~ when the phototube was operated at its maximum rated voltage of 1200V. This low but useable signal is due to the relatively low gain ( - 5 X 10 6) of the 10 stage phototubes. A calculation of the photoelectron yield gives n ;~ 20 photoelectrons. A high photoelectron yield is confirmed by the minimal variation in representative signals shown in fig. 3. Two of the tubes produced a signal <~ 100 mV at
C. Bromberg et al. / Scintillation counter hodoscope
247
this probability at the nominal b e a m intensity ( - - 1 × 10T/s) agreed with that predicted from the b e a m structure. The efficiency of a plane was measured to be > 9 8 % indicating that few, if any, spaces were left between elements. At 10 M H z there was no indication of p h o t o t u b e gain sagging. During a particularly bad excursion in the b e a m intensity, 2.1 X 10 7 c o u n t s (twice the normal) were recorded during a single 1.0 s spill. We t h a n k Mr. F. Wagner and Mr. J. Heltsley for c o n t r i b u t i o n s during the construction of the counters.
References Fig. 3. Typical phototube signals: 10 ns/division, 20 mV/division. The small signal near the baseline is provided as a reference for the oscilloscope trigger which was set at about 2 mV.
the m a x i m u m voltage. We therefore used a × 10 amplifier on all channels. This lowered the typical operating voltage to ~ 1000 V. M e a s u r e m e n t s of the probability for greater than 1 element to be active at low b e a m intensities were consistent with the expected yield (5%) based upon the spatial overlap of n e i g h b o r i n g elements. The increase in
[I] J.E. Elias, G. Mikenberg, T. Minto and G.A. Weitsh, Nucl. Instr. and Meth. 141 (1977) 25. [2] R.R. Crittenden, S.C. Ems, R.M. Heinz and J.C. Krider, Nucl. Instr. and Meth. 186 (1981) 519. [3] NDI20 scintillator obtained from National Diagnostics, 198 Rt. 206 South, Somerville, NJ. This scintillator material is similar to NEI02. [4] Flexi-Grit, RDC Industries, Philadelphia, PA 19432. [5] Phototubes and bases were obtained from Hamamatsu Corporation, 420 South Ave., Middlesex, NJ. The base is a high rate modification of the company's design. We thank H. Gordon of BNL for discussions regarding these phototubes and bases.
A SCINTILLATION
COUNTER
HODOSCOPE
245
FOR I0 MHz BEAMS *
C. B R O M B E R G , S . R . W . C O O P E R a n d R . A . L E W I S Department of Physics. Michtgan State University, E. Lansing. MI 48824, U.S.A. Received 21 January 1982
A scintillation counter hodoscope with greater than 10 MHz capability is described. Each hodoscope plane has complete (100%) coverage for 2 cm across the beam,,contains elements as small as I mm, and interposes only 2 mm of scintillating material into the beam.
1. Introduction
2. Hodoscope construction
A study of large PT direct-photon production in hadron interactions at high energies, Fermilab experiment E629, required a beam intensity above 107 charged particles per second and a beam size of approximately I cm (fwhm). In such a beam the counting rate would exceed the limits of conventional multiwire proportional chambers and would approach or exceed the limits of a single element scintillation counter. For these reasons we chose to build a scintillation counter hodoscope to provide (I) a monitor of the beam profile and beam intensity; (2) latched information on the beam position and multiplicity in each triggered event; and (3) fast signals to veto the presence of two or more beam particles within a single rf bucket ( < 1 ns bursts, separated by 19 ns). Existing counter designs [1,2] for this purpose fell short of our goals for rate, efficiency or ease of repair. The counters described here feature two staggered rows of scintillator elements, set in a precision mounting frame, which allows 100% coverage of the beam while limiting the overlap of adjacent elements to 5% of the covered area. The poor quality control on machining and polishing of small commercially supplied scintillator elements has been a major difficulty in the construction of such hodoscopes. In addition, the amount of scintillating material in each element is quite small. Thus the machining dominates the cost of their preparation. We will discuss an inexpensive technique, developed by us, for the preparation of scintillator elements as small as 1 × 2 mm 2 in cross section.
A major component in the design of this hodoscope resulted from a desire to limit the counting rate on each element to a few X 106/s in normal operation. This rate could be easily handled by a straightforward phototube and base design. To achieve the rate goal, under the constraints of the beam size and intensity, 1 mm hodoscope elements were required in the central region of the hodoscope. Other design considerations were to avoid inefficient regions (holes) and to minimize the overlap
a)
b)
--.4 P ~ Imm
2ram j-_
~
OOOO0
000017
T
T
BEAM
* Research supported by the National Science Foundation. 0167-5087/82/0000-0000/$02.75 ~3 1982 North-Holland
Fig. 1. (a) A plastic window frame scintillator support showing the machined slots. (b) A sketch showing the orientation of the scintillator elements after bolting two frames together.
246
C. Bromberg et a L / Scinttllation counter hodoscope
between adjacent elements for veto purposes. Finally. in the event of a failure, we wished to be able to replace a single element without unduly disturbing the remainder. These properties are incorporated in the design described below. Precision slots were machined in a lucite window frame (shown in fig. la) which positioned the scintillator elements accurately to ~-0.01 mm. Two frames, when bolted face to face, formed two rows of staggered elements as indicated in fig. lb. The spacing of the slots was set such that adjacent elements would overlap by 0.01 ram. Eight 1 mm elements span the central portion of the hodoscope flanked on either side by one 2 mm and one 5 mm element. The slots are machined 2 mm in depth (the beam direction) to accept scintillators of this thickness. The scintillators were ordered machined and polished commercially [3]. The light output from the delivered scintillators was acceptable, however, the machining of the elements did not meet the ±0.01 mm tolerances; many of the elements were 0.05 mm outside of specifications. The surface polish was also clearly below standards. We therefore developed a fast and uncomplicated procedure by which additional rough cut blanks or oversized polished pieces from the original shipment could be brought within tolerances and polished. We have found that mylar backed abrasives available commercially [4] in various grit sizes down to 0.5 ~ m are ideal for the sanding and polishing of small scintillators. Starting with a slightly oversize scintillator blank and working by hand, a series of wet sandings followed by micrometer measurements produced a flat finished scintillator element 1 m m × 2 m m x 35 mm within the ±0.01 mm tolerance in about 15 min. After using the 0.5 p,m grit, a final polishing with ~ 2 aluminum silicate was almost unnecessary. The finished scintillators were wrapped with two layers of 0.008 mm aluminized mylar and sealed with a contact cement at one end and along the edge facing out from the slots. The aluminized mylar wrapping allowed the scintillators to be fit and held in the slots but not tightly enough to prevent easy removal. Approximately 3 mm of scintillator extended beyond the support frame allowing the light guides to be glued on. The light guides, of rectangular cross section, were tapered to approximately match the scintillator size on one end and the active area of the 1 / 2 " diameter phototube [5] on the other end. Before wrapping with aluminum foil and black tape, the light guides were heated and bent to shape in a jig in order to space the phototubes and bases as shown in fig. 2. Space limitations require that the six elements in one row be viewed from one side while the remaining six elements be viewed from the other side. The scintillator support frame, phototubes, and bases were mounted on a I / 4 " aluminum plate with a 6"
BASE
PH01
~TILLATOR ~ME
--
IOcm - - - ~ I
Fig. 2. Mounted locations of the scintillator frame, light guides. phototubes and bases.
square cut-out for the beam and halo. A light-tight box, through which the light guides penetrated, was also mounted on the aluminum plate. The penetrations were sealed with black RTV. In the event of a failure, an entire row of scintillators, light guides, and phototubes could be removed as a unit for repair. No such failures in fact occurred during approximately four months of continuous operation.
3. Operational performance Three hodoscope planes were built; two were placed just before the target ( x , . v ) and a third (x), placed 10m downstream of the target, was used as a tag for a noninteracting beam track. A minimum ionizing particle passing through a 1 mm element produced an output pulse of ~ 200 mV into 50 f~ when the phototube was operated at its maximum rated voltage of 1200V. This low but useable signal is due to the relatively low gain ( - 5 X 10 6) of the 10 stage phototubes. A calculation of the photoelectron yield gives n ;~ 20 photoelectrons. A high photoelectron yield is confirmed by the minimal variation in representative signals shown in fig. 3. Two of the tubes produced a signal <~ 100 mV at
C. Bromberg et al. / Scintillation counter hodoscope
247
this probability at the nominal b e a m intensity ( - - 1 × 10T/s) agreed with that predicted from the b e a m structure. The efficiency of a plane was measured to be > 9 8 % indicating that few, if any, spaces were left between elements. At 10 M H z there was no indication of p h o t o t u b e gain sagging. During a particularly bad excursion in the b e a m intensity, 2.1 X 10 7 c o u n t s (twice the normal) were recorded during a single 1.0 s spill. We t h a n k Mr. F. Wagner and Mr. J. Heltsley for c o n t r i b u t i o n s during the construction of the counters.
References Fig. 3. Typical phototube signals: 10 ns/division, 20 mV/division. The small signal near the baseline is provided as a reference for the oscilloscope trigger which was set at about 2 mV.
the m a x i m u m voltage. We therefore used a × 10 amplifier on all channels. This lowered the typical operating voltage to ~ 1000 V. M e a s u r e m e n t s of the probability for greater than 1 element to be active at low b e a m intensities were consistent with the expected yield (5%) based upon the spatial overlap of n e i g h b o r i n g elements. The increase in
[I] J.E. Elias, G. Mikenberg, T. Minto and G.A. Weitsh, Nucl. Instr. and Meth. 141 (1977) 25. [2] R.R. Crittenden, S.C. Ems, R.M. Heinz and J.C. Krider, Nucl. Instr. and Meth. 186 (1981) 519. [3] NDI20 scintillator obtained from National Diagnostics, 198 Rt. 206 South, Somerville, NJ. This scintillator material is similar to NEI02. [4] Flexi-Grit, RDC Industries, Philadelphia, PA 19432. [5] Phototubes and bases were obtained from Hamamatsu Corporation, 420 South Ave., Middlesex, NJ. The base is a high rate modification of the company's design. We thank H. Gordon of BNL for discussions regarding these phototubes and bases.