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A scintillating gaseous hydrogen drift chamber for use as a position-sensitive target
NUCLEAR
INSTRUMENTS
AND M E T H O D S
165 ( 1 9 7 9 ) 3 5 1 - 3 5 3 ;
t~) N O R T H - H O L L A N D
PUBLISHING
CO.
A SCINTILLATING GASEOUS HYDROGEN DRIFT CHAMBER FOR USE AS A POSITION-SENSITIVE TARGET* B. DIETERLE, PETER DENES
The University of New Mexico, Albuquerque, New Mexico 87131, USA and J.B. DONAHUE
Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, USA Received 14 February 1979 At room temperature and pressure the hydrogen gas gain is 60, and the drift velocity is 0.5 cm//ts for E = 200 V/cm. One MeV of ionization produces 15 photons for 1850 A < ; t < 2 1 0 0 °.
We have constructed a drift chamber filled with 100% hydrogen which has a photomultiplier viewing the drift region. Single u-tracks in the detector give coincidence pulses from the photomultiplier and the sense wire, caused by hydrogen scintillation and charge collection, respectively. When used as a target, a charged particle time and position can be determined using the two signals and the drift velocity of electrons in hydrogen. The device is well suited for neutral beam particle experiments with low energy reaction products such as elastic np or 7p small angle scattering. The neutral beam would not cause a count, but recoil protons would tag interactions in the target with nanosecond and millimeter time and space resolutions. The proton would not have to penetrate target walls to be detected. Similar detectors have been described 1) but none have used the scintillation of hydrogen. Hydrogen gas is in fact a weak scintillator 2) in the visible region and has low gain when used as a gas in a proportional counter 3) or drift chamber. Thus, the sole motivation for pursuing the use of hydrogen is its importance as a proton target. Eventually we hope to detect recoil protons from Mott-Schwinger Scattering 4,5) with this target/recoil proton detector. The test device we constructed is shown in fig. 1. It consists of a drift chamber with one sense wire (6.5 cm long × 20 a m dia.) and a 3.5 x 6.5 × 8 cm drift region which is enclosed in a phenolic box 7.5 × 10 × 15 cm. The drift region is * This work supported in part by the United States Department of Energy.
viewed directly by an RCA 31000Q (1850 A cutoff quartz window) photomultiplier (QPM) inserted through one of two ports and sealed with an O-ring. Hydrogen gas (99.99596 pure) is flowed continuously through the apparatus during operation. Tests with u-particles were performed with a 239pu source (0.1/2Ci) in the box on one side of the drift region and a piece of 3.2 mm thick scintillator on the opposite side. The scintillator was used to seal off the second port and was viewed from the outside of the box by an RCA 6810A photomultiplier (TPM). Alpha particles traversed the box and drift chamber and caused a pulse in TPM which triggered an oscilloscope. Outputs from QPM and the sense wire (SW) were displayed on the screen.
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Fig. 1. The test device. The drift chamber collects electrons on the sense wire (SW) and the RCA 31000Q phototube detects photons from a~-tracks in the drift region. The solid scintillator (NE104) is used to define an a~ track and is covered with thin AI foil so that only the 6810A phototube views it.
352 +HV
B. DIETERLE et al. IOK
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IIIIIIIIIIIIII Fig. 2. Tracings of typical pulses. The oscilloscope was triggered by the RCA 6810A output (TPM). Trace (1) is the sense wire (SW) and trace (2) is the RCA 31000Q output (QPM). The latter detects light in time with TPM due to hydrogen scintillation and light in time with SW. Trace (3) is a typical scintillation pulse with trace (2) expanded.
All tests were at room temperature (21 °C) and pressure (635 mm of Fig). Drift chamber operation was ascertained by observing coincidences between the SW and the TPM with the drift potential set at - 1 8 0 0 V. Breakdown occurred at + 1750 V on the sense wire with stable operation at 1700 V. Signal/ noise was optimized on the QPM by eye with - 1 9 3 0 V on the photocathode using the same mode of trigger. Under these conditions we did observe 2-fold (TPM, SW), 2-fold (TPM, QPM) and 3-fold (TPM, QPM, SW) coincidences. Fig. 2 shows traces obtained by using TPM to trigger the Tektronix 485 oscilloscope and displaying correlated pulses from the QPM and SW. We have performed several tests to show that the interpretation of the pulses is correct: (1) Remove u-source, or fill with air and TPM pulses go away. (2) Cover scintillator plastic with two (instead of one) layers of 0.6 # m AI foil and QPM and SW pulses are unchanged when sweep is triggered by TPM. (3) Remove scintillator from TPM agd triggers go away. Then lower scope trigger threshold until sweeps occur. Now the SW pulses occur but in time with the TPM instead of delayed. (4) Cover QPM with black paper and no signal is observed. (5) Cover source with thin AI foil and QPM pulses remain. Test (1) shows the scope sweep is mainly generated by alphas. Test (2) shows light is not leaking
Fig. 3. Block diagram of the electronics. EF is an emitter follower attached to SW and LA is a wide band amplifier with a gain of - 25.
out of the solid scintillator and making pulses in QPM (as opposed to hydrogen scintillation). Test (3) shows an effect observed by other researche r s l ) - t h e emission of light from the region around the sense wire. Tests (4) and (5) show the QPM signals are not electrical pickup or light from the source. Additional, but incomplete investigations by us show that saturation effects in the drift velocity are present and that the effect of the drift field on the hydrogen scintillation is <20% for E = 1 0 0 to 200 V cm -1 in agreement with other work6). In fig. 2 we show typical pulse heights and time delays for the electronics system shown in fig. 3. The results are: (a) the time delay of the SW is about 9/.ts for a 4.5 cm drift path giving a drift velocity of 0.5 crags -1. (b) The QPM pulses are due to single and double electron emission at the photocarhode, as can be seen from pulse height spectra. Using Poisson statistics and the relative number of zero, one, and two photoelectron events we find that only 2photons c m - ~ or 15 photons MeV- t have been emitted. We have assumed the quantum efficiency of QPM to be 30%, which may be an overestimate since it is falling near the cutoff of 1850,( and we h a v e d e t e r m i n e d that >80% of the photons are in the region 1850/~<2 <2100 A. The evidence for this is the loss of signal when a 3 mm quartz plate is placed over the face of QPM. We expect a significant increase in the light signal when wavelength shifters or LiF windows are used to extend the sensitive region farther into the UV.
A SCINTILLATING GASEOUS HYDROGEN DRIFT CHAMBER
(c) Using a gain of 25 for the amplifier on the sense wire we determine that 1.5 ×10 -~3 C were collected at the sense wire. If the average ionization energy is 36 eV, we obtain a gas multiplication factor of 60. (d) QPM has delayed light pulses which are in time with the SW pulse and are thus thought to be light from the discharge at the SW. Under our conditions this pulse was about six times the size of the gas scintillation pulse. As a comparison Rossi 3) obtains gas gains of about 100 and Rice-Evans 7) quotes drift velocities which we extrapolate to be N 0 . 2 c m / l s -~ for the same E/p (V cm -I mm of Hg). Saturation effects might explain the latter comparison which is not good. We were not able to find equivalent data on hydrogen scintillation light output. Future development of the target will include studies of the following:
353
(1) LiF window PMT. (2) Improved light collection with reflectors and wavelength shifters. (3) Light output versus-electric field. (4) Operation with different density, temperature, and pressure. (5) Better systematics-many of our numbers could be wrong by a factor of 2. References 1) A. J. P. L. Policarpo, M. A. F. Alves and M. Salete S. C. P. Leite, Nucl. Instr. and Meth. 102 (1972) 49. 2) A.E. Griln and E. Shopper, Z. Naturforsch. 6a (1951) 698. 3) B. Rossi and H. Staub, Ionization Chambers and Counters (McGraw-Hill, New York, 1949). 4) N.F. Mott, Proc. Roy. Soc. (London) A124 (1929) 425. 5) j. Schwinger, Phys. Rev. 73 (1948) 407. 6) A. Symanski and J.A. Herman, J. Chem. Phys. 60 (1963) 349. 7) p. Rice-Evans, Spark, Streamer and Drift Chambers (The Richlieu Press, London, 1974).
INSTRUMENTS
AND M E T H O D S
165 ( 1 9 7 9 ) 3 5 1 - 3 5 3 ;
t~) N O R T H - H O L L A N D
PUBLISHING
CO.
A SCINTILLATING GASEOUS HYDROGEN DRIFT CHAMBER FOR USE AS A POSITION-SENSITIVE TARGET* B. DIETERLE, PETER DENES
The University of New Mexico, Albuquerque, New Mexico 87131, USA and J.B. DONAHUE
Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545, USA Received 14 February 1979 At room temperature and pressure the hydrogen gas gain is 60, and the drift velocity is 0.5 cm//ts for E = 200 V/cm. One MeV of ionization produces 15 photons for 1850 A < ; t < 2 1 0 0 °.
We have constructed a drift chamber filled with 100% hydrogen which has a photomultiplier viewing the drift region. Single u-tracks in the detector give coincidence pulses from the photomultiplier and the sense wire, caused by hydrogen scintillation and charge collection, respectively. When used as a target, a charged particle time and position can be determined using the two signals and the drift velocity of electrons in hydrogen. The device is well suited for neutral beam particle experiments with low energy reaction products such as elastic np or 7p small angle scattering. The neutral beam would not cause a count, but recoil protons would tag interactions in the target with nanosecond and millimeter time and space resolutions. The proton would not have to penetrate target walls to be detected. Similar detectors have been described 1) but none have used the scintillation of hydrogen. Hydrogen gas is in fact a weak scintillator 2) in the visible region and has low gain when used as a gas in a proportional counter 3) or drift chamber. Thus, the sole motivation for pursuing the use of hydrogen is its importance as a proton target. Eventually we hope to detect recoil protons from Mott-Schwinger Scattering 4,5) with this target/recoil proton detector. The test device we constructed is shown in fig. 1. It consists of a drift chamber with one sense wire (6.5 cm long × 20 a m dia.) and a 3.5 x 6.5 × 8 cm drift region which is enclosed in a phenolic box 7.5 × 10 × 15 cm. The drift region is * This work supported in part by the United States Department of Energy.
viewed directly by an RCA 31000Q (1850 A cutoff quartz window) photomultiplier (QPM) inserted through one of two ports and sealed with an O-ring. Hydrogen gas (99.99596 pure) is flowed continuously through the apparatus during operation. Tests with u-particles were performed with a 239pu source (0.1/2Ci) in the box on one side of the drift region and a piece of 3.2 mm thick scintillator on the opposite side. The scintillator was used to seal off the second port and was viewed from the outside of the box by an RCA 6810A photomultiplier (TPM). Alpha particles traversed the box and drift chamber and caused a pulse in TPM which triggered an oscilloscope. Outputs from QPM and the sense wire (SW) were displayed on the screen.
/j
/
. ~ -..
/
/
'
/
e
/
_1,"
II I
,'//
,,
I //
Fig. 1. The test device. The drift chamber collects electrons on the sense wire (SW) and the RCA 31000Q phototube detects photons from a~-tracks in the drift region. The solid scintillator (NE104) is used to define an a~ track and is covered with thin AI foil so that only the 6810A phototube views it.
352 +HV
B. DIETERLE et al. IOK
IKpF
]
II II I-IIII
__ --
Te-
- -
TRIGGER
(~)
-
(2)
I
~
i
I tS ---
-~
'
-] I I
II]1
200mV, 2ys/div
II
I I I II
I I I
II
I II
II
II
I
20mV, l O n s / d i v
I1
I I-'J
(3) ~ i
IIIIIIIIIIIIII Fig. 2. Tracings of typical pulses. The oscilloscope was triggered by the RCA 6810A output (TPM). Trace (1) is the sense wire (SW) and trace (2) is the RCA 31000Q output (QPM). The latter detects light in time with TPM due to hydrogen scintillation and light in time with SW. Trace (3) is a typical scintillation pulse with trace (2) expanded.
All tests were at room temperature (21 °C) and pressure (635 mm of Fig). Drift chamber operation was ascertained by observing coincidences between the SW and the TPM with the drift potential set at - 1 8 0 0 V. Breakdown occurred at + 1750 V on the sense wire with stable operation at 1700 V. Signal/ noise was optimized on the QPM by eye with - 1 9 3 0 V on the photocathode using the same mode of trigger. Under these conditions we did observe 2-fold (TPM, SW), 2-fold (TPM, QPM) and 3-fold (TPM, QPM, SW) coincidences. Fig. 2 shows traces obtained by using TPM to trigger the Tektronix 485 oscilloscope and displaying correlated pulses from the QPM and SW. We have performed several tests to show that the interpretation of the pulses is correct: (1) Remove u-source, or fill with air and TPM pulses go away. (2) Cover scintillator plastic with two (instead of one) layers of 0.6 # m AI foil and QPM and SW pulses are unchanged when sweep is triggered by TPM. (3) Remove scintillator from TPM agd triggers go away. Then lower scope trigger threshold until sweeps occur. Now the SW pulses occur but in time with the TPM instead of delayed. (4) Cover QPM with black paper and no signal is observed. (5) Cover source with thin AI foil and QPM pulses remain. Test (1) shows the scope sweep is mainly generated by alphas. Test (2) shows light is not leaking
Fig. 3. Block diagram of the electronics. EF is an emitter follower attached to SW and LA is a wide band amplifier with a gain of - 25.
out of the solid scintillator and making pulses in QPM (as opposed to hydrogen scintillation). Test (3) shows an effect observed by other researche r s l ) - t h e emission of light from the region around the sense wire. Tests (4) and (5) show the QPM signals are not electrical pickup or light from the source. Additional, but incomplete investigations by us show that saturation effects in the drift velocity are present and that the effect of the drift field on the hydrogen scintillation is <20% for E = 1 0 0 to 200 V cm -1 in agreement with other work6). In fig. 2 we show typical pulse heights and time delays for the electronics system shown in fig. 3. The results are: (a) the time delay of the SW is about 9/.ts for a 4.5 cm drift path giving a drift velocity of 0.5 crags -1. (b) The QPM pulses are due to single and double electron emission at the photocarhode, as can be seen from pulse height spectra. Using Poisson statistics and the relative number of zero, one, and two photoelectron events we find that only 2photons c m - ~ or 15 photons MeV- t have been emitted. We have assumed the quantum efficiency of QPM to be 30%, which may be an overestimate since it is falling near the cutoff of 1850,( and we h a v e d e t e r m i n e d that >80% of the photons are in the region 1850/~<2 <2100 A. The evidence for this is the loss of signal when a 3 mm quartz plate is placed over the face of QPM. We expect a significant increase in the light signal when wavelength shifters or LiF windows are used to extend the sensitive region farther into the UV.
A SCINTILLATING GASEOUS HYDROGEN DRIFT CHAMBER
(c) Using a gain of 25 for the amplifier on the sense wire we determine that 1.5 ×10 -~3 C were collected at the sense wire. If the average ionization energy is 36 eV, we obtain a gas multiplication factor of 60. (d) QPM has delayed light pulses which are in time with the SW pulse and are thus thought to be light from the discharge at the SW. Under our conditions this pulse was about six times the size of the gas scintillation pulse. As a comparison Rossi 3) obtains gas gains of about 100 and Rice-Evans 7) quotes drift velocities which we extrapolate to be N 0 . 2 c m / l s -~ for the same E/p (V cm -I mm of Hg). Saturation effects might explain the latter comparison which is not good. We were not able to find equivalent data on hydrogen scintillation light output. Future development of the target will include studies of the following:
353
(1) LiF window PMT. (2) Improved light collection with reflectors and wavelength shifters. (3) Light output versus-electric field. (4) Operation with different density, temperature, and pressure. (5) Better systematics-many of our numbers could be wrong by a factor of 2. References 1) A. J. P. L. Policarpo, M. A. F. Alves and M. Salete S. C. P. Leite, Nucl. Instr. and Meth. 102 (1972) 49. 2) A.E. Griln and E. Shopper, Z. Naturforsch. 6a (1951) 698. 3) B. Rossi and H. Staub, Ionization Chambers and Counters (McGraw-Hill, New York, 1949). 4) N.F. Mott, Proc. Roy. Soc. (London) A124 (1929) 425. 5) j. Schwinger, Phys. Rev. 73 (1948) 407. 6) A. Symanski and J.A. Herman, J. Chem. Phys. 60 (1963) 349. 7) p. Rice-Evans, Spark, Streamer and Drift Chambers (The Richlieu Press, London, 1974).