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A chopped high intensity muon beam at the Stopped Muon Channel at LAMPF
Nuclear Instruments and Methods in Physics Research A 333 (1993) 260-264 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
A chopped high intensity muon beam at the Stopped Muon Channel at LAMPF D. Ciskowski, H. Ahn, R. Dixson, X. Fei, V.W. Hughes and B.E. Matthias ' Department of Physics, Yale University . New Haven, CT 06511, USA
C. Pillai
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
K. Woodle
Brookhaven National Laboratory, Upton, Long Island, NY 1197.3, USA
Received 15 March 1993 A chopped positive muon beam with high intensity, low momentum, and high purity has been developed at LAMPF using a high voltage chopper system . The rise and fall times of the chopped muon beam are approximately 100 ns, the repetition rate extends up to 100 kHz and the extinction ratio for muons is 0 .3% . The beam chopper system is described and the test results are presented. This chopped high intensity W+ beam is to be used to achieve resonance line-narrowing in a precision microwave spectroscopy experiment on muonium
1 . Introduction The Stopped Muon Channel (SMC) [1] at the Clinton P. Anderson Meson Physics Facility (LAMPF) has provided high intensity positive muon beams from several modes of operation including El + beams from ,T+ w+ beams [2] and subsurface decay in flight, surface P, + beams [3]. These beams are customarily provided with the time structure of the LAMPF proton beam which consists of pulses of 600 I.Ls to 800 [Ls duration at a repetition rate of 120 Hz . For some experiments however it is advantageous to have a muon beam with a pulsed time structure. For application to muon spin resonance (wSR) experiments a beam chopper was developed at LAMPF to provide a single muon in a target at a time [4]. Pulsed muon beams are available now at KEK [5] from their 500 MeV proton synchrotron with a beam intensity of 10 4 l.L +/50 ns pulse and a repetition rate of 20 Hz, and at the Rutherford-Appleton Laboratory from their 800 MeV 100 wA fast cycling proton synchrotron with a w+ beam intensity of 8000 I.L +/pulse in a double pulse structure where each pulse has a width of 70 ns and i Present address: Universität Heidelberg, Physikalisches Institut, D-6900 Heidelberg 1, Germany.
the spacing is 340 ns and a repetition rate of 50 Hz [6]. Pulsed muons at a momentum of 25 MeV/c were produced at BNL [7] from the 30 GeV AGS with a beam intensity of 10 4 w -/50 ns micropulse (there are 12 micropulses per AGS cycle) and an AGS repetition rate of 0 .7 Hz . For the muonium spectroscopy experiments, such as a new high precision measurement of the muon magnetic moment and the muonium ground state hyperfine structure interval currently in progress [8], a chopped or pulsed high intensity muon beam is required to allow "old" muonium atoms formed in a noble gas to be selected for resonance line narrowing [9]. These "old" muonium atoms interact with microwaves for more than one muon lifetime of 2.2 lts. The chopped or pulsed muon beam should have rise and fall times much shorter than the muon mean lifetime in order to select the muonium atoms for observation for an experiment at LAMPF. Until a pulsed muon beam now being designed [10] for the LAMPF proton storage ring is available, the high intensity muon beam of 10 7 ll +/s at LAMPF must be chopped to provide the required time structure. The possibility of producing a chopped surface muon beam at TRIUMF was considered recently [1l] . We measured the characteristics of the chopper system including the rise and fall times, the chopping
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D Ciskowski et al. /Chopped high intensity muon beam at LAMPF
frequency or chopper repetition rate and the extinction ratio of muons at the SMC at LAMPF. Positron conlaminations were measured . In this paper we discuss the instrumentation of the chopper system and present our results for the chopped high intensity muon beam test .
2. Experimental arrangement A diagram of the beam line and the apparatus used for the development of the chopped high intensity positive muon beam is shown in fig. 1 . Positive muons from the SMC passed a gas barrier, used to block the radioactive gases from the channel upstream, and entered a static E X B beam separator, used as a velocity selector with the electric field E perpendicular to the magnetic field B. The separator has a gap of 10 .2 cm between two stainless steel plates each with a width of 20 .3 cm and a length of 152.4 cm, and was typically operated at f 70 kV, though the maximum voltage can be more than t 100 kV . The magnetic field was tuned to maximize the muon beam transmission and remove beam positrons. A separator curve, illustrating the dependence of transmitted rates of e + and p + on magnetic field setting for constant E, is shown in fig. 2. The transmitted rates were measured by an ionization chamber which has different detection efficiencies for e+ and I.L +. The separator was conditioned at ±70 kV for 28 MeV/c e + and lt +. The muon beam then passed through a set of quadrupoles (Q1 and Q2) to the chopper where it was either diverted or passed .
26 1
The separator was placed upstream of the chopper to steer away the beam positrons since the electric field in the chopper alone is not sufficient to deflect beam positrons from the beam and to obtain a small beam envelop for the positrons entering the separator. The muon beam spot as it passed through the chopper was typically 8.0 cm in diameter . The chopper was followed by a lead collimator with a diameter of 5.1 cm (removed later to obtain a maximum muon rate), then by a 1 .33 m long vacuum beampipe, and another lead collimator of 7.1 cm diameter . A muon beam counter downstream of the chopper detected W+ as they passed through a 200 wm thick plastic scintillator . A 25 MeV/c muon deposits an average 439 keV in this scintillator where light was monitored by four Amperex XP2020 photo-multiplier tubes (PMTs) . The analog signals from these PMTS were summed and a threshold was set on the sum of the PMT signals corresponding to 3 of the PMTS triggering simultaneously . This counter was used for low rate muon counting. The detector system downstream of the muon counter is shown in fig. 3. It consists of either a beam profile monitor or an ionization chamber used for the beam diagnosis, a scintillation counter S1 to stop muons, and scintillation counters S2 and S3 to detect positrons. The beam profile monitor measured the transverse spot size of the beam with a spatial resolution of 5 mm . The ionization chamber was calibrated against the muon beam counter at low rate and then was used for high rate measurement . Muons passing through the muon counter were stopped in S1 . Beam positrons and decay positrons from stopped muons were observed in S2 and S3 .
Muon Counter
Fig. 1 . Beam line and apparatus for production of a chopped muon beam. The electric field of the separator is perpendicular to the separator plates and the magnetic field is perpendicular both to the electric field and to the paper. The insert for the chopper shows the front view of the chopper plates with two grounding bars to shape the electric field and indicates gaps and widths of the plates and bars . The electric field of the chopper is perpendicular to the chopper plates . The Al foil is used as the vacuum window .
262
D. Clskowskl et al. / Chopped high intensity muon beam at LAMPF
3. Chopper system
20 .0
m 0 ô v
15 .0
10 .0
5.0
50
150
100
200
B (Gauss)
Fig. 2. Measured rate in ionization chamber of positrons and muons with E field at ± 70 kV and B field varying from 0 to 210 G. The positron and muon peaks are indicated. The beamline elements of the SMC were tuned to transport a maximum number of muons for a muon beam momentum of pw = 28 .5 MeV/c with a momentum spread of about 10%. The proton current on the A2 production target (4 cm thick) was 735 I.LA with a duty factor of 5.1%. We obtained an average unchopped muon rate of 9.5 X 10 6 /s which was limited in part by the separator spacing of 10 cm and the collimator diameter of 7.1 cm . Muon beams of pw = 25 MeV/c and 28 MeV/c were used for the chopper tests.
_L
a
IONIZATION CHAMBER
r -~ "11
-i I I
s
N
N
I
BEAM
AI WINDOW
N
N
The central component of the apparatus for producing the desired chopped muon beam is the chopper system [12] consisting of two high voltage (HV) modulators with one for the positive HV supply and the other for the negative HV supply, a HV control unit, two 6 kV power supplies for the cathode bias in the modulators, and two stainless steel plates in vacuum, 100.35 cm X 15 .20 cm, separated by 10 .2 cm . The capacitance of the chopper plates is 13 pF . A schematic electronics diagram of the chopper system with high voltage modulators and chopper plates is shown in fig. 4. Each plate of the chopper can be ramped to ± 20 kV and was run at ± 13 .5 kV, ± 17 .5 kV, and ± 20 kV in our test . With a high voltage of ± 13 .5 kV on the plates a 25 MeV/c muon beam is deflected into a plate near the end of the chopper. The plates are suspended inside a vacuum cavity by ceramic insulators and the high voltage to each plate is provided via a high voltage feedthrough from the high voltage modulator with the power supply for each plate rated to 30 kV at 100 mA. The muon beam is turned off when the high voltage to the plates is on . The high voltage modulator that can switch the high voltage with a pre-set time period is the most critical component of this system. This device [12] was devel-
I
I MYLAR I WINDOWS
I
v
I N I
L _ _ _ J N
30 cm w SCALE
N S1
ti
y n
N S2
N
N
v S3
Fig. 3. Beam diagnosis system consisting of an ionization chamber, a muon stopping counter S1 (0 .95 cm thick), and positron counters S2 (0 .95 cm thick) and S3 (0 .64 cm thick) . A beam profile monitor replacing the ionization chamber was used to measure the transverse spot size of the muon beam .
FIg. 4. Schematic diagram of the chopper electronics system . Eimac YU114 triodes are used as high voltage switching tubes.
D. Ctskowski et al. / Chopped high intensity muon beam at LAMPF
Fig. 5. Phase stable trigger for the chopper system using two delay generators to control HV on-time by gate 1 and HV off-time by gate 2 within the beam gate .
oped for the ion source electrostatic kicker at LAMPF and was designed to operate with real-time control of pulse width and a maximum pulse height of 25 kV . The chopper has two identical circuits using a switch tube
1400 1200 1000-
yc ô U
800 600 400 200 0
r
1200 1000800 -I
500
10 ,00 Delay
(ns)
1500
2000
Fig. 6. Muon counts vs time delay after start of triggers for HV switching . Typical muon beam rise time is 100 ns (a) and fall time is 120 ns (b).
263
assembly which consists of two high power Eimac YU114 triodes in parallel for supplying the positive HV plate and negative HV plate. The cathode bias is obtained from two separate 6 kV power supplies . A phase stable trigger which coincides with the beam gate, shown in fig. 5, was used for the chopper system in our test . The start of the beam gate generates a 50 ns pulse from a discriminator which triggers the gate 1 of a delay generator LeCroy 222 whose delayed output triggers the gate 2 of the second LeCroy unit . The delayed output of the gate 2 then triggers the first gate in order to retrigger the chopper during the beam gate . Both gates are inhibited by anti-beam gate logic when the beam gate is absent so that the chopper triggers only during a beam gate and the first trigger occurs at a pre-set time within the beam gate . The gate 1 output Ll controls the HV on the chopper for time tLl (typically 10 to 20 ws) and the gate 2 output L2 adjusts the HV off time tL2 (typically 1 to 5 ws). Fiber optic links are used to transmit signals from the low voltage triggers to the HV section of the modulators .
4. Results The chopper operates typically at a repetition rate of the order of 100 kHz with adjustable HV on and HV off times and with about 100 ns rise and fall times. Our studies were made with the l.L + beam on (chopper voltage off) for 1 to 5 Ws and with the w + beam off (chopper voltage on) for 10 to 20 ~Ls. Though the HV modulator can switch on as fast as 20 ns [12], the muon beam deflection due to the transit time of the beam through the chopper may have different time structure depending on the beam momentum, maximum high voltage applied on the chopper plates, the length of the chopper plates, and the aperture of the beamline . To measure the time structure of the muon beam with the chopper in operation, the muon counting rate was monitored continuously near the rise and fall edges of the chopper. The muon counter signal (wj) was gated with a width of 20 ns and this gate was timed to the chopper signal and walked across the muon beam pulse. The typical muon beam rise and fall time structures are shown in fig. 6. The rise time (from 10% beam intensity to 90%) was found to be approximately 100 ns . The fall time was about 120 ns (90 to 10%) . The chopper repetition rate, 1/(t Ll + t L2 ), is in the range of 40 kHz to 100 kHz in our test . The chopper duty cycle within the beam gate for w+ is given by tL2/(tLl + tL2) which ranging from 5 to 50%. The effectiveness of the chopper system is characterized by the ability to block the transmission of the muon beam . The muon extinction ratio (i .e . the beam
264
D. Ciskowski et al. / Chopped high intensity muon beam at LAMPF
suppression ratio for the HV on and off) is given by fe =
(Non/Ton) (Noff/ Toff )
where Non(Non) is the number of muons detected with the chopper voltage on (off) and Ton(Toff) is the time that the chopper HV was on (off). The muon extinction ratio was measured to be
fe
= 0.0028(4).
The positron contamination was measured by the
positron telescope (S2 - S3). The positrons come from
the target area where -rr ° decays occur and from muon
decay. It was measured [2] to be e + /w + = 10 at 28 MeV/c at SMC before the use of the static field separator . The measured positron contamination with
the 1 .5 m separator operating at ±70 kV is about 8% . The time structure of the chopped positron background is somewhat different from that of the muon . Rather than an abrupt drop in the counting rate like
the muon beam showed, there was a decay tail in the
positron rate following an initial drop . The rate decays with a time constant compatible with the muon lifetime, suggesting some positrons come from the decay of muons deflected by the chopper.
5. Summary The results of our measurements including the ex-
tinction ratio and rise/fall times for the chopped muon beam are satisfactory for our future muonium experiment . The muon beam chopper system can easily be
set up at other meson beam facilities . The chopper developed for a positive muon beam should also work
for a negative muon beam . The use of a chopped high intensity muon beam would allow other atomic physics
experiments to use line narrowing techniques, and solid state physics and chemistry experiments to use the wSR method with a time dependent muon beam .
Acknowledgements We would like to acknowledge the helpful support of the LAMPF staff especially from Scott Dick, Ted Newlin and Joe Ivie . This work was supported in part by the U.S . Department of Energy .
References [1] P.A . Thompson, V.W . Hughes, W.P . Lysenko and H.F . Vogel, Nucl . Instr. and Meth . 161 (1979) 391 . [2] H.W. Reist, D.E. Casperson, A.B . Denison, P.O . Egan, V.W . Hughes, F.G . Mariam, G. zu Putlitz, P.A . Souder, P.A. Thompson and J. Vetter, Nucl . Instr. and Meth . 153 (1978) 61 . [3] A. Badertscher, P .O . Egan, M. Gladisch, M. Greene, V.W . Hughes, F.G . Mariam, D.C. Lu, G. zu Putlitz, M.W . Ritter, G. Sanders, P.A. Souder and R. Werbeck, Nucl . Instr. and Meth . A238 (1985) 200. [4] R.L. Hutson, D.W. Cooke, R.H . Heffner, M.E . Schillaci, S.A. Dodds and G.A . Gist, Hyperfine Interactions 32 (1986) 893 . [5] K. Nagamine, Z. Phys. C56 (1992) S215 . [6] G.H . Eaton, Z. Phys. C56 (1992) S232 . [7] A. Blaer, J. French, A.M . Sachs, M. May and E. Zavattini, Phys . Rev. A40 (1989) 158. [8] V.W. Hughes, G. zu Putlitz, P. Souder, M. Boshier, D. Ciskowski, S. Dhawan, X. Fei, M. Janousch, W. Liu, W. Schwarz, K. Jungmann, B. Matthias, R. Holmes, J. MeCracken, X. Wang, C. Pillai, O. van Dyck and K. Woodle, LAMPF experiment No . 1054 . ; V.W. Hughes, Z. Phys. C56 (1992) S35. [9] V.W. Hughes and G. zu Putlitz, in : Quantum Electrodynamics, ed . T. Kinoshita (World Scientific, Singapore, 1990) p. 822. [10] D.H. White, Z. Phys . C56 (1992) S255 . [11] J.L. Beveridge, Z. Phys. C56 (1992) S258. [12] G.J . Krausse, Los Alamos National Laboratory report LA-UR-85-2057, presented at the 5th IEEE Pulsed Power Conference, Arlington, VA, June 10-12, 1985.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
A chopped high intensity muon beam at the Stopped Muon Channel at LAMPF D. Ciskowski, H. Ahn, R. Dixson, X. Fei, V.W. Hughes and B.E. Matthias ' Department of Physics, Yale University . New Haven, CT 06511, USA
C. Pillai
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
K. Woodle
Brookhaven National Laboratory, Upton, Long Island, NY 1197.3, USA
Received 15 March 1993 A chopped positive muon beam with high intensity, low momentum, and high purity has been developed at LAMPF using a high voltage chopper system . The rise and fall times of the chopped muon beam are approximately 100 ns, the repetition rate extends up to 100 kHz and the extinction ratio for muons is 0 .3% . The beam chopper system is described and the test results are presented. This chopped high intensity W+ beam is to be used to achieve resonance line-narrowing in a precision microwave spectroscopy experiment on muonium
1 . Introduction The Stopped Muon Channel (SMC) [1] at the Clinton P. Anderson Meson Physics Facility (LAMPF) has provided high intensity positive muon beams from several modes of operation including El + beams from ,T+ w+ beams [2] and subsurface decay in flight, surface P, + beams [3]. These beams are customarily provided with the time structure of the LAMPF proton beam which consists of pulses of 600 I.Ls to 800 [Ls duration at a repetition rate of 120 Hz . For some experiments however it is advantageous to have a muon beam with a pulsed time structure. For application to muon spin resonance (wSR) experiments a beam chopper was developed at LAMPF to provide a single muon in a target at a time [4]. Pulsed muon beams are available now at KEK [5] from their 500 MeV proton synchrotron with a beam intensity of 10 4 l.L +/50 ns pulse and a repetition rate of 20 Hz, and at the Rutherford-Appleton Laboratory from their 800 MeV 100 wA fast cycling proton synchrotron with a w+ beam intensity of 8000 I.L +/pulse in a double pulse structure where each pulse has a width of 70 ns and i Present address: Universität Heidelberg, Physikalisches Institut, D-6900 Heidelberg 1, Germany.
the spacing is 340 ns and a repetition rate of 50 Hz [6]. Pulsed muons at a momentum of 25 MeV/c were produced at BNL [7] from the 30 GeV AGS with a beam intensity of 10 4 w -/50 ns micropulse (there are 12 micropulses per AGS cycle) and an AGS repetition rate of 0 .7 Hz . For the muonium spectroscopy experiments, such as a new high precision measurement of the muon magnetic moment and the muonium ground state hyperfine structure interval currently in progress [8], a chopped or pulsed high intensity muon beam is required to allow "old" muonium atoms formed in a noble gas to be selected for resonance line narrowing [9]. These "old" muonium atoms interact with microwaves for more than one muon lifetime of 2.2 lts. The chopped or pulsed muon beam should have rise and fall times much shorter than the muon mean lifetime in order to select the muonium atoms for observation for an experiment at LAMPF. Until a pulsed muon beam now being designed [10] for the LAMPF proton storage ring is available, the high intensity muon beam of 10 7 ll +/s at LAMPF must be chopped to provide the required time structure. The possibility of producing a chopped surface muon beam at TRIUMF was considered recently [1l] . We measured the characteristics of the chopper system including the rise and fall times, the chopping
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
D Ciskowski et al. /Chopped high intensity muon beam at LAMPF
frequency or chopper repetition rate and the extinction ratio of muons at the SMC at LAMPF. Positron conlaminations were measured . In this paper we discuss the instrumentation of the chopper system and present our results for the chopped high intensity muon beam test .
2. Experimental arrangement A diagram of the beam line and the apparatus used for the development of the chopped high intensity positive muon beam is shown in fig. 1 . Positive muons from the SMC passed a gas barrier, used to block the radioactive gases from the channel upstream, and entered a static E X B beam separator, used as a velocity selector with the electric field E perpendicular to the magnetic field B. The separator has a gap of 10 .2 cm between two stainless steel plates each with a width of 20 .3 cm and a length of 152.4 cm, and was typically operated at f 70 kV, though the maximum voltage can be more than t 100 kV . The magnetic field was tuned to maximize the muon beam transmission and remove beam positrons. A separator curve, illustrating the dependence of transmitted rates of e + and p + on magnetic field setting for constant E, is shown in fig. 2. The transmitted rates were measured by an ionization chamber which has different detection efficiencies for e+ and I.L +. The separator was conditioned at ±70 kV for 28 MeV/c e + and lt +. The muon beam then passed through a set of quadrupoles (Q1 and Q2) to the chopper where it was either diverted or passed .
26 1
The separator was placed upstream of the chopper to steer away the beam positrons since the electric field in the chopper alone is not sufficient to deflect beam positrons from the beam and to obtain a small beam envelop for the positrons entering the separator. The muon beam spot as it passed through the chopper was typically 8.0 cm in diameter . The chopper was followed by a lead collimator with a diameter of 5.1 cm (removed later to obtain a maximum muon rate), then by a 1 .33 m long vacuum beampipe, and another lead collimator of 7.1 cm diameter . A muon beam counter downstream of the chopper detected W+ as they passed through a 200 wm thick plastic scintillator . A 25 MeV/c muon deposits an average 439 keV in this scintillator where light was monitored by four Amperex XP2020 photo-multiplier tubes (PMTs) . The analog signals from these PMTS were summed and a threshold was set on the sum of the PMT signals corresponding to 3 of the PMTS triggering simultaneously . This counter was used for low rate muon counting. The detector system downstream of the muon counter is shown in fig. 3. It consists of either a beam profile monitor or an ionization chamber used for the beam diagnosis, a scintillation counter S1 to stop muons, and scintillation counters S2 and S3 to detect positrons. The beam profile monitor measured the transverse spot size of the beam with a spatial resolution of 5 mm . The ionization chamber was calibrated against the muon beam counter at low rate and then was used for high rate measurement . Muons passing through the muon counter were stopped in S1 . Beam positrons and decay positrons from stopped muons were observed in S2 and S3 .
Muon Counter
Fig. 1 . Beam line and apparatus for production of a chopped muon beam. The electric field of the separator is perpendicular to the separator plates and the magnetic field is perpendicular both to the electric field and to the paper. The insert for the chopper shows the front view of the chopper plates with two grounding bars to shape the electric field and indicates gaps and widths of the plates and bars . The electric field of the chopper is perpendicular to the chopper plates . The Al foil is used as the vacuum window .
262
D. Clskowskl et al. / Chopped high intensity muon beam at LAMPF
3. Chopper system
20 .0
m 0 ô v
15 .0
10 .0
5.0
50
150
100
200
B (Gauss)
Fig. 2. Measured rate in ionization chamber of positrons and muons with E field at ± 70 kV and B field varying from 0 to 210 G. The positron and muon peaks are indicated. The beamline elements of the SMC were tuned to transport a maximum number of muons for a muon beam momentum of pw = 28 .5 MeV/c with a momentum spread of about 10%. The proton current on the A2 production target (4 cm thick) was 735 I.LA with a duty factor of 5.1%. We obtained an average unchopped muon rate of 9.5 X 10 6 /s which was limited in part by the separator spacing of 10 cm and the collimator diameter of 7.1 cm . Muon beams of pw = 25 MeV/c and 28 MeV/c were used for the chopper tests.
_L
a
IONIZATION CHAMBER
r -~ "11
-i I I
s
N
N
I
BEAM
AI WINDOW
N
N
The central component of the apparatus for producing the desired chopped muon beam is the chopper system [12] consisting of two high voltage (HV) modulators with one for the positive HV supply and the other for the negative HV supply, a HV control unit, two 6 kV power supplies for the cathode bias in the modulators, and two stainless steel plates in vacuum, 100.35 cm X 15 .20 cm, separated by 10 .2 cm . The capacitance of the chopper plates is 13 pF . A schematic electronics diagram of the chopper system with high voltage modulators and chopper plates is shown in fig. 4. Each plate of the chopper can be ramped to ± 20 kV and was run at ± 13 .5 kV, ± 17 .5 kV, and ± 20 kV in our test . With a high voltage of ± 13 .5 kV on the plates a 25 MeV/c muon beam is deflected into a plate near the end of the chopper. The plates are suspended inside a vacuum cavity by ceramic insulators and the high voltage to each plate is provided via a high voltage feedthrough from the high voltage modulator with the power supply for each plate rated to 30 kV at 100 mA. The muon beam is turned off when the high voltage to the plates is on . The high voltage modulator that can switch the high voltage with a pre-set time period is the most critical component of this system. This device [12] was devel-
I
I MYLAR I WINDOWS
I
v
I N I
L _ _ _ J N
30 cm w SCALE
N S1
ti
y n
N S2
N
N
v S3
Fig. 3. Beam diagnosis system consisting of an ionization chamber, a muon stopping counter S1 (0 .95 cm thick), and positron counters S2 (0 .95 cm thick) and S3 (0 .64 cm thick) . A beam profile monitor replacing the ionization chamber was used to measure the transverse spot size of the muon beam .
FIg. 4. Schematic diagram of the chopper electronics system . Eimac YU114 triodes are used as high voltage switching tubes.
D. Ctskowski et al. / Chopped high intensity muon beam at LAMPF
Fig. 5. Phase stable trigger for the chopper system using two delay generators to control HV on-time by gate 1 and HV off-time by gate 2 within the beam gate .
oped for the ion source electrostatic kicker at LAMPF and was designed to operate with real-time control of pulse width and a maximum pulse height of 25 kV . The chopper has two identical circuits using a switch tube
1400 1200 1000-
yc ô U
800 600 400 200 0
r
1200 1000800 -I
500
10 ,00 Delay
(ns)
1500
2000
Fig. 6. Muon counts vs time delay after start of triggers for HV switching . Typical muon beam rise time is 100 ns (a) and fall time is 120 ns (b).
263
assembly which consists of two high power Eimac YU114 triodes in parallel for supplying the positive HV plate and negative HV plate. The cathode bias is obtained from two separate 6 kV power supplies . A phase stable trigger which coincides with the beam gate, shown in fig. 5, was used for the chopper system in our test . The start of the beam gate generates a 50 ns pulse from a discriminator which triggers the gate 1 of a delay generator LeCroy 222 whose delayed output triggers the gate 2 of the second LeCroy unit . The delayed output of the gate 2 then triggers the first gate in order to retrigger the chopper during the beam gate . Both gates are inhibited by anti-beam gate logic when the beam gate is absent so that the chopper triggers only during a beam gate and the first trigger occurs at a pre-set time within the beam gate . The gate 1 output Ll controls the HV on the chopper for time tLl (typically 10 to 20 ws) and the gate 2 output L2 adjusts the HV off time tL2 (typically 1 to 5 ws). Fiber optic links are used to transmit signals from the low voltage triggers to the HV section of the modulators .
4. Results The chopper operates typically at a repetition rate of the order of 100 kHz with adjustable HV on and HV off times and with about 100 ns rise and fall times. Our studies were made with the l.L + beam on (chopper voltage off) for 1 to 5 Ws and with the w + beam off (chopper voltage on) for 10 to 20 ~Ls. Though the HV modulator can switch on as fast as 20 ns [12], the muon beam deflection due to the transit time of the beam through the chopper may have different time structure depending on the beam momentum, maximum high voltage applied on the chopper plates, the length of the chopper plates, and the aperture of the beamline . To measure the time structure of the muon beam with the chopper in operation, the muon counting rate was monitored continuously near the rise and fall edges of the chopper. The muon counter signal (wj) was gated with a width of 20 ns and this gate was timed to the chopper signal and walked across the muon beam pulse. The typical muon beam rise and fall time structures are shown in fig. 6. The rise time (from 10% beam intensity to 90%) was found to be approximately 100 ns . The fall time was about 120 ns (90 to 10%) . The chopper repetition rate, 1/(t Ll + t L2 ), is in the range of 40 kHz to 100 kHz in our test . The chopper duty cycle within the beam gate for w+ is given by tL2/(tLl + tL2) which ranging from 5 to 50%. The effectiveness of the chopper system is characterized by the ability to block the transmission of the muon beam . The muon extinction ratio (i .e . the beam
264
D. Ciskowski et al. / Chopped high intensity muon beam at LAMPF
suppression ratio for the HV on and off) is given by fe =
(Non/Ton) (Noff/ Toff )
where Non(Non) is the number of muons detected with the chopper voltage on (off) and Ton(Toff) is the time that the chopper HV was on (off). The muon extinction ratio was measured to be
fe
= 0.0028(4).
The positron contamination was measured by the
positron telescope (S2 - S3). The positrons come from
the target area where -rr ° decays occur and from muon
decay. It was measured [2] to be e + /w + = 10 at 28 MeV/c at SMC before the use of the static field separator . The measured positron contamination with
the 1 .5 m separator operating at ±70 kV is about 8% . The time structure of the chopped positron background is somewhat different from that of the muon . Rather than an abrupt drop in the counting rate like
the muon beam showed, there was a decay tail in the
positron rate following an initial drop . The rate decays with a time constant compatible with the muon lifetime, suggesting some positrons come from the decay of muons deflected by the chopper.
5. Summary The results of our measurements including the ex-
tinction ratio and rise/fall times for the chopped muon beam are satisfactory for our future muonium experiment . The muon beam chopper system can easily be
set up at other meson beam facilities . The chopper developed for a positive muon beam should also work
for a negative muon beam . The use of a chopped high intensity muon beam would allow other atomic physics
experiments to use line narrowing techniques, and solid state physics and chemistry experiments to use the wSR method with a time dependent muon beam .
Acknowledgements We would like to acknowledge the helpful support of the LAMPF staff especially from Scott Dick, Ted Newlin and Joe Ivie . This work was supported in part by the U.S . Department of Energy .
References [1] P.A . Thompson, V.W . Hughes, W.P . Lysenko and H.F . Vogel, Nucl . Instr. and Meth . 161 (1979) 391 . [2] H.W. Reist, D.E. Casperson, A.B . Denison, P.O . Egan, V.W . Hughes, F.G . Mariam, G. zu Putlitz, P.A . Souder, P.A. Thompson and J. Vetter, Nucl . Instr. and Meth . 153 (1978) 61 . [3] A. Badertscher, P .O . Egan, M. Gladisch, M. Greene, V.W . Hughes, F.G . Mariam, D.C. Lu, G. zu Putlitz, M.W . Ritter, G. Sanders, P.A. Souder and R. Werbeck, Nucl . Instr. and Meth . A238 (1985) 200. [4] R.L. Hutson, D.W. Cooke, R.H . Heffner, M.E . Schillaci, S.A. Dodds and G.A . Gist, Hyperfine Interactions 32 (1986) 893 . [5] K. Nagamine, Z. Phys. C56 (1992) S215 . [6] G.H . Eaton, Z. Phys. C56 (1992) S232 . [7] A. Blaer, J. French, A.M . Sachs, M. May and E. Zavattini, Phys . Rev. A40 (1989) 158. [8] V.W. Hughes, G. zu Putlitz, P. Souder, M. Boshier, D. Ciskowski, S. Dhawan, X. Fei, M. Janousch, W. Liu, W. Schwarz, K. Jungmann, B. Matthias, R. Holmes, J. MeCracken, X. Wang, C. Pillai, O. van Dyck and K. Woodle, LAMPF experiment No . 1054 . ; V.W. Hughes, Z. Phys. C56 (1992) S35. [9] V.W. Hughes and G. zu Putlitz, in : Quantum Electrodynamics, ed . T. Kinoshita (World Scientific, Singapore, 1990) p. 822. [10] D.H. White, Z. Phys . C56 (1992) S255 . [11] J.L. Beveridge, Z. Phys. C56 (1992) S258. [12] G.J . Krausse, Los Alamos National Laboratory report LA-UR-85-2057, presented at the 5th IEEE Pulsed Power Conference, Arlington, VA, June 10-12, 1985.