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Measurement of the shape of accelerator beam pulses
NUCLEAR INSTRUMENTS AND METHODS
54 (1967) 329-330 ; © NORTH-HOLLAND PUBLISHING CO.
MEASUREMENT OF THE SHAPE OF ACCELERATOR BEAM PULSES* D. E . FREDERICK
Institute fin- Atomic Research and Department of Physics, Iowa State (Iniversitv, Arnes, Iowa 50010, U.S.A .
Received 20 July 1967 An accelerator timing signal and a logic signal derived from an accelerator-produced nuclear event are used to gate on and off, respectively, a train of crystal-controlled oscillator pulses. The
Accurate determination of the deadtime, a necessary evil in any counting experiment, is made all the more difficult when working with a pulsed accelerator having a nonrectangular beam pulse whose width is greater than the event resolving time. We describe a simple technique to measure the time distribution of X-rays in the beam pulse of a 70 MeV electron synchrotron whose pulse shape is reasonably stable in time. For this purpose, we had available a flexible computer-based data acquisition system hence were able to use computer modules and nuclear counting equipment already on hand. Few other laboratories have, circumstances identical to the above. However, many present generation accelerators have pulsed beams of ? 1 psec duration, a time very much greater than fast pulse electronics resolving times, hence there exists a common need for similar information. Much such installations already have or plan to acquire an on-line digital computer . In lieu of such a computer facility, however, a time of flight measurer such as the TMC Model 201 ADC input unit could be used . Whereas we could use slow equipment already on hand (i.e., a 120 kHz oscillator and a 10 MHz scaler) since our beam burst was ~ 100 psec full width at half maximum (fwhm), it would be necessary to use an oscillator of 20-100 MHz
frequency distribution of the number of gated oscillator pulses yields detailed information about the accelerator beam pulse shape. 120
k HZ
_ W_n!tr :%PUT
OSCILLATOR
_''i RF STOP (FROM SYNCHROTRON)
_
'
'!
SCNMli7 T-1-
__
.r
START
10 m SECj DELAY I
RESET
BEAM GATE -~
SL
1_-
_
Coun+er STC-
_
Comov~e" tNTERRI,'OT
Fig. 2. Block diagram of counter control.
for such modern accelerators as the electron linacs . The philosophy was to turn on a counter + with the electron synchrotron rf STOP sihaped b\ a Schmitt trigger circuit (figs . 1 and 2) which came just before the peak in the accelerator magnetic guide field . At that time, the electron beam would begin t : " spiral in towards a bremsstrahlung target at smaller radius than the accelerating orbit while the pulse counter counted pulses from a 120 kHz oscillator. Bremsstrahlung-produced photoprotons were detected in a Work was performed in the Ames Laboratory of the U. S. solid state detector. The resultant pulses were amplified Atomic Energy Commission . Contribution no . 2127 . and fed into a discriminator to give a prompt standard + TSI Model 1511A Dual PulsCCounter . logic pulse labelled PROTON in fig. 2. To avoid counting background events, we generally enveloped the X-ray beam pulse with a gating signal labelled BEAM MAGNETIC GATE . The PROTON and BEAM GATE pulses Nvere GUIDE FIELD fed into a computer module 2-input AND gate whose ~OmSEC INTENSITY tIlC L l~ t1 output,UIN 1 c1\ .11 V1", LUIilCu L!II output COUNTER 1 RF RAY \.TOP 81 RESE7 i (fig . 2' Simultaneously, the COUNTER STOP sig STOP RF AM produced a computer INTERRUPT signal . 1'ndcr program control, the scaler's contents were read into the computer, and a frequency distribution (if such Fiô. 1 . Timing of logic pulses in relation to acceleration cycle. oscillator pulse counts was accumulated. The rf STOP Electrons are iniected into accelerator at nearly zero field SIGNAL w.as delayed 10 cosec by use of two one-shot intensity and produce bremsstrablung for about 100 psec at constant electra, energy at peak of field. multivibrators in series and OR',d with the lnove .A\I 5
29
T1~1Ti^n
C'TIr"n
.1
IS
L
IIt .I
330
D. E . FREDERICK
gate output . It was also sent to the counter RESET terminal, but it did not generate a computer INTERRUPT s:gnal . Therefore, the counter was always stopped and reset before the next accelerator cycle but only X-ray initiated events produced recorded X-ray
ime distribution data. The 10 ample time for scaler readout.
cosec delay allowed
Thecentroid oftherecorded time distribution was the average time required for the electrons to drift into the target, typically 900 psec Since the bremsstrahlung energy distribution is time independent, the fwhm of the time distribution is just that of the X-ray beam, typically 125 psec. Surprisingly, the beam pulse was more Gaussian than rectangular, contrary to what we had previously thought from visual observations on an oscilloscope . Visual observations led us to believe that the beam pulse was nearly rectangular and nearly 200 usec wide. This may be due to how beam phase space is filled. Some tests of the assumption of timeindependence of beam pulse shape and time-of-occurrence was made. The BEAM GATE term is normally not essential in the logic shown in fig. 2. It was convenient, however, to narrow its width to 1 psec, while holding the PROTON logic level true, and make use ofthefact that its timing is obtained from an accelerator signal different from the rf STOP signal . The resultant time distribution of 13,usec fwhm gave an upper limit to the time jitter of the rf STOP pulse. It was not possible to measure accurately the effect of integrating the X-ray beam shape over a 10 min measurement but,
within statistics, an identical result was obtained in a 1 min beach pulse shape measurement. Visual observation of detector output, for a detector placed very close to the X-ray target, showed no major burst-toburst variation. An oscilloscope photograph integrated over 2 or 3 beam bursts might have been useful, but we did not attempt this . Finally, a test made with the rf power turned off at 45° in the acceleration cycle, instead of nearly 90°, gave a 13 psec fwhm time distribution centered at 200 .usec after rf STOP, as compared to 900,usec drift time noted above. Though an expected radial beam spread of about 1 mm could account for this 13 usec fwhm, the time spread at 45° probably reflects timing pulse jitter which, though sufficiently small for our purposes, could be reduced further if one wishes to use this technique to study the beam dynamics in detail . It should be noted that the need for a test running for many thousands of bursts is dictated by the requirement that the whole beam be sampled, i.e., the deadtime must be small during this test Since we obained no more than one datum per burst, it is straightforward to show that the fraction, f, of PROTON signals lost is just f= Je-8-(1 - E))Is,
where s is the average number of PROTON signals/ burst. For s < 1, fx Je(1- JE). Thus, it is desirable to record fewer than or about one event per 10 beam bursts in order to insure that the beam pulse shape measurement is not itself biased by deadtime effects.
54 (1967) 329-330 ; © NORTH-HOLLAND PUBLISHING CO.
MEASUREMENT OF THE SHAPE OF ACCELERATOR BEAM PULSES* D. E . FREDERICK
Institute fin- Atomic Research and Department of Physics, Iowa State (Iniversitv, Arnes, Iowa 50010, U.S.A .
Received 20 July 1967 An accelerator timing signal and a logic signal derived from an accelerator-produced nuclear event are used to gate on and off, respectively, a train of crystal-controlled oscillator pulses. The
Accurate determination of the deadtime, a necessary evil in any counting experiment, is made all the more difficult when working with a pulsed accelerator having a nonrectangular beam pulse whose width is greater than the event resolving time. We describe a simple technique to measure the time distribution of X-rays in the beam pulse of a 70 MeV electron synchrotron whose pulse shape is reasonably stable in time. For this purpose, we had available a flexible computer-based data acquisition system hence were able to use computer modules and nuclear counting equipment already on hand. Few other laboratories have, circumstances identical to the above. However, many present generation accelerators have pulsed beams of ? 1 psec duration, a time very much greater than fast pulse electronics resolving times, hence there exists a common need for similar information. Much such installations already have or plan to acquire an on-line digital computer . In lieu of such a computer facility, however, a time of flight measurer such as the TMC Model 201 ADC input unit could be used . Whereas we could use slow equipment already on hand (i.e., a 120 kHz oscillator and a 10 MHz scaler) since our beam burst was ~ 100 psec full width at half maximum (fwhm), it would be necessary to use an oscillator of 20-100 MHz
frequency distribution of the number of gated oscillator pulses yields detailed information about the accelerator beam pulse shape. 120
k HZ
_ W_n!tr :%PUT
OSCILLATOR
_''i RF STOP (FROM SYNCHROTRON)
_
'
'!
SCNMli7 T-1-
__
.r
START
10 m SECj DELAY I
RESET
BEAM GATE -~
SL
1_-
_
Coun+er STC-
_
Comov~e" tNTERRI,'OT
Fig. 2. Block diagram of counter control.
for such modern accelerators as the electron linacs . The philosophy was to turn on a counter + with the electron synchrotron rf STOP sihaped b\ a Schmitt trigger circuit (figs . 1 and 2) which came just before the peak in the accelerator magnetic guide field . At that time, the electron beam would begin t : " spiral in towards a bremsstrahlung target at smaller radius than the accelerating orbit while the pulse counter counted pulses from a 120 kHz oscillator. Bremsstrahlung-produced photoprotons were detected in a Work was performed in the Ames Laboratory of the U. S. solid state detector. The resultant pulses were amplified Atomic Energy Commission . Contribution no . 2127 . and fed into a discriminator to give a prompt standard + TSI Model 1511A Dual PulsCCounter . logic pulse labelled PROTON in fig. 2. To avoid counting background events, we generally enveloped the X-ray beam pulse with a gating signal labelled BEAM MAGNETIC GATE . The PROTON and BEAM GATE pulses Nvere GUIDE FIELD fed into a computer module 2-input AND gate whose ~OmSEC INTENSITY tIlC L l~ t1 output,UIN 1 c1\ .11 V1", LUIilCu L!II output COUNTER 1 RF RAY \.TOP 81 RESE7 i (fig . 2' Simultaneously, the COUNTER STOP sig STOP RF AM produced a computer INTERRUPT signal . 1'ndcr program control, the scaler's contents were read into the computer, and a frequency distribution (if such Fiô. 1 . Timing of logic pulses in relation to acceleration cycle. oscillator pulse counts was accumulated. The rf STOP Electrons are iniected into accelerator at nearly zero field SIGNAL w.as delayed 10 cosec by use of two one-shot intensity and produce bremsstrablung for about 100 psec at constant electra, energy at peak of field. multivibrators in series and OR',d with the lnove .A\I 5
29
T1~1Ti^n
C'TIr"n
.1
IS
L
IIt .I
330
D. E . FREDERICK
gate output . It was also sent to the counter RESET terminal, but it did not generate a computer INTERRUPT s:gnal . Therefore, the counter was always stopped and reset before the next accelerator cycle but only X-ray initiated events produced recorded X-ray
ime distribution data. The 10 ample time for scaler readout.
cosec delay allowed
Thecentroid oftherecorded time distribution was the average time required for the electrons to drift into the target, typically 900 psec Since the bremsstrahlung energy distribution is time independent, the fwhm of the time distribution is just that of the X-ray beam, typically 125 psec. Surprisingly, the beam pulse was more Gaussian than rectangular, contrary to what we had previously thought from visual observations on an oscilloscope . Visual observations led us to believe that the beam pulse was nearly rectangular and nearly 200 usec wide. This may be due to how beam phase space is filled. Some tests of the assumption of timeindependence of beam pulse shape and time-of-occurrence was made. The BEAM GATE term is normally not essential in the logic shown in fig. 2. It was convenient, however, to narrow its width to 1 psec, while holding the PROTON logic level true, and make use ofthefact that its timing is obtained from an accelerator signal different from the rf STOP signal . The resultant time distribution of 13,usec fwhm gave an upper limit to the time jitter of the rf STOP pulse. It was not possible to measure accurately the effect of integrating the X-ray beam shape over a 10 min measurement but,
within statistics, an identical result was obtained in a 1 min beach pulse shape measurement. Visual observation of detector output, for a detector placed very close to the X-ray target, showed no major burst-toburst variation. An oscilloscope photograph integrated over 2 or 3 beam bursts might have been useful, but we did not attempt this . Finally, a test made with the rf power turned off at 45° in the acceleration cycle, instead of nearly 90°, gave a 13 psec fwhm time distribution centered at 200 .usec after rf STOP, as compared to 900,usec drift time noted above. Though an expected radial beam spread of about 1 mm could account for this 13 usec fwhm, the time spread at 45° probably reflects timing pulse jitter which, though sufficiently small for our purposes, could be reduced further if one wishes to use this technique to study the beam dynamics in detail . It should be noted that the need for a test running for many thousands of bursts is dictated by the requirement that the whole beam be sampled, i.e., the deadtime must be small during this test Since we obained no more than one datum per burst, it is straightforward to show that the fraction, f, of PROTON signals lost is just f= Je-8-(1 - E))Is,
where s is the average number of PROTON signals/ burst. For s < 1, fx Je(1- JE). Thus, it is desirable to record fewer than or about one event per 10 beam bursts in order to insure that the beam pulse shape measurement is not itself biased by deadtime effects.