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Pileup rejection in pulsed beam experiments
Nuclear Instruments and Methods in Physics Research A264 (1988) 407-409 North-Holland, Amsterdam
407
P I L E U P R E J E C T I O N IN P U L S E D B E A M E X P E R I M E N T S S. SEN, D.R. C H A K R A B A R T Y *, P. P A U L , J. S T A C H E L a n d M. T H O E N N E S S E N Department of Physics, State University of New York at Stony Brook, Stony Brook, N Y 11794, USA
Received 10 June 1987
A simple method of pileup rejection suitable for pulsed beams is described, and its sensitivity and time resolution are analyzed.
1. Introduction Spectral distortion in high counting rate experiments has led to the use of a variety of pileup rejecting circuits. A common procedure [1] is to form a bipolar pulse and detect the time between the leading edge and the point of zero-crossing. If the detector pulse shape is constant this time is invariant with respect to amplitude for single pulses. A second type of rejection circuit [2] uses semilogic pulses produced by hard clipping the leading edge of the detector pulse to define the pulse arrival time. The circuit then checks in an appropriate time interval before and after a good event for the presence of a pileup pulse. Ref. [2] discusses the shortcomings of the crossover technique. The leading edge method is limited by the rise time of the pulse. We describe here a simple method allowing efficient elimination of pileup independent of the rise time of the pulses which conveniently uses the intrinsic time structure of an accelerator beam, such as that of a heavy ion linac. The method uses two charge-integrating ADCs timed relative to the beam burst which integrate over the early part and the complete pulse, respectively. In the absence of pileup the ratio of the two ADC outputs has a definite value essentially independent of pulse height. We describe the method with the example of a measurement of high energy gamma rays produced in heavy ion fusion reactions. In these experiments gamma rays were detected in a 10 in. x 15 in. NaI crystal at a distance of 60 cm from the target. The time-bunched beam was obtained from the superconducting linac at Stony Brook with a bunch separation of 106 ns and a bunch width of - 400 ps. Pileup in these experiments comes from three sources: (i) from gamma rays in the same beam bunch; (ii) from neutrons in the same beam bunch; and (iii) * Permanent address: Nuclear Physics Division, Bhabha Atomic Research Center, Bombay, India. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
from gamma rays and neutrons in earlier or later beam bunches. The present procedure is sensitive to pileup of classes (ii) and (iii).
2. Electronics Fig. 1 describes schematically the electronics used for pileup rejection. The pulse from the NaI is split into two parts. One is the linear pulse used for energy measurement. The other is clipped and used for timing. The linear part is split again and fed to two charge-integrating ADCs (Lecroy 2249W). The ADC gates are generated in coincidence with the beam burst. One gate is - 600 ns wide and integrates over the entire pulse. The other is - 150 ns wide and covers the leading edge. The gate signal starts the integration. Both ADC outputs are recorded on tape for each event during the experiment. The ratio of these two ADC outputs is then computed for the whole set of events during offline analysis. Fig. 2 shows a typical ratio spectrum. The method of using this ratio spectrum to reject pileup and its sensitivity to pileup amplitude and time are described in the following sections. Linear
signal
)
Fig. 1. Electronic block diagram.
S. Sen et aL / Pileup rejection in pulsed beam experiments
408 3. P r i n c i p l e
of operation
I ....
Let A(E, t) and B(E, t) be the fractions of energies deposited in the wide gate and narrow gate, respectively, by a pulse of energy E arriving at time t with respect to the start time of the gate. Experimentally A(E, t) and B(E, t) were found to be nearly independent of energy. Let us consider the pileup between a pulse of energy E 1 starting at t = 0 and a pulse of energy E 2 at time t. Here t can range from - o o to + 600 ns where the negative values of t represent pulses from earlier beam bunches. For this event the ratio of the A D C outputs is E,A(0) + E 2 A ( t ) R = e,B(0) + E~8(t) '
(1)
and the total energy E measured by the A D C with the wider gate is E = E,A(O) + E2A(t ) = E, + E2a(t),
B(0) 1 +
E2a(t))
(3)
E2b(t)/(E- E2a(t))"
The ratio spectrum (fig. 2) will therefore be centered around Ro=A(O)/B(O ) for all values of E. Pileup events from earlier bunches will produce smaller values or R (left of R0); delayed pileup from the same beam bunch or later bunches will lead to larger values (right of R0). Pileup is rejected during offline data analysis by placing an acceptance window around the peak in the ratio spectrum. However, pulse shape fluctuations will 10"
10 3
,-.-! o r_)
10 ~
10 ° 320
360 Ratio
400 (xlO0)
1.6
Lb(t
'
'
I
)
1.2
08
/
0.4-_
\
-
o. o - -
\
~
~----,~ ,
200
--i00
0
i00
Time
(ns)
200
I-_ 3OO
Fig. 3. Quantities a(t) and b(t) plotted against the time of arrival of the pileup pulse.
(2)
where a ( t ) = d ( t ) / A ( O ) and we have used A ( 0 ) = I . Similarly defining b ( t ) = B(t)/B(O), we can write
R = A(O) 1 + E 2 a ( t ) / ( E -
# ' ' ' ' 1 ' ' ' ~ 1 ' ' ' ' 1
440
Fig. 2. A ratio spectrum. The dashed vertical lines define an acceptance window of 8R/R o - 0.04.
produce a width to the ratio peak even in the absence of pileup and thus good events will be rejected if the window chosen is too narrow. In our sample experiment we chose a width of 6 R / R o = 4% as shown in fig. 2. The following section analyses the sensitivity towards pileup rejection based on this sample width.
4. Sensitivity analysis From eq. (3) we can compute the values of E 2 and
Eza(t) which satisfy the condition that the ratio R falls within the acceptance window. Here E z is the maxim u m energy of a pulse occurring at time t that can pile up without being recognized, and E2a(t ) is the upper limit by which the true energy E 1 is altered. The sensitivity of the method obviously depends on the quantities a(t) and b(t) which contain the experimental conditions and were experimentally determined. For measuring a(t) both A D C gates were kept 600 ns wide and one signal was delayed with respect to the other. The ratio spectrum was then centered around A(t)/A(O). By varying the time delay of the second pulse, a(t) was obtained over the entire range of interest. Similarly with two 150 ns wide gates b(t) was also measured. A plot of these quantities is given in fig. 3. The resultant limiting values E 2 and E 2 a (t) are listed in table 1 as a function of the total observed energy E and the time delay t between E t and E:. The table can be interpreted as follows. The set of E 2 limits allows one to estimate the importance of a particular spectral section toward unrecognized pileup, whereas E2a(t ) gives the actual energy distortion produced by that pileup. For example, for a 22 MeV pulse the setup is insensitive to pileup pulses of 510 keV or less from a
S. Sen et al. / Pileup rejection in pulsed beam experiments
409
Table 1 Listing of energy E 2 (MeV) of a pileup pulse producing a ratio R within the acceptance window (4%) as function of total energy E and time delay t between the pulses. Values in parenthesis are the energy Eza(t ) (MeV) actually added in the pileup event. E
t(ns)
(MeV)
- 210
- 102
13
52
102
204
306
450
22
0.59 (0.29) 0.53 (0.26) 0.47 (0.24) 0.42 (0.21) 0.36 (0.18) 0.31 (0.15)
0.51 (0.45) 0.46 (0.40) 0.41 (0.34) 0.36 (0.32) 0.31 (0.28) 0.27 (0.24)
3.08 (2.97) 2,78 (2.68) 2.49 (2.40) 2.18 (2.11) 1.90 (1.83) 1.61 (1.55)
0.77 (0.70) 0.69 (0.64) 0.62 (0.57) 0.55 (0.50) 0.47 (0.43) 0.40 (0.37)
0.58 (0.48) 0.52 (0.43) 0.47 (0.39) 0.41 (0.34) 0.36 (0.29) 0.30 (0.25)
0.70 (0.43) 0.63 (0.39) 0.57 (0.35) 0.50 (0.30) 0.43 (0.26) 0.37 (0.22)
1.15 (0.43) 1.04 (0.39) 0.92 (0.35) 0.81 (0.30) 0.70 (0.26) 0.60 (0.22)
2.34 (0.43) 2.11 (0.39) 1.89 (0.35) 1.66 (0.30) 1.44 (0.26) 1.22 (0.22)
20 18 16 14 12
burst occurring 102 ns earlier and such pileup will add at most 450 keV. It is apparent from table 1 that the present method is effective in eliminating pileup from a pulsed beam source with a burst separation of > 100 ns. In that case unrecognized pileup can increase the energy by at most 2%. This could be further reduced by narrowing the acceptance window below 4%. The virtue of this method is its simplicity and the fact that the pileup rejection window can be chosen during data analysis at will. It can be seen from table 1 that pileup of neutrons with gamma rays from the same beam burst is more
troublesome. Since it may involve time differences down to - 10 ns the minimum recognized piled up energy is - 3 MeV. A careful analysis involving the time of flight and energy deposited by the neutrons in the detector is necessary to assess the importance of such pileup.
References
[1] M.G. Strauss, Rev. Sci. Instr. 34 (1963) 335. [2] S.L. Blatt, J. Mahieux and D. Kohler, Nucl. Instr. and Meth. 60 (1968) 221.
407
P I L E U P R E J E C T I O N IN P U L S E D B E A M E X P E R I M E N T S S. SEN, D.R. C H A K R A B A R T Y *, P. P A U L , J. S T A C H E L a n d M. T H O E N N E S S E N Department of Physics, State University of New York at Stony Brook, Stony Brook, N Y 11794, USA
Received 10 June 1987
A simple method of pileup rejection suitable for pulsed beams is described, and its sensitivity and time resolution are analyzed.
1. Introduction Spectral distortion in high counting rate experiments has led to the use of a variety of pileup rejecting circuits. A common procedure [1] is to form a bipolar pulse and detect the time between the leading edge and the point of zero-crossing. If the detector pulse shape is constant this time is invariant with respect to amplitude for single pulses. A second type of rejection circuit [2] uses semilogic pulses produced by hard clipping the leading edge of the detector pulse to define the pulse arrival time. The circuit then checks in an appropriate time interval before and after a good event for the presence of a pileup pulse. Ref. [2] discusses the shortcomings of the crossover technique. The leading edge method is limited by the rise time of the pulse. We describe here a simple method allowing efficient elimination of pileup independent of the rise time of the pulses which conveniently uses the intrinsic time structure of an accelerator beam, such as that of a heavy ion linac. The method uses two charge-integrating ADCs timed relative to the beam burst which integrate over the early part and the complete pulse, respectively. In the absence of pileup the ratio of the two ADC outputs has a definite value essentially independent of pulse height. We describe the method with the example of a measurement of high energy gamma rays produced in heavy ion fusion reactions. In these experiments gamma rays were detected in a 10 in. x 15 in. NaI crystal at a distance of 60 cm from the target. The time-bunched beam was obtained from the superconducting linac at Stony Brook with a bunch separation of 106 ns and a bunch width of - 400 ps. Pileup in these experiments comes from three sources: (i) from gamma rays in the same beam bunch; (ii) from neutrons in the same beam bunch; and (iii) * Permanent address: Nuclear Physics Division, Bhabha Atomic Research Center, Bombay, India. 0168-9002/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
from gamma rays and neutrons in earlier or later beam bunches. The present procedure is sensitive to pileup of classes (ii) and (iii).
2. Electronics Fig. 1 describes schematically the electronics used for pileup rejection. The pulse from the NaI is split into two parts. One is the linear pulse used for energy measurement. The other is clipped and used for timing. The linear part is split again and fed to two charge-integrating ADCs (Lecroy 2249W). The ADC gates are generated in coincidence with the beam burst. One gate is - 600 ns wide and integrates over the entire pulse. The other is - 150 ns wide and covers the leading edge. The gate signal starts the integration. Both ADC outputs are recorded on tape for each event during the experiment. The ratio of these two ADC outputs is then computed for the whole set of events during offline analysis. Fig. 2 shows a typical ratio spectrum. The method of using this ratio spectrum to reject pileup and its sensitivity to pileup amplitude and time are described in the following sections. Linear
signal
)
Fig. 1. Electronic block diagram.
S. Sen et aL / Pileup rejection in pulsed beam experiments
408 3. P r i n c i p l e
of operation
I ....
Let A(E, t) and B(E, t) be the fractions of energies deposited in the wide gate and narrow gate, respectively, by a pulse of energy E arriving at time t with respect to the start time of the gate. Experimentally A(E, t) and B(E, t) were found to be nearly independent of energy. Let us consider the pileup between a pulse of energy E 1 starting at t = 0 and a pulse of energy E 2 at time t. Here t can range from - o o to + 600 ns where the negative values of t represent pulses from earlier beam bunches. For this event the ratio of the A D C outputs is E,A(0) + E 2 A ( t ) R = e,B(0) + E~8(t) '
(1)
and the total energy E measured by the A D C with the wider gate is E = E,A(O) + E2A(t ) = E, + E2a(t),
B(0) 1 +
E2a(t))
(3)
E2b(t)/(E- E2a(t))"
The ratio spectrum (fig. 2) will therefore be centered around Ro=A(O)/B(O ) for all values of E. Pileup events from earlier bunches will produce smaller values or R (left of R0); delayed pileup from the same beam bunch or later bunches will lead to larger values (right of R0). Pileup is rejected during offline data analysis by placing an acceptance window around the peak in the ratio spectrum. However, pulse shape fluctuations will 10"
10 3
,-.-! o r_)
10 ~
10 ° 320
360 Ratio
400 (xlO0)
1.6
Lb(t
'
'
I
)
1.2
08
/
0.4-_
\
-
o. o - -
\
~
~----,~ ,
200
--i00
0
i00
Time
(ns)
200
I-_ 3OO
Fig. 3. Quantities a(t) and b(t) plotted against the time of arrival of the pileup pulse.
(2)
where a ( t ) = d ( t ) / A ( O ) and we have used A ( 0 ) = I . Similarly defining b ( t ) = B(t)/B(O), we can write
R = A(O) 1 + E 2 a ( t ) / ( E -
# ' ' ' ' 1 ' ' ' ~ 1 ' ' ' ' 1
440
Fig. 2. A ratio spectrum. The dashed vertical lines define an acceptance window of 8R/R o - 0.04.
produce a width to the ratio peak even in the absence of pileup and thus good events will be rejected if the window chosen is too narrow. In our sample experiment we chose a width of 6 R / R o = 4% as shown in fig. 2. The following section analyses the sensitivity towards pileup rejection based on this sample width.
4. Sensitivity analysis From eq. (3) we can compute the values of E 2 and
Eza(t) which satisfy the condition that the ratio R falls within the acceptance window. Here E z is the maxim u m energy of a pulse occurring at time t that can pile up without being recognized, and E2a(t ) is the upper limit by which the true energy E 1 is altered. The sensitivity of the method obviously depends on the quantities a(t) and b(t) which contain the experimental conditions and were experimentally determined. For measuring a(t) both A D C gates were kept 600 ns wide and one signal was delayed with respect to the other. The ratio spectrum was then centered around A(t)/A(O). By varying the time delay of the second pulse, a(t) was obtained over the entire range of interest. Similarly with two 150 ns wide gates b(t) was also measured. A plot of these quantities is given in fig. 3. The resultant limiting values E 2 and E 2 a (t) are listed in table 1 as a function of the total observed energy E and the time delay t between E t and E:. The table can be interpreted as follows. The set of E 2 limits allows one to estimate the importance of a particular spectral section toward unrecognized pileup, whereas E2a(t ) gives the actual energy distortion produced by that pileup. For example, for a 22 MeV pulse the setup is insensitive to pileup pulses of 510 keV or less from a
S. Sen et al. / Pileup rejection in pulsed beam experiments
409
Table 1 Listing of energy E 2 (MeV) of a pileup pulse producing a ratio R within the acceptance window (4%) as function of total energy E and time delay t between the pulses. Values in parenthesis are the energy Eza(t ) (MeV) actually added in the pileup event. E
t(ns)
(MeV)
- 210
- 102
13
52
102
204
306
450
22
0.59 (0.29) 0.53 (0.26) 0.47 (0.24) 0.42 (0.21) 0.36 (0.18) 0.31 (0.15)
0.51 (0.45) 0.46 (0.40) 0.41 (0.34) 0.36 (0.32) 0.31 (0.28) 0.27 (0.24)
3.08 (2.97) 2,78 (2.68) 2.49 (2.40) 2.18 (2.11) 1.90 (1.83) 1.61 (1.55)
0.77 (0.70) 0.69 (0.64) 0.62 (0.57) 0.55 (0.50) 0.47 (0.43) 0.40 (0.37)
0.58 (0.48) 0.52 (0.43) 0.47 (0.39) 0.41 (0.34) 0.36 (0.29) 0.30 (0.25)
0.70 (0.43) 0.63 (0.39) 0.57 (0.35) 0.50 (0.30) 0.43 (0.26) 0.37 (0.22)
1.15 (0.43) 1.04 (0.39) 0.92 (0.35) 0.81 (0.30) 0.70 (0.26) 0.60 (0.22)
2.34 (0.43) 2.11 (0.39) 1.89 (0.35) 1.66 (0.30) 1.44 (0.26) 1.22 (0.22)
20 18 16 14 12
burst occurring 102 ns earlier and such pileup will add at most 450 keV. It is apparent from table 1 that the present method is effective in eliminating pileup from a pulsed beam source with a burst separation of > 100 ns. In that case unrecognized pileup can increase the energy by at most 2%. This could be further reduced by narrowing the acceptance window below 4%. The virtue of this method is its simplicity and the fact that the pileup rejection window can be chosen during data analysis at will. It can be seen from table 1 that pileup of neutrons with gamma rays from the same beam burst is more
troublesome. Since it may involve time differences down to - 10 ns the minimum recognized piled up energy is - 3 MeV. A careful analysis involving the time of flight and energy deposited by the neutrons in the detector is necessary to assess the importance of such pileup.
References
[1] M.G. Strauss, Rev. Sci. Instr. 34 (1963) 335. [2] S.L. Blatt, J. Mahieux and D. Kohler, Nucl. Instr. and Meth. 60 (1968) 221.