Fast timing from a fission ionization chamber

N U C L E A R I N S T R U M E N T S A N D M E T H O D S 99

(197z) 477-486; ©

NORTH-HOLLAND

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FAST TIMING FROM A F I S S I O N I O N I Z A T I O N CHAMBER* H. ROSLERt, J. K. MILLARD and N. W. HILL

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. Received 22 October 1971 The timing resolution of a 2~Cf-loaded fission ionization chamber connected to a new current preamplifier has been tested by looking at coincident fission fragment pulses and pulses fl'om fission 7-rays which are detected in a plastic scintillator. A time resolution of 1 nsec fwhm could be achieved.

1. Introduction In recent years electron linear accelerators have become a powerful tool for investigating cross sections for neutron induced reactions. By bombarding a target with a pulsed beam of highly energetic electrons short polyenergetic bursts of neutrons are produced. The difl'erent neutron energies are separated by time of

THIN WINDOW

flight. The energy resolution which can be achieved is determined by the flight path length, which is limited from intensity considerations, by the accelerator pulse * Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation. t Visiting scientist from Reaktorstation Garching, Mtinchen, Germany.

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Fig. 2. The assembled ionization chamber with the cap removed. width, and by the accuracy with which the time of occurrence of an event can be measured. The latter is desired to be a fraction o f the burst width, For charged particle producing reactions such as neutron induced fission it is desirable to detect all of the reaction products independently of their direction

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of emission. Two types of detection devices accomplish this, both of which provide good timing. These are either gaseous scintillation detectors 1) or ionization chambers. Gas impurities tend to deteriorate the timing capability of gaseous scintillators, so that for fast timing the use of ionization chambers may be advantageous. In the present report the timing capability of a fission ionization chamber loaded with spontaneously fissioning 2s2Cf has been tested in connection with a newly developed fast current sensitive preamplifier. Similar chambeis and preamplifiers will be used in fission cross section experiments at the Oak Ridge Electron Linear Accelerator. 2. The ionization chamber In contrast to reports on very small, low capacitance Cf chambers, the one used in the experiment reported here is an identical replica of a chamber to be used in actual cross section measurements (fig. 1). The design is similar to the one used, in ref. 2. Eleven isolated aluminum plates, each O.O05" thick and 3.5" in diana-

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eter are supported by nylon rods between two grounded aluminum rings. Nylon spacers maintain a gap of 0.05" between the plates. Every second plate including one which carries 0.00012 #g 2 5 2 C f o n a 0.5 cm diameter surface is connected to a separate BNCconnector. A common connector provides negative voltage to the remaining plates. In this scheme the signal plates are dc-coupled to the preamplifier eliminating the need of coupling capacitors and bias supply [esistors with their attendant noise generation in the preamplifiers. The pulses ate routed to the connectors by short pieces of shielded cable. The shielding is grounded to the chamber and connected to both adjacent plates via 0.025 #F capacitors to avoid cross talk, i.e., a response by capacitive coupling from one signal plate to another. Fig. 2 shows the chambe[ with the cap removed. The assembled chamber was ew~cuated for several days and then filled with a

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open-loop input capacitance of the stage. If, for convenience, Ai is chosen equal to Co/Cf, the bandwidth simply becomes g,,/Ci,. Evidently, the bandwidth of the gain section (Ai) must be significantly larger than gm/C~.. Under the same conditions on A~ the input impedance is l/g m over the full bandwidth. This low value allows stages to be compatibly cascaded, since the output end of the load network Zo requires a low impedance. With the current gain dependent only on the ratio of Zr/zo, the value of Rf can be equal to that of conventional charge-sensitive preamplifiers• Consequently, after signal integration, the energy resolution of this preamplifier is comparable to available chargesensitive designs. A complete diagram of the preamplifier is shown in figs. 4a and 4b. The total current gain of 5000 is obtained from three stages having respective current gains, beginning with the input, of 25, 20, and 10.

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ments) operating in parallel. Biased with a combined drain current of 20 mA, these devices yielded a cornposite transconductance of about 50 mmho. An energy signal was obtained from the linear current output by placing a 5.1 z 10 - 9 F capacitor between this point and ground. Routing this integrated current signal to a 100 f2 input impedance main amplifier with all differentiation switched out and with the integrator set for 2/tsec yields a linear energy

With no detector capacitance the 10 to 90% rise time of the preamplifier could be adjusted to about 5 nsec. With a 100 pF simulated detector capacitance the rise time measured by providing pulses at the input through a 15 kf2 resistor, was degraded to 15 nsec. This result was also verified with fission pulses from the ionization chamber (fig. 7a). To achieve this rise time with the large detector capacitance required two high-transconductance input FET's (SFB8558 of Texas [nstru-

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4. Experimental setup The timing performance of the ionization chamber and preamplifier was tested by starting a time-to-amplitude-converter with the fragment pulse from a ZSZCf spontaneous fission event in the ionization chamber, and providing the stop signal from aplasticscintillator detecting the prompt fission gamma rays. To avoid a broadening in the time spectrum, the distance between fission chamber and scintillator must be sufficient to clearly separate the 7-rays from the fission neutron pulses due to their different flight time. The electronic set-up as shown in the block diagram of fig. 5 is conventional. The fast current signal from the preamplifier triggers a constant fraction timing discriminator working on 50°,~, fraction of pulse height. At the stop side a 1.5" diameter × 1" high Naton 136 scintillator (referred to as "big scintillator") is mounted on a RCA 8575 phototube connected to a constant fraction timing photomultiplier base. Both linear signals are checked by single channel analyzers; if beth signals fall within the selected range of pulse height, a properly delayed coincidence signal opens the gate of the analyzer. Thus it is possible to restrict the dynamic range on both sides. In order to correct forthe finite timing resolution of the scintillator, a second experiment was performed in which the fission chamber was replaced by a 8 diameter × ~ high Naton scintillator (referred to as "small scintillator"), both scintillators detecting coincident ?-rays from a 6°Co source. The changes in the

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5. Results Fig. 6a shows the pulse height distribution from the chamber. By cutting off all pulses above the pronounced dip in the distribution, the number of coincidences per chamber pulse dropped from 3.6 x 10-2 to 8 x 10-4, indicating that the pulses below the dip are from particles. A clear separation between fission- and a-pulses is expected due to the very thin layer of Cf. The shape of the fission pulse height spectrum results from the fact that a) the specific ionization of heavy and light fragments is not much differenta), b) the distance between two plates in the ionization chamber is only 5% of the average range of the fragments in argon at 1 atm. Thus, the pulse height distribution is totally governed by the geometry. According to Fulmer 4) and Lassen 5) the differential energy loss of fission fragments has an approximately linear dependence on the travelled flight distance x dE -

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F w h m near peak (nsec)

"°Co, full dyn. range on both detectors ~°Co, restr, dyn. range on big N a t o n 136 scint. a~Co, restr, dyn. range on small N a t o n 136 scint. 6°Co, restr, dyn. range on both detectors ~ C f , full dyn. range on both detectors e'~zCf, restr, dyn. range on fission c h a m b e r es"Cf, restr, dyn. range on both detectors

f(p) proportional to liP 2 with a sharp cutoff at the pulse height corresponding to a flight distance equal to the gap between adjacent plates. When discriminating against co-particles, the fission pulse rate as a function of applied voltage yields a plateau slope of 3%/100 V 20,000 r~ ........

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over more than 350 V starting at 150 V. The timing experiment was performed with 415 V. No pulses from plates without Cf were observed. Crucial to the final time resolution achieved is a careful walk adjustment of the constant fraction timing ]

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modules, i.e., to adjust for remaining differences in the cross-over time for different pulse heights. This was done by dividing the pulse height distributions into three equal ranges and adjusting the discriminators in such a way that the time distributions from the smallest and from the largest pulses peak at the same channel. Fig. 7 shows the fission chamber pulses at the output of the preamplifier and at the monitor output of the constant fraction timing discriminator. The figure shows the variety of puke shapes and pulse he~[ghts thus indicating the problems inherent in proper timing from the parallel plate chamber. Linear pulse height distributions from the fission

chamber and from both scintillators looking at 6°Co y-rays are shown in fig. 6. The full distributions define the "full dynamic range", while the range of pulse heights the arrows indicate the "restricted dynamic range" as cited in table 1. In the scintillators the full dynamic range is about 60: 1. For the strongly peaked distribution of pulse heights from the fission chamber a dynamic range is not easily defined. Replacing the pulse height distribution by a rectangular distribution of equal total number of pulses with a height of half the peak height and a lower margin just above the c~-pulses would cover a dynamic range of about 3: 1. The time distributions from the 7-7-experiment and

486

H. ROSLER et al.

from the Cf chamber are shown in fig. 8 and fig. 9, respectively. The calibration in all cases is 0.0383 nsec/channel. Note, that the distance between the Cf source and the scintillator is different for fig. 9a and fig. 9b. The widths of the time distributions were calculated from the y-ray peak in case of the fission chamber and from the y-y-coincidence peak for the two scintillators. All time distributions, except for the ones with restricted dynamic ranges on start and stop side, are skewed, an effect is due to an early triggering of the discriminators by the very smallest pulses. This effect was significantly enhanced for the smaller scintillator and could not be corrected by "walk adjustment" without destroying the proper timing for larger pulses. In order to calculate the width of these skewed distributions by fitting a curve rather than by evaluating the root mean square deviation from the average value, the following procedure was chosen: First a Gaussian curve was fitted to the points having an absolute distance from the peak less than one-half of the fwhm (full width at half maximum). Then, keeping the peak position fixed, the curve was symmetrized by averaging points with equal distance from the peak on both sides. Finally, a second Gaussian was fitted to the points of the symmetrized curve lying within one fwhm from the peak. The fwhm of both Gaussians are compiled in table I. The indicated errors are the maximum possible errors compatible with the statistical uncertainties of the data points.

in the 7-y-experiment an average fwhm of 0.31 nsec was achieved. This remaining time spread is due to fluctuations in electron transit times in the phototubes and to finite life times in the scintillator material 5) rather than being dependent on scintillator size. A fwhm of 0.3 l/~/2 = 0.22 nsec was therefore attributed to each scintillator as intrinsic resolution at small dynamic range. With this value a resolution width for the fission pulses alone of 1.065 nsec l\vhm is calculated, which can be reduced to 0.89 nsec by redticing the dynamic range according to fig. 6a. This resolution width is considered sufficiently small compared to lower limits on electron accelerator pulse width of 3-5 nsec.

6. Discussion

:') N. O. Lassen, Dan. Mat.-Fys. Medd. 25 (1949) 11. ~) F. J. Lynch, IEEE Trans. Nucl. Sci. NS-15 (1968) 102.

With restricted dynamic range on both scintillators

H. R6sler gratefully acknowledges a NATOFellowship from the German Academic Exchange Service. He wishes to thank the Oak Ridge National Laboratory for its kind hospitality. References

1) See for example: J. L. Teyssier, D. Blanc anti A. Godeau, J. Phys. Radium 24 (1963) 55. ~) L. W. Weston, R. Gwin, G. deSaussure, R. R. Fullwood and R. W. Hockenbury, Nucl. Sci. Eng. 23 (1965) 45 :3)j. K. Millard, An investigation of broadband current preamplification for obtaining simultaneous high-resolution energy and time information from nuclear radiation detectors, Thesis, ORNL-TM-3252 (University of Tennessee, 1971). ~) C. B. Fulmer, Phys. Rev. 108 (1957) 1113.