The use of silicon heavy ion detectors in high radiation fields

NUCLEAR

INSTRUMENTS

AND METHODS

THE USE OF SILICON

HEAVY

83 (1970) 208-212;

ION DETECTORS

J. J. WESOLOWSKI, Lawrence

Radiation

Laboratory,

University

0 NORTH-HOLLAND

IN HIGH

PUBLISHING

RADIATION

co.

FIELDS*

W. JOHN and J. HELD of California,

Livermore,

California

94550, U.S.A.

Received 10 February 1970 Techniques have been developed to permit measurements of fission fragment energies with Si detectors in the presence of a thermal neutron flux of 10” n/cm2.sec and associated radiations.

1. Introduction It is necessary to have a high thermal neutron flux available for measurements of small fission cross sections or for fission measurements with very small samples. Well collimated thermal fluxes of large intensity, say of 10” n/cm2*sec, are not readily available. Uncollimated fluxes of this magnitude exist inside the thermal columns of many reactors. Unfortunately they are accompanied by a flux of fast neutrons and a copious supply of gamma rays. The response of a conventional heavy ion detection system, i.e., a surface barrier or phosphorous-diffused Si junction with charge sensitive preamplifiers and slow electronics, in such a hostile environment, is very poor. Even if such a system does detect fission fragments it will have poor resolution due to large leakage current caused by the gamma rays which come not only from the reactor itself but also from various neutron induced reactions in the silicon and the materials near the detectors. We have developed a system using cooled phosphorous diffused Si detectors, voltage sensitive preamps and fast electronics which maintains good resolu* Work performed

under the auspices of the U. S. Atomic Energy Commission. 7 Fabricated commercially to our specifications.

tion for a long period of time even when placed inside the thermal column of a reactor in a slow neutron flux of 10” n/cm2. set (cadmium ratio = 600). The system which will be described is capable of measuring the energies of coincident fission fragments, thus allowing the determination of fragment mass distributions. Fast electronics is used rather than the conventional fastslow system in order to minimize pile-up and to optimize the signal to noise ratio’). The latter is important in the high radiation field. The excellent pile-up rejection also makes it useful for cross section measurements of elements with very high alpha activity. 2. Detectors The detectors t are phosphorous-diffused silicon of 300 ohm-cm resistivity with a surface area of 300 mm2. The total thickness is approximately 125 pm. The Si wafers are mounted on open-backed aluminum holders with silver epoxy. A thin gold ring approximately 1 mm wide, 1000 A thick and slightly smaller in diameter than the detector is evaporated on the front side (N side) of the detector. Two parallel signal leads are ultrasonically bonded to the gold. The gold ring was found to consistently provide the low contact resistance necessary for short pulse decay

Fig. I. Fission fragment linear signal for as2Cf; horizontal 50 nsec/cm, vertical 10 mV/cm.

208

USE

Fig. 2. Example

of linear

OF

signal

Si

HEAVY

from

ION

detector

DETECTORS

IN HIGH

with too large contact

Fig. 1 shows the linear pulse from a good detector while fig. 2 shows the pulse from a detector having a large contact resistance. Two such detectors are placed on opposite sides of a target foil holder which can be rotated to any one of four positions. The faces of the detectors are z 2 mm from the target foil. The assembly is mounted inside a small evacuated aluminum target chamber which is then placed inside the thermal column of the reactor. A metal O-ring is used for the vacuum seal. A 6-ft-long evacuated tube carries the target shaft, detector leads, cooling lines, and thermocouple leads outside the thermal column. The detectors are cooled to about - 16” C by circulating refrigerated alcohol through the cooling lines which are attached to the detector mounting plates. Outside the reactor the cooled detectors have leakage currents of less than 1 PA at 200 V reverse bias and the resolution of a 252Cf fission spectrum is good. In the 10” n/cm”. set flux the leakage rises to about 6 ,uA initially, increasing to about 10 PA after two weeks. Tnside the reactor the resolution is measured by means of a calibration target consisting of 2 nanograms of 235U placed on a thin metal foil. The resolution is not quite as good (peak to valley for 235U is z 10: 1) in the neutron flux as outside. It is not clear how much of the difference is caused by the large leakage and how much by the large solid angle subtended by the detectors. Fig. 3 shows the energy spectrum of one of the detectors which has been in the flux for one month. time.

3. Electronics The experimental signal desired is a pulse whose amplitude is proportional to the energy deposited in the Si detector. Furthermore, good pile-up rejection is required. The approach adopted was to amplify a fast

RADIATION

resistance;

209

FIELDS

horizontal

50 nsec/cm,

vertical

5 mV/cm.

(short rise time and decay time) linear signal, pass it through a fast linear gate and then stretch it for subsequent pulse-height analysis. Pile-up occurring within the gate interval would be vetoed by suitable logic. Irradiation is avoided by locating the preamplifiers

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Fig. 3. Energy spectrum for 23SU fission for a detector in the flux of 1O’l n/cm2.sec. The detector had been in the flux for one month. The sample consisted of 2x 1O-g g of 235U on a thin foil.

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detector

Anti noise detector

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Fig. 4. Block diagram of fast electronics used in super-heavy element experiment,

amp

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USE OF si HEAVY

ION

DETECTORS

approximately 6 ft from the detectors. Termination of the long cables with 50 Sz produced signals from the detector of very low amplitude, typically 3 mV/lOO MeV at the cable termination. Amplifiers of the voltage amplification type can be designed with rise-times of a nanosecond and gains of the order of 20 dB, but generally their stability and linearity are not comparable to charge amplifiers. However, for fission fragment work the required energy resolution of the electronics is only * 1%. Therefore we selected a voltage preamplifier design’), commonly referred to as a White follower. The current from the detector is integrated on the total input capacitance, and the resulting voltage signal is amplified. If the time constant composed of the detector equivalent circuit (including the series bulk resistance of the detector and contact resistance) and the amplifier input circuit is kept small, the signal from the detector is fast. Fig. 4 is a block diagram of the electronic system. Coincident fission fragment signals were processed through identical parallel channels. The preamplifiers providing a gain of 10 were followed by another set of fast voltage amplifiers 1 and 1’ with a variable gain of from 1 to 10. A fast fan-out linear amplifier provided linear signals to four different loads enabling proper line terminations to eliminate reflections. Parallel paths from the fan-out amplifiers delivered the signals to amplifiers 2 and 2’ also fast with variable gain. Amplifiers 2 and 2’ feed the linear signals to the linear gate stretchers. Delay was introduced so that the linear signals would arrive in coincidence with the gating pulse to the linear gate. The generation of the logic gating pulses to the linear gates begins at the fan-out amplifiers which drive variable gain amplifiers 3 and 3’. These amps provide inputs to discriminators 4 and 4’, which are variable threshold leading edge discriminators whose threshold is set near the noise level. The outputs are fast logic pulses whose pulse width is adjustable from 5 nsec to 150 nsec and produce an output for each threshold crossing. They are operated with pulse widths preset to 10 nsec. If coincidence exists for a fission event, the twofold coincident unit provides a gating pulse to the two linear gates whose pulse width is preset to the pulse width of the linear signal. The linear gates convert the input voltage pulses to current pulses which are integrated for a time period equal to the gating pulse width. The stored charge, which is proportional to the fission fragment energy, is again converted to a voltage pulse and shaped within the linear gate to properly satisfy the input requirements of a multiparameter pulse height analyzer.

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211

4. Pile-up and noise pickup rejection Pile-up that may exist during the linear gating interval is detected and both the original and pile-up linear events are rejected. Pile-up rejection is accomplished with a pile-up module that samples each output of discriminators 4 and 4’. The unit generates an antiinput to the coincident module if a second output is seen from either discriminator during the gating interval. The delay between discriminators 4 and 4’ and the coincident module is equal to the linear pulse width or gating interval to allow the pile-up rejector time to look for multiple inputs. From the standpoint of pile-up rejection, it is desirable to adjust the discriminators to as low a level as possible. However, there tends to be a small reflection on the tails of the pulses from the termination. Care must be taken to see that the discriminators are above the reflection so that good pulses are not rejected. Environmental noise was a problem before it was virtually eliminated by a third fission detector placed near the coincident detectors but unable to “see” the source. The output of this anti-noise detector, after suitable amplification and discrimination, was sent to one of the two inputs of the pile-up gate. A noise event processed in the anti-noise detector and the fission detectors was rejected by simulating pulse pile-up. The signals from the fission detectors were delayed and arrived at the pile-up gate about 20 nsec later than a pulse from the anti-noise detector. The pile-up gate thus assumed that pile-up occurred and completely rejected the event. For experiments involving very low counting rates, additional precautions are taken. In order to check the authenticity of coincident fragment events, a picture of the analog signals of both coincident detectors is taken. This is accomplished by routing outputs from the fan-out amplifiers through adequate delay and into the dual trace inputs of an oscilloscope. Mounted to the scope is a 35 mm strip camera with high speed film moving at a pre-set rate. The scope is triggered from a coincident unit whose inputs are discriminators 5 and 5’, and the output of the primary coincident unit. Timing marks are placed on the film. Each event stored by the multi-dimensional analyzer is tagged with the clock time. The analyzer output is dumped on magnetic tape for computer analysis. Interesting events can then be identified and examined on the film. 5. Application of the technique to search for super-heavy elements A system similar to that described above, but without

J. I. WESOLOWSKI

212

was used in a search for supernoise suppression, heavy element 110 in platinum ore via neutron induced fission3). The technique used in the search was to expose a sample to thermal neutrons and to measure the kinetic energies of the resulting fission fragments. The kinetic energies of the fragments of a super-heavy element are much larger than those of contaminant materials such as 235U. If the super-heavy element exists and has a reasonable neutron induced fission cross section, it could be easily distinguished from lighter contaminants. The system operated satisfactorily in a thermal flux of 10” n/cm”. set for six weeks. In connection with a subsequent experiment, the present system with the noise suppression has been operated in the same flux for eight weeks and showed no appreciable sign of deterioration. Fig. 5 shows two-parameter pulse height spectra of

et al.

the coincident fragment detectors. The target this time was Pb ore of 1500 A thickness sandwiched between 1000 A of pure aluminum. Element 114 might be present in Pb ore because of its analogous chemistry. The of only two peaks correspond to a 235U contamination lo-l3 g, illustrating the high sensitivity. As was the case with platinum, the present experiment gives no indication of fission fragments from the neutron induced fission of a super-heavy element. One of the several detectors used in these experiments would occasionally give a very large amplitude signal (sometimes an order of magnitude larger than the average signal) when struck by a fission fragment. The shape characteristics of these pulses were very similar to normal pulses. This occurred at a frequency of less than one large pulse for every 2000 fission fragments, but of course was of concern in these superheavy element experiments. A scan of the surface of the detector with a well collimated 252Cf fission source indicated that the large pulses came exclusively from a small area of the detector (much less than 0.06 in diameter) located off the center. The count rate of the large pulses was not significantly changed by drastic reduction of the bias voltage thus making it unlikely that an avalanche breakdown occurred. It is surmised that amplification via a transistor action takes place at this small spot owing to faulty manufacture. The large pulses were not observed in the companion detector and were apparently unrelated to radiation damage. We thank P. Dickey, R. Knief and W. Singleton for assistance in various phases of this work. H. Spracklen for suggesting the gold ring contact, and W. Wade for design and construction of mechanical parts.

I

I 20

10 Detector

No. I channel

I 30 number

Fig. 5. Pulse-height data array of coincident fission fragments from a Pb ore foil irradiated by thermal neutrons. Numbers labeling the contours and those outside the contours give the number of events per channel squared. The counts are from an impurity of lo-rsg of 2ssU. The counts were obtained in a 20 h run.

References 1) F. Goulding, in Semiconductor

detectors

for nuclear spectro-

University of California Radiation Laboratory Report no. UCRL-16231 (1965). 2) H. Jackson, in Lawrence Radiation Laboratory Counting Handmetry,

book, Ccl-8 3)

(1966).

J. J. Wesolowski, W. John, R. Jewel1 and F. Guy, Phys. Rev. Letters 28B (1969) 544.