Fission fragment detection by thin film capacitors—II

Ntwlear Track Detection. Vol. 1. No. 1, pp. 71-74. Pergamon Press, 1977. Printed in Great Britain.

FISSION F R A G M E N T D E T E C T I O N BY THIN FILM C A P A C I T O R S - - I I CURRENT

PULSE COUNTER

AND MECHANISMS

N. KLEINand P. SOLOMON Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa, Israel

and l,. T()\I\I ~%IN() Laboratorio l)o~,inlctria c Biofisica dellc Radiazioni, CNEN-Casaccia, Roma, Italy (Received 15 N o r e m b e r 1976)

Abstract--Fission fragments produce measurable current pulses in thin film capacitors at electric fields which are high, but still below the breakdown range for fragments. This paper describes the use of silicon dioxide capacitors for the detection of fission fragments by such current pulses. With capacitor areas of 2 x l0 -2 cm 2, the pulses were insignificant when the oxide was relatively thin, but with 3800A thick oxide, fission fragments produce detectable pulses of about I0- ~sC. The mechanisms producing the current pulses by fission fragments are discussed. 1. I N T R O D U C T I O N

able, however, with several thousand /~ thick oxide layers. Current pulses were first registered with a 3800/~-thick SiO 2 layer as reported recently by Klein et el. (1975). The 3800-,~-thick SiO 2 layer was thermally grown on a p+ degenerate silicon substrate (0"006 ohm-cm resistivity). The capacitor area was 2 × 10 -2 cm 2, the aluminium counter-electrode 200-Athick, and the capacitance about 150 pF. In order to avoid charge carrier depletion in the Si substrate the polarity of the counter electrode was kept negative. Figure l shows schematically the apparatus used for the current-pulse detection. For amplification a Brookdeal type-452 Precision a.c. amplifier was used. The amplifier output was connected to a storage

IN ANearlier paper on thin film capacitors a new fission fragment counter was described, which registers crossing fragments by nonshorting breakdowns (Tomlnasino et al.. 1975). This breakdown detector has finite applicability owing to gradual destruction by counting events. We are reporting here an alternative use of a thinfilm capacitor as fission-fragment counter. Electric fields applied to the capacitor are lower than those which cause breakdowns by fragments, and the fragments crossing the detector are registered by current pulses (Klein et al., 1975). 2. T H E D E T E C T O R A N D ITS CIRCUIT

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Initial experimen'ts, carried out with SiO 2 capacitors used for breakdown counting, failed since the fission fragment-induced current pulses were submerged in the noise of the detecting system, which had an overall noise figure better than 3dB. These detectors had areas larger than 8 × 10 2 cm 2 and oxide layers a few hundred to a thousand A thick. The signal-to-noise ratio of these counters was to low owing to their large capacitance and small sensitive thickness. The fission-fragment-induced pulses became detect-

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N. KLEIN. P. SOLOMON and L. TOMMASINO

oscilloscope and fission fragment-induced current pulses were counted and measured on the screen. Complete details about the radiation source and experimental procedures are described elsewhere (Klein et al., 1975). 3. RESULTS A typical voltage pulse recorded on the detector C d , after the passage of a fission fragment through the oxide, is shown in Fig. 1. For a given applied field to the detector, there was an appreciable scatter in pulse heights due to the spread in energy losses in the SiO 2 experienced by the different types of fission fragments. Measurements resulted in data on mean pulse voltages, AV, caused by the fission fragments as function of,applied voltage. Mean pulse-charges were calculated from the relation AV ~ N e / ( C a + Ci,); here e is the electronic charge, N the number of collected electrons, and C d and (7in are the detector and the preamplifier input capacitances, respectively. Figure 2 shows the mean charge/pulse vs the applied voltage. This mean pulse charge is in the 10- ~sC range and appears to increase rapidly with voltage. We plot in Fig. 2 also the number of current pulses registered in 100sec irradiation time vs the applied voltage. The curve for the number of current pulses reaches a plateau above 220 V. The largest pulse height registered corresponded to a charge of about 7 x 10- ~sC. Larger pulses resulted in breakdowns, and these occurred above 230 V at a rate increasing with 'voltage. At 240V an average of six breakdowns was observed in 100sec. Without radiation, breakdowns started to occur above 330 V. We were interested also in the spectrum of the pulse-

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FIG. 2. Mean pulse charge (crosses) and number of current pulses/100sec (circles), as functions of voltage applied to a silicon dioxide detector on irradiation by fissionfragments.

height distribution, and for this purpose the sensitivity of measurements was improved. The detector capacitance was 40 pF, the oxide thickness 4500 A, and the activity of the z52Cf source was about 2x 104 disintegrations/sec. The detector was coupled to a charge-sensitive Ortec 109 PC type preamplifier, the output of which is directly proportional to the charge input and does not depend on detector capacitance. The amplifier output was connected to a multi-channel analyser. Figure 3 shows a spectrum of pulse-height distribution vs the channel number produced by fission fragments crossing the 40pF detecting capacitor. The electric field applied to the capacitor was about 4MV/cm. This spectrum consists of two partially ~

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FIG. 3. Spectrum of pulse-heightdistribution vs the channel number. overlapping peaks, due to light and heavy fission fragments, respectively (Klein et al., 1975). After the exposure of the detector to a fluence of approximately 5 x 10~ fission fragments/cm 2, the shape of the pulseheight distribution spectrum becomes distorted. Also the pulse heights decrease with increase in the total fission fragment fluence received by the detector (Tommasino and Zapparoli, 1976). 3. DISCUSSION AND CONCLUSIONS We investigated recently the effect of thermal and ionization spikes, which accompany the passage of a fission fragment through the detector, on fissionfragment-induced breakdowns (Tommasino et aL, 1975). Results of these studies are useful also for the

FISSION F R A G M E N T D E T E C T I O N - - II interpretation of the current pulses produced by fission fragments. Let us consider the thermal spikes first: there is a large uncertainty regarding the cross section S of the spike, and the calculations extended from 3 × 10 - l a < S < 1 0 -11 cm 2 (Tommasino et al., 1975). The temperature pulse of such spikes was found to dissipate in times z, with 6 x 10-13 < z < 2 x 10-1 ~ sec. The temperature pulse greatly enhances the electrical conductance of the spike and a current pulse is expected to arise owing to the applied field. If the pulse is ascribed to thermionic emission from the aluminium electrode, the current densities J due to a field of 5.4 MV/cm are calculated to vary for the above range ot S from 5 x 101° > J ~>0 (in A/cm2). These values of J and z correspond to the discharge of 70 000 > N > 0 electrons through the detector, the figure 70000 relating to S = 3 x 10-~3 cm 2. For the small values of S the charge e N is close to the values measured (Fig. 2). Owing to space charge limitations, however, it is not likely that current densities could reach magnitudes larger than 107 A / c m 2. For this current density e N is much smaller than the charges measured, and it is rather doubtful that the current pulses are due to thermal spikes. The effect of the ionization spike can be connected with the positive charge produced around the track by impact ionization of secondary electrons. The magnitude of this charge may be estimated: the light fragments of the fission of 252Cf lose through the silicon dioxide on the average 1060eV/A, and the heavy fragments 1200eV/A (Tommasino et al., 1975). The losses are mainly caused by ionization. The magnitude of the ionization energy E i for the creation of an electron-hole pair by electronic impact in SiO 2 is little known. We may infer it from theoretical considerations for semi-conductors, for which E~ ,~ 3E o + 1 eV, E o being the band-gap (Klein, 1968). Applying the relation to SiO 2 with E0~9eV, E i ~ 2 8 e V is estimated. The passage of a heavy fission fragment through the 3800 A thick oxide should produce, then, about 160000 impact ionizations. The density of ionization decreases rapidly with distance from the track (Katz and Kobetich, 1968; Fleischer et al., 1975). Irrespective of whether we assume a radius of 100A or several hundred A for the ionization spike, the positive charges produced cause instantaneous electric fields much larger than the 5-6 MV/cm field applied to the detector. The ionization spike is followed immediately E*

73

by charge neutralizations, which may be geminate (Hughes, 1973), or plasma recombinations with the secondary electrons, but also recombinations with electrons injected from the aluminium electrode. These processes are very complex, and we can describe them only qualitatively: Owing to the fact that, relative to electrons, positive charges are extremely immobile around the track (Di Stefano and Eastman, 1971; Solomon, 1974), the field is greatly enhanced at the cathode and diminished or even reversed at the anode. In consequence, an electronic current-pulse is injected by tunneling from the cathode or even the anode into the oxide around the track, and the net charge lost by the capacitor registers as a voltage pulse in the external circuit. The pulse continues until electron-hole recombination, and possibly some hole drift, decreases the positive charge around the track to a small value. The pulse magnitude depends on a number of parameters, and in this regard silicon dioxide seems to have some advantages for the detection of fission fragments by current pulses: (a) The threshold field for breakdowns is large and the leakage current is very small in the range of fields used for detection. (b) The current injected at the cathode rises steeply with field, which powerfully enhances the magnitude of the current pulses. (c) The electron-hole recombination cross section is small (10-1Scm 2) and the electron sweep-out time only a few psec (Solomon, 1974). A disadvantage is the large band-gap and ionization energy, which have the effect of a decrease in the magnitude of the current pulse. The exposure of the detector to an irradiation of 5 × 1 0 7 fission fragments/cm 2 resulted in a distortion in the shape of the pulse-height spectrum and a decrease in the mean pulse height. This appears to be a consequence of the radiation damage in the silicon dioxide, and may possibly be connected with enhanced electron trapping. Trap generation is known to occur on ion implantation and electron irradiation in silicon dioxide (Johnson, 1975). The effect of this radiation damage has to be accounted for, when the silicon dioxide detector is used for d E / d x measurements. It seems at this early stage in the development of the insulator current-pulse counters that they offer some advantages over existing detecting systems: SiO 2 pulse counters can be easily obtained with thicknesses appreciably lower than those of the semiconductor detectors and can withstand much larger applied electric fields; these characteristics are useful for timing and dE/dx measurements of highly ionizing particles.

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N. K L E I N , P. S O L O M O N a n d L. T O M M A S I N O

The associated signal processing e q u i p m e n t is simpler t h a n for scintillation detectors. The thin insulator counters do not require thick windows as gas counters. The main p r o b l e m in the use of thin insulator current-pulse counters appears to be the achievement of satisfactory signal-to-noise ratios. F o r optimization it will be of interest to investigate other insulators a n d their c o m b i n a t i o n s with varying electrode metals.

REFERENCES Di Stefano T. H. & Eastman D. E. (1971) Photoemission measurements of the valence levels of amorphous SiO 2. Phys. Rev. 27, 1560-1562. Fleischer R. L., Price P. B. & Walker R. M. (1975) Nuclear Tracks in Solids: Principles and Applications. University of California Press, Berkeley.

Hughes R. C. (1973) Charge-carrier transport phenomena in amorphous SIO2: direct measurements of the drift mobility and life time. Phys. Rev. Lett. 30, 1333-1336. Johnson W. C. (1975) Mechanisms of charge build-up in MOS-insulators. IEEE Trans. Nucl. Sci. NS-22ql, 2144-2150. Katz R. & Kobetich E. J. (1968) Formations of etchable tracks in dielectrics. Phys. Rev. 170,401 ~,05. Klein C. A. (1968) Band-gap dependence and related features of radiation ionizing energies in semiconductors. J. appl. Phys. 39, 2029-2038. Klein N., Solomon P. & L. Tommasino (1975) Thin insulator current pulse counter of fission fragments. Nucl. Instrum. Meth. 129, 119-121. Solomon P. (1974) D.Sc. Thesis, Technion-Haifa. Tommasino L., Klein N. & Solomon P. (1975) Thin-film breakdown counter of fission fragments. J. appl. Phys. 46, 1484-1488. Tommasino L. & Zapparoli G. (1976) Unpublished.