Bolometers

Nuclear Instruments and Methods in Physics Research A2g9 (1990) 504-511 North-Holland

504

Section V. Detectors for general physics experiments OLO

E

S

A . ALESSANDRELLO *, C . BROFFERIO, D .V . CAMIN, O . CREMONESI, E. FIORINI, A . GIULIANI, G. PESSINA and E . PREVITALI Dipartimento di Fisica dell'Uniuersith and Sezione dell'INFN, Milano. Italy Presented by A . Giuliani

Results on the trust advanced bolometers for particle detection are reported . A microcalorimeter able to reach resolutions better than 20 eV for energy depositions of the order of 1-10 keV is briefly described . We report the performances of three massive detectors, which are capable to reveal low- and high-energy -y-rays as well as low-ionizing nuclear recoils. For the first time results obtained with a superconductive absorber (molybdenum) are presented, showing that there are serious difficulties in thermalizing the deposited energy, which could prevent from employing superconductors in a bolometerc approach.

1 . Introduction Low-temperature calorimeters, also called "bolometers" by analogy with the astronomy infrared detectors from which they derive, are not a novelty anymore in particle detector field and are now considered as peculiar and promising devices to investigate single-particle interactions at low energy 111. Their operating principle, which was proposed some years ago [2,3], is quite simple : the energy deposited by a single particle in a diamagnetic dielectric crystal, cooled at temperatures lower than 1 K in order to get a low heat capacity according to the Debye law, is able to determine an appreciable temperature increase, which is usually measured by means of the subsequent resistance change of a doped semiconductor thermistor kept in thermal contact with the crystal (fig. 1) ; other methods for measuring the temperature change have been proposed with encouraging experimental results [4] . The advantages of this technique with respect to conventional detectors can be summarized in the following three points : i) not only the ionizing fraction, subjected to statistical fluctuations, but the whole deposited energy contributes to the signal formation, making resolution of 1 eV-fractions in principle achievable [3] ; ii) non-ionizing events are detectable ; iii) the choice of the material for the energy absorber is much wider than in conventional ionization detectors, being the only requirements that it be crystal line and have a so high Debye temperature that,

Presently at Laboratorio Nazionale del Gran Sasso, Assergi, L'Aquila, Italy.

given a certain volume, the heat caF city is low enough [2] . The listed points make bolometers particularly attractive for the investigation of crucial problems in

physics [5], like the determination of the neutrino mass through the calorimetric measurement of tritium 0spectrum, the detection of dark matter weakly interacting massive candidates and the measurements of double beta decay rates for nuclei whose corresponding elements are unsuitable for conventional detector construction . However, what is still lacking is a physics experiment running with a bolometer : this article, far from being a review, tries to present those results which tell us how far and how close we are to this final goal, devoting particular attention to the work developed by the Milan group to which the authors belong ; experiments aimed at particle detection through nin-equilibrium phonon production ar-- not considered in order to limit the already wide topic .

2 . Experimental results The results obtained by three groups are described here : as far as small-mass bolometers are concerned ( < ï mg), the impressive energy spectra presented by THERN11STOR

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0168-9002/90/$03 .50-) 19910 - Elsevier Science Publishers B .V . (North-Holland)

Fig. 1 . Schematic view of a bolometer.

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the Wisconsin- Goddard collaboration will be reported, while for large-mass bolometers (from - 1 g up) the detectors developed by the BONUS collaboration and the Milan group will be described . 2 .1 . Small-mass bolometers

The Wisconsin-Goddard collaboration has developed several microcalorimeters aimed at X-ray detection in the few keV range. The best result in terms of energy resolution was achieved by a composite detector, in which the part for energy absorption consists of a TeCdHg crystal (size: 4 X 10-° mm3) glued with a silver epoxy onto a sensing element, consisting of a silicon crystal (size : 0.5 X 1 x 0.075 mm3) doped with phosphorus by implantation in a small region in order to make the thermistor [6]. The choice of the absorber comes from the observation that the main reason why in previous work [7] this group was not able to push resolution close to the theoretical limit was that a large fraction of the electron-hole pairs generated by the ionizing particle are trapped by impurities whose energy levels lie in the bandgap of silicon and germanium, which were used as energy absorbers ; TeCdHg has a very small bandgap (a few meV) with a consequent reduction of the importance of this phenomenon: the price to pay, however, is a higher heat capacity of the detector . The trapping could in principle be completely avoided by employing a metal as absorber, but in this case the electronic contribution to specific heat would make the detector total heat capacity too high. The choice of a small bandgap semiconductor represents therefore an advantageous compromise between two contradictory requirements. The detector is operated with a dilution or adiabatic demagnetization refrigerator at a bath temperature of

about 100 mK, at which the the istor exhibits a resistance of several MSZ and a sensitivity, defined as Id log R/d log T 1, of about 5. In these conditions, the thermistor, thermalized at the bath through thin aluminium wires, is able to stand a voltage bias of - 10 mV with a temperature increase with respect to the base temperature of - 10% ; an energy deposition of - 5 keV determines a voltage pulse across the detector of - 500 ILV [8]. The detector was irradiated with an 55 Fe X-ray source, which emits photons of 5.9 (K .line) and 6.4 (K ß -line) keV . The resulting spectrum is a dramatic demonstration of the potential of this technique: the .-line is - 17.4 eV and the FWHM resolution for the K two components of the K .-line begin to be resolved (fig. 2). The resolution achieved is an order of magnitude better than that given by conventional Si(Li) detectors . The limit of these devices, which prevents their immediate utilization in a neutrino mass experiment, is the relative slowness of the pulse risetime (- 100 ps) incompatible with the high rate of tritium ß-decays necessary to have sufficient counts at the end-point region . A possible solution is the development of a single detector with two thermistors, the first one giving slow but low-noise pulses, the second one giving noise but fast pulses to be used for time discrimination in order to reject the unavoidable pileup . 2 .2. Large-mass bolometers

In small-mass bolometers like the one described in the previous section, the goal is to push detector resolution to around 1 eV; in larger-mass devices the main aim is to exploit other peculiar properties of bolometerc detection, like the capability of detecting non-ionizing events and the possibility of employing unusual materials as energy absorbers which are interesting for double

COUNTS 150 120

a

90 Nin

60

üa 1 1 N1 n K ¢i

30 0

5 .8

6 .0

Fig. 2. (a) Spectrum of a

6 .2 55

6 .4

6 .6 ENERGY (keV)

of two components in the K .-line Fe X-ray sourcL obtained :vith a microcalonmeter ; (b) the evidence . of 17 .4 eV FWHM indicates a resolution V.

Pli~' Slç-S

A. Alessandreilo et al. / Balorneters

506

beta decay or for dark matter detection . In the latter case, resolution is less important, in the sense that even a detector with a resolution of the same order of magnitude as a conventional one would be a powerful device to study the mentioned phenomena . The main difficulty in operating large devices is the necessity to work at much lower temperatures (even 5-10 mK), which is not itself a problem, but which has as physical consequences an increase of the decoupling between the various thermal systems which make up the detector: energy absorber, glue, thermistor lattice, thermistor electrons . This decoupling leads to very long pulse risetimes with a consequent reduction of the pulse amplitude and sometimes prevents one from applying high enough biases to the thermistor, whose electron temperature might increase too much because of the electrical power dissipated in it. Bolometers performances cannot therefore be trivially extrapolated from "high" temperature and small volume to low temperature and "large" volume . In spite of this, some "large" bolometers have been realized and the performances, if they are regarded as test devices, are encouraging. 2.2.1 . A 0.7 g a- and nuclear-recoil-detector

At the end of 1987 the Milan group realized the first "massive" bolometer [9]. The particle absorber was a pure (100) germanium crystal (size: 11 x 4 x 3 mm3 ) and the sensor was a small germanium , rystal (size: 3 x 0.5 x 0.5 mm3 ) doped presumably with gallium and arsenic (the sensor being commercial, we have no information at all about the doping method : anyway, we characterized nearly 20 thermistors of the same type, finding with good reproducibility an R(T) behaviour of the form R (T) et exp(T,/ T )0.5, where the exponent 0.5 is usually typical of NTD [81). The crystal was mounted in a ;old frame which introduced only point contacts with it, while the frame

COUNTS

was firraly clamped to the bath. The sensor was glued onto the crystal, the contact surface being the 3 x 0.5 mm2 one ; the thickness of the glue, a two-component nonconductive epoxy, was about 50 Rm. A couple of 40 urn diameter gold wires, 3 mm long, assured the readout of the signal: the wires were cold-welded by indium to two large copper electrodes, thermally connected as firmly as possible to the bath. The thermal conductance between sensor and bath, measured at the operation temperature, was in this configuration very similar (within the experimental errors) to that measured for the sensor alone, suspended by its own wires, showing that the thermal impedance provided by the point contacts between gold frame and crystal dominates that given by the gold wires : this was exactly what one was looking for, in order to force the heat generated in the crystal to flow through the sensor . The detector was operated at different base temperatures, all around 50 mK, and with different bias levels, which raised the thermistor temperature to values between 60 and 80 mK. The best performances were obtained with a bias level of 19 mV, which lead the detector temperature to 71 mK corresponding to a resistance of 1 .2 MSZ ; this operation point was far from optimum in a pure bolometerc analysis [3], which would suggest a temperature of about 55 mK, and it corresponded to a negative slope in the I- V curve of the thermistor . However, the high parasitic capacitance (500 pF) forced us to work with resistances not much higher than 1 MSZ ; furthermore, the detector a-particles released a relatively high energy which would have determined a nonlinear response at lower temperatures. That shows how our detector was a crude device working in nonoptimum conditions: however, the results were satisfactory. An implanted ß-source of 228Ra, in equilibrium with its 228 Th daughters, which gives therefore a cascade of a-decays with 5 main lines between 5 and 9 MeV, was

COUNTS

Ib

7

8

9

Fig. 3 . Spectra of a ` z "Ra source with high satellite peaks (a) and low satellite peaks (b).

A. Alessandrello et aL / Bolometers

507

Table 1 Nuclear recoil detection EQ [MeV]

Er [MeV]

Channels

5.4233 5.6855

0.101 0.114 0.125 0.166

1484 1554 1586 1653 1712 1745 1841 1877 2354 2397

-

6.06 6.2883 6.7785 8.7844 -

placed nearby the detector. The FWHM resolution is -- 50 keV at - 5 MeV (fig. 3b) . An interesting phenomenon is offered by the small satellite peaks which are visible at the high-energy side of some of the lines: these peaks are explainable admitting that sometimes instead of an a-particle it is an unstable nucleus which reaches the detector from the source: in that case the nucleus is subjected to an a-decay inside the bolometer, allowing the detection of the entire energy of the transition, which is about 2% higher than the a-energy only. This effect is not evident with a conventional detector, because the low-ionizing recoiling nucleus does not give a high enough signal. This observation is the first clear proof of the superiority of bolometers as far as nonionizing and low-ionizing energy detection is concerned . In order to make the phenomenon even more evident, we have pre-implanted the detector exposing it to the source under vacuum for several days without the collimator, raising very much the height of satellite peaks . A spectrum in which the main peaks and the satellite peaks have the same height is shown in fig. 3a. Of course, as the lifetime of the parent implanted nucleus is - 3 days, after 20 days the spectrum reassumed the characteristics of fig . 3b. A couple of interesting considerations can be made from the spectrum in fig . 3a: i) the first line is not doubled, as it corresponds to the first a-decay which implants the parent nuclide in the detector, and so the corresponding nuclei have no way of going into the detector; ii) the distances in energy, ®, between the satellite peaks and the main peaks are not the same for all the energies and do not exactly correspond to that expected from the mass ratio between a-particle and recoiling nucleus ; the distance is always a bit larger, suggesting that the recoiling nucleus detection efficiency (,E,) is higher than the a-particle one (E.). If Er is the recoil energy and E. is the a-energy, calling E the main peak

E [MeV] 5.4155 5.6823 5.861 6.0652 6.2935 6.4211 6.7925 6.9317 8.7769 8.9432

a [MeV] 0.124 0.128 0.139 0.166

C. /(r

._

0.814 0.891 0.901 0.999

energy as obtained by detector calibration, it is possible to write (10): . EIE« __ EI E« AIE , Er ErErle .E. showing therefore that the detection-efficiency ratio Cu/Cr is measurable from the spectrum in fig. 3b. This ratio turns out to be a function of energy (table 1); at low recoiling energy (100 keV) a-detection-efficiency is only 81% of nucleus-detection-efficiency while at high recoiling energy (160 keV), the two efficiencies are comparable : this can be interpreted assuming that the absolute detection efficiency of the nucleus gets lower increasing the energy, as the ionizing part of the deposited energy increases determining a higher probability of electron- or hole-trapping, with delayed or hindered thermalization as a consequence . 2.2.2. A 25 g a- and low-energy -y-detector The BONUS collaboration developed several bolometers, using Ge-doped sensors of different volumes (111. The most important and original results obtained by this group concerns the realization of a massive detec-

Fig. 4. The most massive bolometer in the wodii . (1) 25 g sapphire absorber ; (2) - 5 mm3 Ge thermistor; (3) suspending nylon threads; (4) sapphire strip for the heat link ; (5) thermal bath . V. GENERAL PHYSICS

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A. Alessandrello et al. / Bolometers

tor, consisting of a 25 g sapphire crystal as an absorber and a large-volume (5 mm3) Ge-thermistor as a sensor . The crystal is of cylindrical shape, with a length and diameter of 2 cm; it is suspended in the bath by 32 nylon wires of only 60 ~tm diameter, which introduce a negligible thermal conduction ; a thin sapphire strip links the system to the bath at 100 mK, determining a 30 ms time constant (fig. 4). The thermistor is biased to 40 mV (a very high value, allowed probably by the large sensor volume which assures a reasonable thermal coupling between lattice and carriers in the thermistor) and has an operating resistance of 2.7 MSE. 24'Am source which Tests were performed with a emits a's and low-energy y's and, at reasonably high -y-energy, with a ('°Co source which gave high pulses but released with a very low probability the full energy in the detector, which did not allow the resolution t) be monitored. The a's at 5.48 MeV were on the contrary detected with an excellent resolution of 35 keV FWHM ; the low-energy Y's (59 .54 keV) gave a peak in the right position with respect to 4x-calibration, showing the wide linearity of the device, with a FWHM resolution of 16 keV . The bolometer described is the most massive one ever realized anywhere in the world: the good resolution, the low threshold and the low Z of the constituting material make the device a possible dark matter detector as it is now, provided it is operated in the proper low-background environment . 2 .2.3. An 11 g high-energy y-detector

The Miian group has realized the massive bolometer 1121 with the highest V/(® 3) ratio in the world, which is the parameter that defines most correctly the " thermal mass" of a crystalline device . This detector consists of (100) 11 g Ge crystal as an absorber, while the sensor is a doped Ge crystal (size : - 1 mm3). The absorber volume is 20 x 10 x 10 mm3, and the mass 11 g. The detector was mounted inside a copper envelope, well-clamped at the heat sink, and the mechanical connection between the envelope and the crystal was realized by four aluminium points, introducing as usually a negligible thermal conduction to the bath: two gold wires (diameter : 40 ~Lm) from the sensor assured the elecCrical readout and the heat-sinking of the device . The bolometer was operated at a base temperature of - 25 mK and it was biased to 500 ILV, raising the temperature to -- 28 mK: a clear limit of the device was that the low resistance of the thermistor (250 kQ at the base temperature) joined with the anomalous weak thermal conductivity at very low temperatures prevented from introducing too much power in the sensor, limiting very much the tolerable bias level. Nevertheless, the bolometer was able to detect y-rays from 120 keV to 2.6 MeV, with 80 keV FWHM resolu-

COUNTS

1594 keV 1

COUNTS

b 496 .

356 .

216 .

79 . 0

COUNTS 166 .

ENERGY

122 keV 1

C

120 .

00 .0

48 .6

- .6®®

ENERGY

Fig. 5. Spectra obtained with an 11 g Ge detector irradiated by 232Th (a), 6°Co (b) and "Co (c) y-sources .

tion almost independently of energy. The resolution is clearly measurable through the double-escape peak coming from pair production under 2.6 MeV photon irradiation (fig. 5a) ; it is possible to see also the small full-energy peak, as in the case of 1.3 MeV line from a 6°Co source (fig. 5b) . The detector was irradiated also with low energy Y's (120 keV) from a 57Co source and, as visible in fig. 5c, the photoelectric peak is evident . The threshold measured from the baseline random noise is around 50 keV .

A . Alessandrello et al. j Bolometers Resistance (kQ) xE-i

2080 .

HAI

4 .590

3 .50ü -f-

a

2 .588 ,

r

1 .580

.5800 -i

Temperature ( m K)

s .l~9g

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.? 8

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V IV)

Fig. 6. (a) Resistance-temperature curve for the Ge thermistor used for a molybdenum detector ; (b) voltage-current curve for the molybdenum bolometer : a mark indicates the chosen operating point. 2.2.4.A 24 g superconductive absorber detector The question is open whether a superconductor is a suitable material for an absorber in a low-temperature calorimeter. From the point of view of heat capacity, if the temperature is sufficiently lower than the critical temperature T, the contribution to specific heat coming from electrons is negligible with respect to the lattice contribution, as it is governed by the law (BCS theory) : cs

yTc 1 .34(1 .76TVT) 31z exp( -1 .76T,/T ),

=

where cs is the superconductive electron specific heat and y is the temperature proportionality coefficient for normal electron specific heat. However, since an ionizing particle excites initially a phonon system whose average energy is higher than the superconductive energy gap, the produced phonons break Cooper pairs before thermalizing ; the effective

thermalization time of the quasiparticle being probably much longer than the total response time of the detector, there is the risk that most of the deposited energy is lost as far as thermal detection is concerned . In order to test experimentally the behaviour of a superconductive absorber, we have developed a detector consisting of a cylinder of molybdenum (Tc = 0.92 K), 12 mm high, with a diameter of 5 mm and a mass of 2 .4 g; as a sensor we have used a Ge-doped crystal with a total volume smaller than 1 nun3 ; the thermistor characteristic is shown in fig. 6a. The detector was mounted very similarly to the 0.7 g Ge bolometer described above. The base temperature was stabilized at 60 mK; an V V curve of the thermistor was measured (fig. 6b) and an operating point was chosen at a thermistor temperature of 71 mK. We would like to remark that these

PULSE AMPLITUDE

1 .40

.6130

. ;log 15 0 S0 .

25UB

35 N

45U0

TIMEOIS)

Fig. 7. Average pulse (a) and spectrum (b) obtained with the molybdenum bolometer irradiated by a

232Th

-y-source .

v. GENERAL PHYSICS

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A . Alessandrello et al. / Bolometers

conditions are better from any point of view than those achieved with the Ge 0.7 g detector, and that a signal across the sensor as high as - 400 IN was expected for an energy deposition of 2.5 MeV, in the hypothesis of complete thermalization of the released energy . 232Th source, which The detector was irradiated by a gives several -y-lines . The most intense ones are at 238, 911, 969 and 2614 keV. The detector was sensitive to the source. An average pulse is shown in fig . 7a, while a spectrum of 2300 min is reported in fig . 7b. However, the detector performances are unsatisfactory and cast a shadow over the possibility of employing superconductors as energy absorbers, as explained by the following considerations : i) the highest pulses visible with the source irradiating the detector (which should correspond to 2.6 MeV photons) have an amplitude not higher than - 50 RV, i.e. one order of magnitude lower than expected, suggesting a very poor thermalization of the deposited energy; ii) no meaningful structure is visible in the spectrum, while, with a reasonable 5% resolution, the double-escape peak from 2.6 MeV photons should be evident, as shown in the Monte Carlo simulation of fig . 8a; iii) there are clear adications that the detector is not sensitive in the full volume: the measured counting rate (which is only indicative since we do not know exactly the energy threshold as the energy calibration of the spectrum is very difficult) is after background subtraction only 1 .3 x 10 -2 Hz. The source intensity is - 3 l.Ci over 4,rr and the solid angle is about 3 x 10 -3. Under these conditions the expected counting rate in the whole detector taking into account the main lines of the sources is 0.58 Hz, while for the sensor alone it is 4.15 x 10-3 Hz, as Monte Carlo simulations show. Therefore we can conclude that there is probably a part ~arei .u ..u. .l .aalW4. "" n " . " . " 1uWrAa " .VL.1aW1Wi1.Yl+li1"" w1

of the absorber which is sensitive, but this part is a very thin layer close to the sensor ; this hypothesis was directly confirmed placing an a-source close to the absorber but at the opposite side with respect to sensor position and obtaining in that way a counting rate compatible with background . Assuming that only a small layer close to the thermistor, as thick as 0.3 mm, is sensitive and that the resolution is about 20%, one obtains by a Monte Carlo simulation a rate comparable to that effectively observed and a spectrum similar in shape to the experimental one (fig. 8b) . For the above reasons, superconductive absorbers seem to give more problems than they can solve: their use is in fact mainly linked to the hope of exploiting new "source-detector" materials for double beta decay search, as for example molybdenum. In most cases, however, the question can be "short-circuited" simply by using a proper dielectric compound including the interesting element, for instance molybdenum carbide or molybdenum oxide. This is the direction that our group decided to follow after the experimental tests on pure molybdenum. 3. Perspectives As far as the future of bolometers is concerned, it is necessary to divide the possible developments of massive bolometers from those of microcalorimeters. In the latter case, the devices of the Wisconsin-Goddard collaboration have already reached a performance level which would allow their meaningful use in a physics experiment, for example in astronomical }I;-ray detection or in a neutrino mass measurement ; what is in progress now is the specific preparation of the experiment.

.Wlri.u.. ..la.iW ..ua+~ilrl~arW1

GO

l'20 40

40

.

0 . . . . . . .. . .. . . .. . .. . .. . . .. . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . .. . . 16011 I SOO 2111111 1 100 2200 2400

. . . . .. . . . . . . . . . . . .. 2800 '21r0û

ENERGY (keV)

I)

2UU

GOO

luau

.~ . . 1400

. . . . . . ., . . . . . . . . . : . .a . . 1800 2200 2600

ENERGY (keV)

Fig. 8. (a) Spectrum which molybdenum bolometer should give when irradiated by a 232Th - y-source in case of full volume sensitivity and 5% resolution (Monte Carlo simulation) ; (b) spectrum expected if only a thin absorber layer. 0.3 mm thick, is sensitive and resolution is 20% (Monte Carlo simulation) .

A . Alessandrello et al. / Bolometers

In massive bolometers, a further increase of one order of magnitude of the total mass would be useful, and the materials for double beta decay or dark matter searches are still to be tested. The Milan group is going to test a massive LiF crystal for dark matter. Furthermore, a low activity cryostat placed in a low cosmic background environment is necessary to start with reasonable sensitivity the experiments mentioned. A special refrigerator with the required characteristic, constructed by Oxford Instruments under continuous checks on material radioactivity performed by the Milan group, is now nearly operational in the Gran Sasso laboratory. Acknowledgements As far as the Milan group activity is concerned, we would iûie to thank R. Benedet, R. Cavallini, S. Latorre and S. Parmeggiano for their skillful help; we would also like to acknowledge useful discussions with N .W. Ashcroft, L. Gonzales-Mestres and D. Perret-Gallix about energy thermalization in superconductors . References

E. Fiorini and T.G Niinikoski. Nucl. Insu. and Meth. 224 (1984) 83. (3] S.H. Moseley, J.C. Mather and D. McCammon, J. Appl. Phys. 56 (1984) 1257. [4] M. Biihler and E. Umlauf, Europhys. Lett. 5 (1988) 297 . [5] E. Fiorini, Cryogenic Detectors and Materials Research in Physics and Astrophysics, in : Superconductive and Low Temperature Particle Detectors, eds. G . Waysand and G. Chardin (Elsevier, Amsterdam, 1989). [6] D. McCammon, M. Juda, J. Zhang, S.S. Holt. R.L. Kelley, S.H. Moseley and A.E. Szymkowiak, Jpn. J. Appl. Phys. 26 (1987) suppl. 26-3. [7] D. McCammon, M. Juda, J. Zhang, R.L. Kelley, S.H. Moseley and A.E. Szymkowiak, IEEE Trans. Nucl. Sci. NS-33 (1986) 236. [8] D. McCammon, private communications. [9] A. Alessandrello, D.V . Camin, E. Fiorini and A. Giuliani. Phys. Lett. B20 2 (1988) 611 . [10] A. Alessandrello, D.V. Camin, E. Fiorini, A. Giuliani, M. Buraschi and G. Pignatel, Nucl. Instr . and Meth. A279 (1989) 142. [111 N. Coron et al., Thermal Spectrometry of Particles and y-Rays with Cooled Composite Bolometers of Mass up to 25 g, in: Superconductive and Low Temperature Particle Detectors, eds. G. Waysand and G. Chardin (Elsevier, Amsterdam, 19891. [12] A. Alessandrello, C. Brofferio, D.V. Camin, E. Fiorini and A. Giuliani, IEEE Trans . Nucl. Sci . NS-36 (1989) 141 . [2]

[1] G.F. Knoll, Radiation Detection and Measurements, 2:ßd edition (Wiley. 1989) p. 688.

V. GâNERAL PH V SICS