A high efficiency, low background neutron and gamma detector for cold fusion experiments

Nuclear Instruments and Methods in Physics Research A 35.5(1995) 609-617

NUCLEAR INSTRUMENTS &Mm IN PHYStCS T!?zZn

EL.SENIER

A high efficiency, low background neutron and gamma detector for cold fusion experiments B. Stella a,b,*f F. Celani ‘, M. Corradi b, F. Ferrarotto b, N. Iucci ‘, V. Milone b,t, A. Spallone ‘, G. Villoresi d a Dipartinento di Fisica “E. Amaldi” - Unioersith di Roma Ill, Italy h INFN Sezione di Roma I, at: Dipartimenio di Fisica Unioersit2 “La Sapienza “, P.le A. Moro 2 - 00185 Roma, Italy ’ Laboratori ~~io~ali INFN di Frascati, V.le E. Fermi - Fraxati, Roma, Italy ’ ht. Fisica Spazio Interplanetario - CNR Frascati, Roma, Italy Received 27 December 1993: revised form received 18 July 1994

Abstract The present apparatus (named by the acrostic “FERMI” also to celebrate the 60 yr of the discovery, by Enrico Fermi and collaborators at Rome University, of the effects of moderation of neutrons) is mainly a moderated neutron detector developed for the search of cold fusion events. It is based on 7 BF, and 2 3He proportional counters with detection efficiency for neutrons 40%-8% in the range 1 keV-20 MeV, pulse shape acquisition and good time resolution for neutron bursts; it also allows us to perform a good reconstruction of the average original neutron energy. The neutron background measured in the Gran Sasso INFN underground laboratory is about 0.09 Hz. Gamma rays are revealed mostly by a complementary low background NaI detector with 26% solid angle coverage. The performances are controlled by a full MC simulation, experimentally tested. A high multiplicity (up to - 100) neutrons’ event has been detected during background runs. The system is being upgraded by the detection and identification of charged hadrons.

1. lntr~uction Unambiguous detection of “nuclear ashes” (neutrons, protons, tritium and gamma rays, as well as 3He and ‘?ie) is fundamental in search for cold fusion. The present apparatus has been designed to detect possible neutrons and gamma’s (plus tritium off-line) coming from “cold” nuclear fusion phenomena inside it. To minimise the neutron background and the noise the detector has been placed inside the INFN Gran Sasso underground ~boratory at Assergi (AQ), where 1200 m of solid rock shield us from cosmic rays, reducing the neutron rate by about lo3 respect to normal external conditions. The name FERMI is an Italian acronym for “Electrochemical Fusion with Interdisciplinary dedicated Research” and refers to the collaboration of specialists in different branches (particle and nuclear physicists, chemists, material engineers), particularly needed in this field.

* Corresponding author. Tel. +39 6 49914344, fax +39 6 4957697, e-mail stella~romal.infn.it. * Deceased.

Relevant features of the apparatus are: high efficiencies, pulse shape acquisition and good time resolution (sensitivity to neutron bursts); it can also perform a good re~nstruction of the average original (before moderation) neutron energy by comparison with a full simulation. Neutrons are detected by seven BF, and two 3He proportional counters, after they have been thermalized in a big ~lyethylene structure (which acts both as a moderator and a mechanical support) covering about 99% of the total solid angle. Gamma rays are detected by one big NaI crystal placed inside at one end of the neutron detector near the experimental sample. A small efficiency for y’s is also achieved by the proportional counters. The electronics system is highly redundant and uses fast scalers, pulse height ADCs and Flash ADCs (FADC) separately for all counters, for a better check of spurious and systematic effects. A full automatic acquisition is performed for each “event”. We describe the expe~mental setup in Chapter 1 and the electronics and data acquisition system in Chapter 2. Detector characteristics and performances are explained in Chapter 3, including the experimental detection of a very high multiplicity neutrons event. Chapter 4 is about the

0168-9~2/95/$09.50 Q 1995 Elsevier Science B.V. Al1 rights reserved SSDI 0168-9002(94)01147-8

B. Stella er al. / Nucl.

instr.andMeth. in Phys.RexA 355 (199.7)609-617

100 cm

i

Fig. 1. Two cross sections of the apparatus: upper, longitudinal; lower, transverse. The star indicates the position of the sample.

Monte Carlo simulation and the reconstruction of neutron energy. In Chapter 5 we summarise the FERMI’s “virtues” and the future upgrades.

2. The detectors Two vertical (longitudinal and transverse) cross sections of the apparatus are shown in Fig. 1. The neutron detectors’ characteristics are listed in Table 1. The seven large (180 cm length * 15 cm 0) low pressure (0.33 atm) SF3 proportional counters are embedded inside a big (2 m length, 1 m large) structure of polyethylene moderator, in a compact geometry to optimise the

solid angle and reduce the characteristic diffusion times of neutrons. The counters are shielded from external neutrons with energy less than some MeV by an outer structure of polyethylene, acting also as a moderator-reflector for neutrons produced inside the geometry. This part of the apparatus has already been used to measure solar neutron fluxes [I] and neutron background inside the Gran Sasso laboratory 121. Signal readout on the BF, counters is made by wire readout through a fast charge preamplifier specifically designed by us; the inner cylindrical cathode is at a negative working voltage. For all neutron detectors the shaping times have been adjusted to about 1 psec and their responses have been equalised adjusting the amplified signal heights on the peak with a neutron source for each detector to give a signal of about 1 V. An external Al case with 20 cm diameter has been grounded to ensure both electromagnetic shielding and protection from high voltages. For detectors 5 and 7 (Fig. I) the gap between the cathode and the Al case has been filled by N 2 cm polyethylene grains to optimise the detector efficiency as a function of energy and allow in this way for a determination of the average original energy (see Chapter 4). In the inner hole of the moderator structure, two high pressure (3 atm) ‘He tubes (100 cm length *5 cm 0, see Table 1) are placed, one on each side of a longitudinal wheeled cart used to insert the experimental samples in the apparatus (Fig. 1). Two slides of polyethylene 2 cm thick are mounted on the cart in front of each ‘He to improve the thermaIisation of neutrons directed onto them; they contribute to have different energy responses at the various counter locations. The signal readout is made on the wire, with the same charge preamplifiers used for the BF, counters. The inner minimal polyethylene thickness in front of the detectors is 2 cm. Surrounding the detectors we have at least 6 cm of polyethylene to moderate also higher energy neutrons.

Characteristics of the proportiona counters. The sensitivity for an isotropic FWHM is obtained from neutron source measured spectra (Figs. 4b-Sbl

flux of thermal

neutrons

is as declared

Detector

RF,

%e

Type Diameter [cm] Sensitive length [cm] Gas Pressure [cm Hg] Working voltage [VI Sensitivity [cps/nvI Resolution (FWHM) [%I Intrinsic Background [Hz] Pulse rise time [(us]

BP28 Chalk River 15 180 BF 196% “‘B) 20s - 2850 165 IO ? x IO-’ 0.5-2.5

Centronics 5 100 ‘He 278 - 1x90 433 11 5 x10-a 0.5-1.0

by each firm. The

B. Stella et al. / Nucl, Instr. and Meth. in Phys. Res. A 355 (I 995) 609-617

Gamma rays are mostly revealed by a cy~indrica1 NaI crystal (5 in. diameter, 5 in. thickness), covering 26% of the solid angle, equipped with a special low noise photomultiplier and amplifier. It is placed close to the center of the neutron detector, inside its axial hole (Fig. 1). The polyethylene structure is further encased on its side and bottom faces by a “box” made by 5 cm thick low radioactivity lead blocks used mostly for better shielding the NaI detector from the external y background: in this way we obtain a 50% back~ound rate reduction.

3. Electronics and data a~q~isjtion (DAQI The expected (or found) peculiarities of neutron emission in cold fusion phenomena require a dedicated acquisition electronics and event trigger logic. It is possible that the neutron emission occurs in a very short time (a “burst” lasting a few microseconds) 131 and each burst could produce up to hundred neutrons of a few MeV energy. Because of the moderating material (polyethylene) in our detector, the therma~isation time for these neutrons can require from tenths to hundreds of microseconds. (Neutrons’ thermalization time results to be 7 = 180 bs in gross average from our Monte Carlo simulations of the complicated moderators.) Moreover, it is of course very important to distinguish between electromagnetic disturbances and true signals, and among them (neutrons, alphas, gamma rays), by Looking at the signal shape. Fig. 2 shows an example of the pulse shape acquisition of a neutron signal from BF, detectors. An example of a high multiplicity event will be displayed in Fig. 7. 3.1, Electronics In our apparatus each detector, after preamplification, is acquired by: 1) a 12 bit peak sensing ADC (LeCroy 2259B); 2) a 8 bit, 2 KByte memory, flash ADC (FADC, CAEN c194); 3) a fast scalar counter (IO0 MHz-ungated). The electronic block scheme, for all detectors, is reported in Fig. 3a, white in Fig. 3b the trigger logic detail for a single detector is shown. The FADC makes available to each detector a memory of 2048 bytes: an internal selectable clock f4 MHz in our selection) cycling acquires the signals and writes to this memory. It is possible, in this way, to define the ranging time of this memory: in our case we have 512 ks available (i.e. 0.25 p,s per memory step). ff no external trigger (event-triggers stops this clock, the memory is cycling, over-writing itself. it is possible, also, to select the number of sample steps, after the event-trigger, to stop the memory over-writing: we divided the 512 p.s full memory time by 16 ps before and 496 ps after the event-trigger signal.

611

3.2. Trigger logic The trigger logic, developed to recognise and acquire a burst neutron event, is specific for this apparatus. We can separate this logic in two parts: a hardware eiectronics assembly to identify a burst and a software acquisition control to select and acquire data. When a signal (discriminated at a threshold of 400 mV, just at the lower end of the wall effect, far away from the noise) comes from a detector at any time, it is counted by the SCALER and converted by the ADC and FADC. This shaped signal enables the TIMER (no retriggerable 500 ps monostable: “gate”) which enables the ADC conversion and sends to the FADC an event trigger signal to stop the delayed cycling memory. After this gate the TIMER is disabled by the OUTPUT REGISTER module controlled by the computer (to avoid restarts if the burst lasts more than 500 p,s) and allows the acquisition to complete the cycle. If other signals occur (within 500 ps after the first one) they are counted by a main scaler (M-SCALER) which gives the multiplicity of the event. A logic AND module assures that the M-SCALER counts only those signals occurring in a time window of 500 (LS. It is not relevant whether the signals come from one or more detectors because they are counted by the M-SCALER throughout a logic OR module (GLOBAL OR) connected to all neutron detectors. The program acquisition is based on a CAMAC Look At Me (LAM) waiting loop (Fig. 3~). After a LAM occurred, the OUT_REGISTER is set to disable the TIMER. The ADC pulse height (200 ns conversion time) of the first signal is read in and recorded and the program waits at least 500 ps to allow the M SCALER to count the following signals. A software mult~li~ity threshold for the burst registration can be set to a suitable value II, typically n = 2.

Fig. 2. Typical FADC

display of a BF, neutron pulse; horizontal

scale: 250 ns per channel.

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3. Stella et al. / Nucl. Instr. and Meth. in Phys. Res A 355 f19951 609-617

CAMAC

BUS

Fig. 3. (a) Global scheme of the electronics.

(b) Trigger logic for one channel. (c) Scheme of the CAMAC data acquisition

loop.

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After that the M-SCALER is acquired (and reset) and we distinguish two cases: 1) M_SCALER < n: not enough neutron signals occurred and we are not interested to acquire the FADC memory (to save time); 2) M-SCALER 2 n: a burst occurred and the FADC memory is acquired (300 ms needed) and recorded in a RAM buffer array. Then all detectors’ SCALERS are acquired (and reset). If a burst lasts more than 500 ps, the shapes of the remaining signals are not recorded, but we can know how many signals occurred during this long acquisition time. The DAQ loop restarts setting on the OUTPUT_REGISTER to enable the TIMER and synchronise the electronics read-out to the acquisition program. This logic was tested by background events with singles or multiples, and using an electronic pulse generator to simulate a single or a burst event. A physical burst event was detected during a background run (see Chapter 3) and it can be considered as a real test of the apparatus’ function. Due to the separate acquisition for each detector and the pulse shape acquisition, we can easily identify radio frequency disturbances, amplifier oscillations or discharges due to humidity or powder, and exclude these events (or, if needed, one particular detector) from the analysis. Pulse height measurement of the y detector is made by a separate multichannel ADC, giving only the spectrum per every run. The NaI signal is also sent to a FADC separately on a tenth channel using a time window of 2 ms and a clock of 1 MHz, beginning the event sampling 1 ms before the neutron “start” (this is one of the features of our FADCsl. This enables us to check eventual prompt gamma emissions correfated with neutrons (observed iater due to the moderation time). As DAQ processor we have used a Macintosh II interfaced to a MacCC crate controller via a MICRON card. The data acquisition program has been developed with MacUAl environment and written in Real Time Fortran [4]. For speed reasons, all CAMAC module readout has been done in a non-standard way, through direct memorymapped access to CAMAC bus. The electronics and DAQ readout reliability and stability have been tested in extensive data taking runs over one year of measurements. Background and neutron source rates are completely reproducible within the statistical errors.

Table 2 Background rates and efficiencies for each neutron detector. The f, background rate is obtained by using the full energy spectrum (from the counting rates on the scalers). The fz rate is obtained from the ADC pulse height spectra by integrating only the zone under the calibrated neutron peak (95% neutron acceptance). The efficiency is a MC evaluation for 2.45 MeV neutrons

hHz1

Detector

f,

BF, 1 Be; 2

12.9 + 0.3 2x9+0.4 45.0 + 0.6 25.5 + 0.4 22.2 + 0.4 25.3 i 0.4 11.8*0.3 4.9 f 0.2 4.9+0.2 177.3 + 1.1

BF, 3 BF, 4 BF, 5 BF, 6 BF, 7 ‘He 1 “He2 Total

E * 102

fi Hz1 sLi+o.2

2.28 3.14 3.02 2.23 2.23 3.88 2.24 2.17 2.78 24.7

10.6 + 0.3 21.2t0.4 11.3io.3 18.7 f 0.4 15.1+0.3 5.9rtO.2 0.58 + 0.06 0.39 + 0.05 89.4 + 0.8

above the discrim~ator threshold (Fig. 4a); this may be reduced to * %I mHz if we consider only the part of the spectra lying under the “neutron peak” as calibrated by an Am-Be neutron source (Fig. 4b). The BF, background spectra of the ADC are nearly totally due to the a activity of the detector materials, giving a broad pulse height distribution. The 3He spectra (Figs. 5a and 5b) show that, in this case, the (Y background goes almost completely in overflow in the ADC and is well separated from the n-source peak. For background events the number of events with multiplicity > 1 is small (typically one event with two signals on the “master scaler” per SOOOOs) and follows, as expected, a Poisson distribution due to uncorrelated

counts 1000.~

4. Detector performances.

The “super-event”

Details for the single neutron detectors’ background rates and efficiencies are given in Table 2. The total neutron background rate measured by the full apparatus in the Gran Sasso INFN underground laboratory amounts to - 0.18 Hz if we consider all the signal area

500” 0 500

1000

ADC Channels ADC for: (a) run by an Am-Be

Fig. 4. Typical BF, spectrum on the peak sensing background run (152710 neutron source.

s); (b) calibration

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B. Stella et al. / Nucl. Instr. and Meth. in Phys. Nes. A 355 (199.5) 609-617

500-

Fig. 5. Typical ‘He spectrum on the peak sensing ADC for: (a) background run (152710 s), with mostly saturated pulses: (b) calibration run by an Am-Be neutron source.

events from the intrinsic detector activity. However an unexpected very high multiplicity event (observed multiplicity = 16), called “superevent”, probably due to cosmic ray, has been detected during some week of background monitoring. The use of thermal neutron detectors gives us a high detection efficiency in a wide range of energies, as needed to explore a variety of nuclear phenomena: it ranges from 40% at 100 keV to 8% at 20 MeV (Fig. 61, being about 25% at 2.5 MeV (i.e. the energy of neutrons from deuteron-deuteron fusion). We cannot reconstruct the energy of the individual emitted neutrons, but we have achieved a determination of the average energy of them by comparing the counting rates of detectors placed by purpose behind different moderator thicknesses, by means of a careful detailed simulation of the apparatus and of the moderation by Monte Carlo method, as we will show in the next section.

0.1

10-l

100

101

E [Mev] Fig. 6. Total apparatus efficiency as a function of neutron energy, as obtained by detailed Monte Carlo calculations.

Time

[bs]

Fig. 7. FADC display, for the 9 detectors, of the high multiplicity “super-event” revealed in the apparatus. The first pulse on BF, #S (and cross talk on SF3 #4, BF, #6 + #7 and ‘He # 1) is not a neutron signal, whiie the following 12 pulses (shaded) are characteristic of neutron capture in detectors. The small pulse on ‘He# I at about 280 p,s corresponds to a y signal.

The full signal shape acquisition provided by FADCs enables us to monitor very finely the detector response in all registered events. It also allows an improved discrimination between y, neutron and a signals. Due to the signal shaping and the separate channel readout, the FADCs enable the observation of “burst” events with a time resolution of 6 (BF,) and 4 (“He) ps (due to the preamplifier shaping time) for two neutron signals on the same detector and only the FADC clock timing (250 ns) for signals on different detectors. How well we can record such burst events is shown by the registration of the background “super-event” with 12 neutron signals observed within 500 ys in our 9 detectors (Fig. 7). The start signal (BF, #5) in this case was not a neutron. The tota multiplicity of this event is 16, due to the 12 neutrons detected during the FADC gate, to the start signal with very high pulse height (and some cross talk) and to other 3 signals (likely to be neutrons) registered on the scalers after the 500 t.~s gate, during the FADC readout

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B. Stella et al. / Nucl. Instr. and Meth. in Phys. Res. A 355 (1995) 609-617

1

2

3

4

5

MeV

Fig. 8. Typical y rays background rate and spectrum detected by our NaI crystal in our laboratory in the Gran Sasso tunnel.

(two on BF, #2 and one on BF, #3). A few gammas from radiative capture of neutrons (hardly visible in Fig. 7) have also been detected by the proportional counters in the same time interval. With the large available information content, our preferred interpretation of such event is a hadronic shower, initiated by a high energy muon, developed inside the detector material, or a muon bundle, accompanied by many (N 100) neutrons (with an inelastic interaction as a start). Stand-alone gamma detection is made through the peak sensing spectra, recorded on a VARRO multichannel analyser, by our NaI detector. In Fig. 8 we can see that, for typical natural radioactivity lines in background runs (about 5 h), our sensitivity (in a similar integration period) ranges from _ lo-’ Hz/60 keV at about 300 keV to better than 10e4 Hz/60 keV in the region 3 MeV-20 MeV. The typical resolution we obtain is crw 150 keV, almost constant over a large range of energies. (We normally neglect the detection of y rays by proportional counters). Gamma detection by NaI correlated with neutron emission is registered separately through a dedicated FADC channel, as explained in the previous section.

We have compared with the Monte Carlo expectations: 1) the sensitivity to thermal neutrons (as declared by the respective firms) for both detectors; 2) the efficiency for neutrons coming from Am-Be and Cf 2s2 sources, with known activities of (266 + 1O)lO’ s- ’ and (259 f 9)lO’ s- ’ respectively, placed inside the apparatus; 3) the measured (by TDC) time distribution between two separate neutrons detected on the same counter from a Cf’52 source (Fig. 9). Experimental results are in good agreement with the simulation, except for the 3He sensitivity to thermal neutrons: the MC calculated value is lower by 30% respect to the one given by Centronics, but complies well with values for similar detectors given by Reuter-Stokes. The total efficiency to calibrated sources is comparable with MC evaluation within the experimental errors. The counting share for each detector is always within 5% of MC value. The MC estimate of neutrons’ die-away time in the apparatus is T w 180 p,s, supporting our choice for a 500 ps time gate acquisition. The polyethylene thicknesses in front of each neutron detector (including grains in the cylindrical gap of counters 5 and 7) have been optimised to obtain the maximal efficiency and energy dependence of it. A statistical reconstruction of neutron energy is performed in the following way. The 9 counters are summed in 5 groups according to similar material thickness in front of them; the 5 measured counting rates rI (i = 1 . . .5) are compared to the energy dependent efficiency of each group e;(E) obtained by

9

\$ E! ; a 111 B LI 2

*lo

-4 10 8” 7 6

9

5

5 z

4

2

3

;

5. MC simulation and reconstruction

of neutron energy

P

2

0

250

500

750 T2-Tl

The detector has been fully simulated by Monte Carlo method (using the MCNP code [5]), including the moderation and interaction of neutrons. Comparisons with various experimental data have been used to tune and improve the reliability of the simulation, which then has been used to compute the efficiency and to optimise various detector parameters.

1000

1250

1500

[ps]

Fig. 9. Distribution of the time elapsed between two consecutive detected neutron signals on the same BF, detector from a Cf”’ source, as measured by a multichannel TDC (histogram) and from MC simulation (diamonds). Both superimposed fits (solid line for data, dashed for MC) are the sum of two exponentials. The distributions are normalised to the same area, corresponding to detector efficiency.

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0.5

Source

ndutron energy [F&V]

Fig. 10. Energy dependence of the neutron efficiency for five groups of detectors, as calculated by Monte Carlo. The different behaviours are exploited to reconstruct the average energy of the detected neutrons.

The statistical resolution on the reconstructed energy was obtained by applying this method to the five counting rates obtained in MC simulations with monoenergetic sources. The distributions of reconstructed energy for different source energies are shown in Fig. 11. The resulting statistical resolution at 2.5 MeV, is u,/E = (11 f l)/@’ (N = total number of detected neutrons), that is uTE/E = 40% with 7.50, u,/E = 20% with 3025 and uE/E = 10% with 12000 detected neutrons. Obviously, in real experiments with background subtraction, the resolution worsens. Considering the much higher efficiency f = 1000 times) of our system compared with those presently used in spectrometric (coincidence time of flight) measurements [6], for the same neutron emission FERMI has a better resolution for the average energy.

6. Conclusions

cubic spiine interpolation between IO). The log-likelihood function

MC calculations

(Fig.

(where rtot and E&E) are the total counting rate and the total MC efficiency respectively) can be minimised with respect to the parameter E, giving the reconstructed energy.

To summarise, FERMI’s “virtues” are: 1) high neutron efficiency (up to 40%) in a wide energy range; 2) sufficientIy good resolution on the average neutron energy and on y rays; 3) low background (0.09 n/s in the Gran Sasso tunnel); 4) good time resolution and sensitivity to neutrons bursts; 51 multihit feature (events with up to 12 neutrons within 500 ps or 16 within 1000 ps have been observed); 6) redundancy; 7) full pulse shape acquisition; 8) full and reliable MC simulation (experimentally checked); 9) parallel detection of y’s and other nuclear products in the axial gap. As for upgrades, to enlarge the sensitivity to nuclear products of d-d fusion, we are implementing a system for the detection and identification of a few MeV charged hadrons, possibly coming out of a strictly adjacent Pd electrode. The present detector has been used up to now in two different experiments [7].

References

Fig. 11. Reconstruction of average energy for three different source neutron energies (1, 2.5, 5 MeV from top to bottom). The number of entries and the average reconstructed energy with its root mean square are shown on each histogram. Each entry corresponds to the reconstructed energy for a single MC run of 5000 emitted neutrons (N - 1600, - 1250, - 950 detected neutrons respectively).

[I] N. Iucci, F. Signoretti and G. Villoresi, Proc. Workshop on Solar Neutrons, Moscow (1987) p. 190. [Z] P. Beili et al., Nuovo Cimento 1OlA (1989) 959. [3] De Ninno et al., Europhys. Lett. 9 (1989) 221; H.G. Menlove et al., AIP Conf. Proc. 228 (1990) p. 287; and H.O. Menlove and M.C. Miller, Nucl. Ins&. and Meth. A 299 (1990) 10. [4] Ma&Al 5.0 - March 1990 - UAl internal note; H. Van der Schmitt: “RTF/68K - Real Time FORTRAN”UAl. [5] J.F. Briesmeister (ed.1, MCNP-A General Monte Carlo Code for Neutron and Photon Transport, version 3B, Monte Carlo

B. Stella et al. / Nucl. Instr. and Meth. in Phys. Rm A 355 (I 995) 609417 Section, Los Alamos Radiation Transport group. Distributed by Radiation Shielding Information Centre (RSIC), P.O. Box X, Oak Ridge, TN USA 37831. [6] T. Bressani et al., Nuovo Cimento 104A (1992) 1413. [7] B. Stella et al., Proc. 3rd ht. Conf. on Cold Fusion, Nagoya,

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Japan, 21-25 Oct. 1992, Frontiers of cold fusion (Univ. Acad. Press) pp. 503-506; B. Stella et al., ibid pp. 437-440; M. Alessio et al., Proc. Rome Workshop on the Status of Cold Fusion in Italy, Rome, 14-16 Feb. 1993, ed. B. Stella, pp. 93-109, and M. Corradi et al., ibid. pp. 110-125.