Time and space resolved neutron diagnostic systems for magnetically confined plasmas

Nuclear Instruments and Methods215(1083)443 452 North-Holhmd Publishing Company

TIME

AND SPACE RESOI,VED

MAGNETICAI.I.Y I).R.

CONFINED

SI.AUGIITER.

443

NEUTRON PLASMAS

H.S. S P R A ( ' K L E N

DIAGNOSTIC

SYSTEMS

FOR

*

a n d R. D F L V A S T O

(:ntrcr~ttl oJ (al(/ornia, Lawrence I.irernl.re Naltcmal l.ahoratorv, lat'erm~Jre. ('/| 94550, LS,4

Received 25 March 1993

~entron spectrometer/counter s~'stenls ha',e been developed for diagnostic ineasurements on nlagneticall} c,onfine,,t deuteriunl

and deuterium-tritium plasmas. An earl 5 version ',,,as used to determine the mean ion energy as ,*ell as the ion-energ> confinement time in the endplugs of the Tandem Mirror Experiment. A recent upgrade includes a 10 MHz pulse height anal',sis and particle tdentffication capability for time- and space-resolved neutron spectrum measuremems. Resolution is adequate for at neutron Doppler-width measurement of mean ion energy in near-future confinement experiments such as the Mirror Fusion Test Facilit',,.

I. Introduction Neutron generation rates due to deuterium and deu t e r i u m - t r i t i u m fusion are abundant in recent and pl'anned experiments. As a result, it is now possible to apply spectroscopic and collimation techniques used in nuclear physics to neutron diagnostic measurements in advanced fusion experiments. This is important and timely since improved ion confinement and increased density arc expected to reduce the escape of ions and charged reaction products from the central plasma in these devices. Also, the nearly complete ionization of most impurities in the central plasma is expected to reduce the characteristic photon radiation. As a result. neutron diagnostic techniques are expected to complement or replace some of the more traditional diagnostics in the determination of central plasma parameters. Several kinds of neutron measurements have been studied at the l.awrence l.ivermore National Laboratory ( L L N L ) as potentially useful diagnostics for hot magnetically confined deuterium and d e u t e r i u m - t r i t i u m plasma. Candidate diagnostics have included yield measurements [I]. spatially resolved methods [2]. and neutron spectroscopy [2,3]. As an early result of these ,studies. a prototype neutron c o u n t e r / s p e c t r o m e t e r was installed near the endplug plasmas of the T a n d e m Mirror | ' x p e r i m e n t (TMX) at I.I.NL for evaluation. Experience gained on T M X led to the development of a more advanced collimated spectrometer which is capable of time-resolved neutron spectrum measuremenls simultaneously on several narrow spatial fields of " Work performed under the auspices of the U.S. Department

of Energy by l.awrence givermore National Laboratory under contract -,=W-7405-[:.ng-4g. 0167-5087/83/000()

0000/$03.00 ": 1983 North-Holland

viev.. Energy resolution obtained in this instrument is adequate to determine the Doppler-broadening in the neutron spectrum due to ion motion, to sinmhaneouslv determine the mean ion energy m several plasma volumes with - 10 cm spatial resolution, and to make these measurements with time resolution as good as I(R) ms. This instrument ,,,,ill be used for mean ion-energy and ion energy confinement time measurements in the Mirror Fusion Test Facility ( M F I ' F ) at LI.NI.. ] h i s paper describes the design and use of these systems as diagnostic probes of current magnetic fusion experiments. The principal focus will be on diagnostics for mirror-confined deuterium plasmas and application of some of these instruments on TMX will also be described.

2. Plasma reactivity measurements Deuterium and d e u t e r i u m - t r i t i m n plasmas generate neutrons upon fusion so that neutron yield data may serve as an indication of the temporal behavior of the fusion rate. Neutron .',/ield from a deulerium plasma whose density profile is axially symmetric with radial scale length R, axial scale length 1., and whose speed distribution is spatially homogeneous is given by:

y,, = ! f, 2( r. : ) ( o l : ) d 3 r = ~
n(r.:)dr

(1)

where ( o c ) is the D(d, n) reactivity, .f*~n(r, : )dr the line density, and YR, Y/ are radial and axial geometry factors which depend on the form of the plasma density

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Combining tile measured neutron yield with plasma line density and density profile measurements the reactivity. ( o r ) , may be determined from eq. (I). Reactivity is a well known and very sensitive plasma thermometer. l:ig. I shows the relation between reactivity and mean ion energy for a Maxwellian speed distribution 14]. Measuring the plasma reactivity is a u,ell established method for determining mean ion energy [5- 15].

f~ n ( r ) d r

profile. Table 1 gives values of YR and Y/ for several plausible profiles. The important feature to notice in the table is the relatively weak sensitivity of 7 to variations in the functional form of the plasma density profile. This allows the volume integral to be replaced, approximately by 7 z L and the square of the radial line integral. The radial line d e n s i t y . . [ ~ n ( r ) d r may be determined experinaentally from microwave interferometry or neutral beam attenuation. Neutron yield may be estimated experimentally from a point measurement using eq. (2) below assuming that the detector is several plasma scale lengths from the source and that the yield is relatively isotropic. 4rrR 2 e-"x E,(t) ...... .,V(t),

(2)

where N = neutron detection rate, R = distance from the monitor to the center of the plasma, x, = neutron attenuation coefficient in diagnostic window. X = diagnostic window thickness, t = neutron detection efficiency (counts per n / c m : ) , -q = neutron scattering correction factor (ratio of total neutron detection rate to virgin neutron detection rate)•

3. Time resohed reactivit?,measurement An earl_,,', uncollimaled neutron yield monitor was employed on the endplugs of TMX for evaluation. T M X confined high temperature deuterimn plasma in a long solenoid [16] v,.ith two "'mirror-confined'" plasnlas acting as "'endplugs'" to reduce escape of ions along the magnetic axis. The " c n d p l u g plasmas" were heated, and usually fueled, by trapping reJected neutral deuterium at energies of 3 40 keV. Injection rates were equivalent to 75-250 A per endplug. Deuterium fusion rates in the "IMX endplug plasmas were sufficiently high to allow time-resolved measuremenls of the 2.5 MeV neutron generation. Neutroq measurements w e r e made near tile east a n d ".aCSl e n d plugs of T M X using instruments shown schematically in fig. 2 and described in some detail elsewhere [1]. A schematic of TMX and the location of ttle neutron detectors is shown m fig. 3. Insets in tile figure show typical traces of neutron yield vs time v.here the plasma reaches stead,,' state during neutral beam injection, then decays upon termination of tile beams. A typical plasma had a duration of - 25 ms and neutron generation rate 10 'a 10 H n / s . The data acquisition electronics used commercially a,,ailable modules as indicated in the block

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c o n t r i b u t i o n of scattered n e u t r o n s to the m e a s u r e m e n t on T M X was d e t e r m i n e d f r o m a n e u t r o n s p e c t r u m m e a s u r e m e n t m a d e at the diagnostic location using the

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d e u t e r i u m p l a s m a as a source. ] ' h e ability to d e t e r m i n e this correction factor in situ w i t h o u t resorting to installation of a s o u r c e w i t h i n the v a c u u m vessel is one of the a s s e t s of this i n s t r u m e n t . T i m e resolved n e u t r o n yield m e a s u r e m e n t s were used. together with line densily m e a s u r e m e n t s [17] to p r o v i d e reactivity a n d m e a n ion energy data [18]. Results of these m e a s u r e m e n t s indicate that fusion reactivity in the T M X e n d p l u g s is c o m p a r a b l e to that expected in d e u t e r i u m p l a s m a at m e a n ion energies in the range 3 - 1 5 keV. T h e highest reactivities c o r r e s p o n d to m e a n ion energies a p p r o x i m a t e l y the s a m e as the m e a n energy of the injected neutrals. M e a n ion energies derived from m e a s u r e d reactivity are in a g r e e m e n t with o t h e r energy m e a s u r e m e n t s based on p l a s m a d i a m a g -

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lletisnl a n d v, ilrl e s t i m a t e s b a s e d on p o w e r b a l a n c e c a l c u l a t i o n s . Ion e n e r g y is o b s e r x e d to r e m a i n app r o x i m a t e l y c o n s t a n t d u r i n g n e u t r a l b e a m injection. t h e n il decaL',s foiler, r a g b e a m t e r m i n a t i o n . "rhc time c o n s t a n t for ion e n e r g y decay. % . is a n i n l p o r t a n t p l a s l n a p a r a m e t e r . . M a n \ , deuteritinl p l a s m a s are b o t h h e a t e d a n d fueled bv d e u t e r i u m n e u t r a l b e a m m.iection. W h e , i becIm injection is t e r m i n a t e d , the d e n s i t y deca~,s ~ ith time c o n s t a n t rp a n d the nle;.in energ,< d e c a \ s with t i m e c o n s t a n t r t . Both effects r e d u c e the n e u t r o n x ield so that it d e c a y s with time c o n s t a n t %. Since tile reacti,,it\ scales as (see fig. I1

.~<,,. ) - c<>nst(

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(3)

o~er a n a r r o ~ e n e r g y r a n g e a n d yield scales as d e n s i t \

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T h e c o n s t a n t of is the slope o b t a i n e d f r o m fig. 1 for the a p p r o p r i a t e r e a c t i o n at lhc p r o p e r energ',.. I"or tieu t e r i u m p l a s m a s o f c u r r e n t interest, ¢~ = 3 so that the n e u t r o n d e c a y time % is a sensitive m e a s u r e of % ill t i l e a b s e n c e of h e a t i n g a n d fueling. T h e p r i n c i p a l l i m i t a t i o n o f this t e c h n i q u e is that the m e a s u r e m e n t m u s t be m a d e w h e n the p l a s m a is not in e q u i l i b r i u m . D e c a y of n e u t r o n vield in the T M X east e i l d p l u g folh)wing the t e r m i n a t i o n of n e u t r a l b e a m h e a t i n g is s h o w n in fig. 5. A p p l i c a t i o n of eq. (4) to t h e s e d a t a i n d i c a t e rl: - 3.5 m s i m m e d i a t e l ) following p l a s m a eqt, ilibrium. T h i s is c o n s i s t e n t v.'ith v a l u e s inferred f r o m p o w e r b a l a n c e c a l c u l a t i o n s [18].

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hm nlotion causes a l)oppler shift in the neutron emission spectrum. If the ion speed distribution were isotropic and Maxwellian in fornl there would also be a dispersion, or "'Doppler width", in tile neutron spectrum which is substantially larger and related directl',, to the ion temperature [6 8] by eq. (5) below. / 16 In 2m.F~r,T,

1013

25 26 27 28 29 30 t (ms)

Fig. 5. l)eca', o f neutron generation rate irl the cast endplug following l e r m i l l a l i o l l o f lleUlrtll beam injection at [ = 25 ms. l h c dcca), of plasma line density is shown for comparison. I.inc densiI', is indicated b,, ( × ) and neutron yield by (I).

in "I'MX from n e u t r o n D o p -

,,E,, = V

/.pl rl ~- 1~--i'"

= 67.4 keV ,t:', = 144.5 keV

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n).

(5)

] ) R+ .~'/uNs,'Jllcr ¢'I a/.

~ h c r c 1-, and 7; arc tile m e a n ion energy, a n d t e m p e r a ture ill keV. m,, and .91,, are the masses o f the neulr(m a n d reaction p r o d u c t ~l-le o r a l l e . and J / : , , is the full-width-at-half-nmximun~ (f`*hm) of the n e u t r o n spectral peak ttt e n t r y 3 1:,,. F.xperinlental m c a s u r e r n c n t s of n e u t r o n energ!, spectra have been m a d e before [9--15. 19-221 in o r d e r to v e r i f \ the thermonL,clear origin of n e u t r o n s generated in plasmas. Five of these e x p e r i m e n t s c o n f i r m e d the therm o n u c l e a r origin (11"the n e u t r o n s and r e p o r t e d D o p p l e r `*idth m e a s u r e m e n t s [12.14 15. 19.221.1to`*ever. a l l b u t o n e [221 ,*ere inert,ally c o n f i n e d p l a s m a s where ifigh resolution n e u t r o n s p e c t r a could be o b t a i n e d h \ timeof-flight techniques. M a n y of these tneasurenlent~, sho,*ed that thc o b s e r v e d n e u t r o n g e n e r a t i o n ~ a s not d u e to t h e r t n o n u c l e a r fusion [9-. l l.13l. -

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N e u t r o n energy, spectra `*ere LU..'CUnlLI]IIIcd o n "]~]X for both east and `*cst e n d p l u g p l a s m a s using tile s',,ten1 illustrated in figs. 2 - 4 . Relativel,, Io`* courH rates `*ere o b s e r v e d d u r i n g these runs so that tile stringent statistical r e q u i r e m e n t s for spectral u n f o l d i n g ~ e r c met only sifter pulse height spectra ~ e r e accumtilatcd o'~er 30 discharges. t h e results sire s h o w n in fig. 6. Tile e x p e r i m e n t a l n e u t r o n peak width is ob~,er,.ed to he 211 keV in the west plug and 322 keV in the east. l)ata in tile figure also sho'0, s a s,.2condar\ peak tit about 2.0 Me\" which is an artifact of tile L,r|folding m e t h o d used [231.

I n s t r u m e n t line ~ i d t h u n d e r these c o n d i t i o n s ~ a s d e t e r m i n e d at tile LI.N [. rotating target n e u t r o n source [2] alld ',~.as found to he all: ,n.~ - 160 201/ keV at 2.5 Me\". T h e r e has been no s,,stenlatic c o n f i r m a t i o n of that line width in the I M X e n v i r o n m e n t . Hm~e',er. the smallest line `*idth o b s e r v e d on 'FMX d u r i n g Iov, 3ield. high density shots wa.s in tile range 160 21.10 keV. indic a t i n g that tile illstrunlent p e r f o r m a n c e was not degraded. l h e energy, d i s p e r s i o n o f I M X n e u t r o n emi,,sion `*as d e t e r m i n e d , a s s u m i n g it to bc u n c o r r e l a t e d v.ith intrinsic i n s t r u m e n t dispersion, by eq. (6).

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F]tJclualJons in the spectra are due to sialislical noise which i~, amplified by the derivative unfolding method used. The ele',ated intensitv at 2.0 MeV ix an artifact of the unfolding method in which neutron scattering from carbon ill the spectrometer is not compensated. (at East endplug. (b) West endplug.

A/:',.,t - intrinsic i n s t r u m e n t lilac ~ i d t h . A E . , p - line width m m e a s u r e d spectrum. T h e data in fig. 6 indicate a mean ion energ) in the east c n d p l u g / 7 , - 10 13 ke\" (after correction for a n i s o t r o p y in tile speed distribution} while the energ?, in the v.eM e n d p l u g ~ a s too Io`* to be d e t e r m i n e d from these data ( t ; ~ 4 keV). It is not vet certain that eq. 151. deri~ed for X.lax~ellian plasmas, may be applied to the some`*hat a n i s o t r o p i c a n d n o n - M a x ~ e l l i a n mirror c o n f i n e d plasma, but tile o b s e r v e d width in tile cast e n d p l u g s p e c t r u m was c o n s i s t e n t with the highest reacti,.it\ observed during the 30 plasnlsis over `*hich tile data ,,,,six acCUlllul:.lted a n d ~,~,ith earlier charge e x c h a n g e measurem e n t s on 2XIIB u n d e r similar c o n d i t i o n s [24]. It is also in a g r e e m e n t with m e a s u r e d disunagnetism. l h e fact that the m e a n energ 3 d e t e r m i n e d this way agrees with the highest m e a s u r e d reactivities is e x p e c t e d since those p l a s m a s c o n t r i b u t e heavily to tile a c c u m u l a t e d data while cooler plilsnlas hap.e a reduced n e u l r o n \ i e l d ,lilt] c o n t r i b u t e IllUC]l less to tile accunlulated data. For tile siitllc r,2asotl, if the ion energy ",,,ere spatiall?, irdlornogeneous, the n e u t r o n D o p p l e r - w i d t h `*ouh.t be r e p r e s e n t a tive o f the hottest plasma.

S. r i m e resolved sptut'troscop) N e u t r o n s p e c t r u m r n e a s t t r e m e n t s it1 t h e paM have not been tlulle resolved due to tile relatively, l o w COtlnt

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Fig. 7. Diagram of the high rate neutron spectrometer. A low gain PMT is used to avoid count-rate-induced drift. The detector and amplifier are in a collimated enclosure to provide spatially resolved measurements. Optical b,olation via a fibre-optic link prc'.ent~, ground loops and transient coupling bet,a,een the experiment and the data acquisuion sv.,tem. Both neutron and gamma ray spectra ma', be acquired simuhaneously.

rates observed and to data rate limitations in tile electronics. Experiments tinder construction such as MI-:II: [25] are expected to generate much larger neutron intensities due to their larger plasma mass and higher ion energy. In anticipation of this. a high data rate pulse h e i g h t / p u l s e shape discrimination system has been developed [26] which allows pulse height spectra with good statistics to be acquired in intervals as short as 50 100 ms. The pulse height analysis part of this instrument resembles one developed for T F R [27]. and another design reported more recently [28] uses the same components. However, our instrument provides two additional functions which are critical in this application. It identifies proton and electron interactions at the maxim u m data rate using pulse shape discrimination in a N['213 organic scintillator, and it provides a fast highgain pre-amplifier in the front end so that a low gain photomultiplier may be used with the detector. The latter feature is i m p o r t a n t since dynode heating and fatigue in a high gain p h o t o m u h i p l i e r result in significant gain instability at high data rates. While this ins t r u m e n t does not have the good energy resolution observed in the time-of-flight system developed for the Joint European Torus (JET) [29] it incorporates recent inlprovements in scintillator resolution by using banded light pipes [30] and is able to acquire spectra during very short time intervals. Finally. this system, whose architecture is shown in fig. 7, is entirely computer controlled. Adjustment of gain, bias. sample intervals a n d status checking is directed by digital controls from a local LSI-II microcomputer. All system parameters are then recorded with the data from each shot. An example of a pulse height spectrum obtained at

tile LLNL Rotating Target Neutron Source (RTNS)[31] and tile corresponding unfolded neutron spectrum is shown in fig. 8. Tile data shown were acquired in an interval of about 200 ms at a total (neutron plus gamma) count rate of 5 MHz. It is expected that this system will be useful in making time-resolved measurements of neutron Doppler width and ion energy during plasma build-up and decay. Fig. 8a shows the raw pulse height data. The inherent differential linearity of the pulse height digitizer is very poor [26,28] and is the source of the a p p a r e n t noise in the spectrum, ltowever, the linearity characteristic is stable and may be corrected for [26,28]. The corrected spectrum is shown in fig. 8b. Pulse height data are then unfolded to obtain the neutron spectrum shown in fig. 8c using the FLYSPE(" data reduction program [23]. Examination of the figure shows that the spectrometer energy resolution at the m a x i m u m data rate is degraded to about 250 keV at 2.45 MeV. In addition the spectrum is distorted at low energies by flaws in the linearity correction. Nevertheless. these preliminary data support our expectation that useful doppler width measurements may be obtained with this instrument in 50 100 ms for M F T F plasmas where 1/, > 10 keV.

6. Spatially

resolvedspectroscopy

The spectrometer system described above is housed in a steerable collimator to allo~ simultaneous doppler v, idth and neutron yield measurements along several l0 cm diameter chords through the MF'I"F plasma. Fig. 9 shows a picture of the collimator and the detector

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7. Conclusions 0

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vulnerable to d i s t o r t i o n due to b a c k g r o u n d and n e u t r o n scattering so that a b s o l u t e yield m e a s u r e m e n t s should bc possible a i t h high accurac,,. Time resolved m e a s u r e m e n t s allow d e t e r m i n a t i o n of the time c o n s t a n t s for ion cncrg,, g r o w t h a n d decay d u r i n g p l a s m a b u i l d u p and following the t e r m i n a t i o n o f p l a s m a heating. Finally, all o f these m e a s u r e m e n t s are i m p o r t a n t in d e t e r m i n i n g the p o w e r b a l a n c e in r e a c t o r scale e x p e r i m e n t s w h e r e only the u n c h a r g e d reaction p r o d u c t s escape the central p l a s m a readily. This work was s t i m u l a t e d and e n c o u r a g e d by m a n ' , c o n t r i b u t i o n s a n d d i s c u s s i o n s at several laboratories. T h e principal individuals w h o kindly gave their time in t u t o r i n g the a u t h o r s in p l a s m a physics were D. (_'orrcll, R.P. Drake. J. Foote, D. G r u b b , a n d T . ( ' . S i m o n e n at L L N L : D. P a p p a s at MIT; T. Elevant, H. H e n d e l , S. Seilcr. and J. S t r a c h a n at PPPI,. T h e i r help is gratefully acknov~ledged. This work was s u p p o r t e d by the Experim e n t a l R e s e a r c h Branch o f the Division of A p p l i e d Plasma Physics. Office of F u s i o n Energy.

References

[1] D.R. Slaughter. LLNL Report UCRL-52923 (1980). [2] I).R. Slaughter. IEEE Trans. Nucl. Sci. NS-26 (1979) g02. [31 I).R. Slaughter. LLNL Report UCRL-82158 (1979).

[4] [5] [6] [7] [g] [9] [10] [11] [12]

[13] [14] [15] [16]

[17] [18] [19]

[20]

[21] [22] [23J

D.R. Slaughter, J. Appl. Phys. 54 (1983) 1209. T.('. Simonen. pri'~ate communication. G. Lehner and F. Pohl. Z. Ph,.sik 207 (1967) 83. M.MR. Williams, J. Nucl. Energy 25 (1971) 489. II. Brysk. Plasma Phys. 15 (1973) 611. B. Rose. A.E. "laylor and E. Wood. Nature 181 (195g) 1630. R . A . ( ' o o m h e a n d B.A. Ward, Plasma Ph',s. 5(1963) 273. M.J. Bernstein and F. Hat, Phvs. l,ett. 31A (197()) 317. V.W. Slivinski. H.C;. Ahlstrom, K.C;. l'irsell. J. l,arsen. S. (ilaros, fk. Zimmerman and H Sha~, Phys. Re'.'. Lett. 35 (1975) 1083. E. Pecorella, M. Samuelli, A. Messlna and C. Strangio, Phys. Fluids 20 (1977) 675. R.A. I,erche, LW. Coleman, J.W. tloughton, I).R. Speck and E.K. Storm. Appl. Phys. l.ett. 31 (1977) 645. D.S. Pappas and R R . Parker, MIT Report PF('/RR-78-5 (1978). T.('. Simonen et al. Plasma physics and controlled nuclear fusion research 1980, IAEA-CN-38/F-I (IAEA, Vienna. 1981) p. 97. J.H. Foote, A.W. Molvik and W.C. Turner, I,LNL Report U('RI.-88129 (1982). l-).P. (;'rubb et al. LLNL Report U('RI,-86476 (1982). L.W. Coleman, Diagnostics for fusion experiments, eds.. E. Sindoni and C. Wharton (Pergamon, New York, 1979) p. 483. J.D. Strachan, P. ('olest~k, It. Eubank, L. Grisham, J. Hovey. G. Shilling, L. Stev, art, W. Stodiek, R. Slooksberry and K.M. Young, Nature 279 (1979) 626. D.S. Pappas, F.J. Wysocki and R.J. Furnstahl, MIT Report PE('/RR-82-14 (1992). W.F. Fisher, Bull. Am. Phys. Soc. 27 (1982) 938. D. Slaughter and R. Strout It. Nucl. Instr. and Meth. 198 (1982) 349.

452

D.R. Shzughter et aL / Neutron diagnostw Xw'tem~

124] W.E. Nexsen, W.C. Turner and W.F. Cumnlins. Re','. ~'i. Instr. 50 (1979) 1227. [25] K.I. Thomassen, l.l.NL Report U('II)-18948(1981). [26] tI.P. Spracklen. IEEE Trans. Nucl. Sci. NS-29 (1t,~82) 896. [27] M. Chatelier, J. Idmtal, A. I.agattu and A. l.cpers. Nucl. Instr. and Meth. 190 (19811 107. [28] A. Berry, M.M. Przybylski and I. Sumner. Nucl. Instr. and Meth. 201 (1982) 241. [291 T. I'levant, Nucl. Instr. and Meth 185 (19811 313.

130} H. Klein and H. Scholermann, IFIEE Trans. Nucl. Sci. NS-2fl (1979) 373. 131] R. Booth, J.C. Davis, (.'.l,. Itanson. J.l,. Held, C.M.I.ogan, J.IL Oshcr. R.A. Nickerson, B.A. Pohl and B.J. Schureacher, Nucl. Instr. and Meth. 145 (1977) 25. 1321 M.J. Bernstein. Rev. Sci. Instr. 40 (1969) 1415. [33] D.W. Glasgow, [).E. Vclkley. J.D. Brandcnbcrgcr, M.I. Mcl:,llistrcm, l I.J. 11cnnccke and D.V. Breitenbccher. N ucl. Instr. and Meth. 114 (1974) 521.