Fission fragment spectroscopy with large surface barrier detectors

558

Nuclear Instruments and Methods m Phystcs Research 221 (1984) 558- 563 Nor)h-Holland, ~msterdam

F I S S I O N F R A G M E N T S P E C T R O S C O P Y WITH LARGE SURFACE BARRIER D E T E C T O R S * K. P A A S C H , H. K R A U S E

a n d W. S C O B E L

I lnstttut fur Expenmentalphystk der Unwersltat Hamburg, D-2000 Hamburg 50, Fed Rep Germany Received 4 July 1983

Large area (A = 8.5 cm2) surface barrier detectors have been tested for consistent time-of-flight and pulse height spectroscopy of fission fragments Tmung was measured with fast and narrow IR laser diode pulses, ~t yielded a resoluuon of (97+ 14) ps and an integral width of (228 + 58) ps fwhm with detectors cooled to -20°C. Mass dependent plasma delay differences were obtained from correlated 252Cf fragment determination and linear momenta from two tlme-of-fhg,ht measurements for different distances. Pulse height spectroscopy performed with the Schmttt cahbratlon constants yletded hnear momenta that were 1 2% higher; thus discrepancy ,s removed ff the rewsed cahbratlon constants derived with fragments of known energies of Welssenberger et al are used

1. Introduction The first chance fission probabflmes Pf of reaction systems with moderate excitation energies (E*>~ 15 MeV above the fission barrier) indicate that the fission decay seems to become increasingly "slower" - or the c o m p e t i n g deexcltation mechanisms hke neutron evaporation " f a s t e r " - t h a n at lower excitation energies. Several possible theoretical explanations are presently under discussion [1]. One tool to get additional experimental reformation on the energy dependency P f ( E * ) is the study of the ratio of pre- to postfission neutron ermssion. These components can be separated by measunng the correlauon of neutrons and fission fragment pmrs in reduced fission [2]. For the purpose of fragment spectroscopy, essentially two types of detectors have been used, namely plastic scintillators and surface barrier (SFB) detectors. Scintillation detectors prowde a fast timing signal and allow primary fragment mass deterrmnaUon from a fragment time-of-flight (TOF) measurement, mass resolutions of 2 - 3 amu can be obtained [3] with flight paths of 20 cm, if only the ume resolution ms better than 300 ps. These detectors do not suffer from lrradmUon damage; they are, however, not capable of an accurate kmeUc energy measurement. Surface barrier detectors are preferable m this respect, but cannot withstand intensive lrradmuon. The total dose per area may be reduced by increasing the d~stance target-detector from typxcally < 5 cm to 20 cm and, to compensate for the loss of neutron-fission coincidences, by using large area detectors for both * Work supported by the Bundesrmmstermm fur Forschung und Technologic 0167-5087/84/$03.00 © Elsevier Science Publishers B.V (North-Holland Physics Publishing Diwslon)

neutrons [4] and fission fragments. Such SFB detectors may favourably replace scintillation detectors if, in addmon, their time resoluuon is ~< 300 ps inctudmg all mass dependent corrections for mass dependent plasma delay differences. In this paper, we report on Ummg and pulse height measurement of fission fragments from 252Cf with self fabricated large area SFB detectors of 8.5 cm 2 acuve area, n-type material of 1000-1500 D cm specific resistance, 170/~m thickness and 400 p F capacity. Section 2 ~s devoted to the experimental set up and data analys~s In sect. 3, we discuss the measured timing properties of the detector as obtained with a fast infra-red (IR) pulser, the pulse height spectroscopy of a 252Cf s.f. source, and the plasma delay differences between correlated fission fragments and their dependence on the fragment mass. In sect 4, we shall demonstrate by a comparison of the linear momenta of fission fragments derived from either the T O F or the pulse height informaUon that both measurements are not consistent on a 1% level. The discrepancy will be traced back to shortcomings of the Schmitt [5,6] calibration procedure.

2. Experimental setup and program 2 1 Setup The experimental set up for coincident fission fragment detection is simdar to that of ref. [7] and is schematically presented in fig. 1. Detector D1 had a fixed position, whereas detector D2 was placed in a vertically movable frame at a horizontal distance S t + S 2 = 22 cm from D1. Both D1 and D2 could be cooled with Peltter elements P to - 3 0 ° C . A 252Cf s.f. source

K Paasch et al / Ft~lon fragment spectroscopy a)

c)

~ -H-~b- - ~

'~1crn

559

11crn

DELAY

DELAY

[ ~

b)

'LI

LI_ ,I

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F~g 1 Set up for a) differential tlnung measurement with an IR laser pulser, b) mechamcal modifications for coincident detection of fission fragments from the movable 252Cf source, and c) block dmgram of the assocmted electromcs

on a 222 # g / c m 2 NI foil could be moved along the D1 D2 axis such that the T O F path of D1 could be varied from 8 to 15 cm The axis was defined by a 5 mm aperture in front of detector D1 if not otherwise mentioned. The detector signals were amplified in fast, charge sensiuve preamphfiers (Ortec H242B) and fed into a linear and a tlrmng circuit of conventional design. The crucml trigger point setting of the C F D s has been carefully checked for pulse height independency by means of 10 db attenuators inserted for tins purpose (ref. [8]). The time walk could be mimrmzed (~< 30 ps) by tumng the walk adJust, if the C F D threshold was set to typically 0.2-0.3. Based upon the experience of ref. [6], timing calibration has been performed with an electronic time calibrator (Ortec 462). Differential rime resolution and its spatial distribution were investigated for a detector in position D2 with short IR (k, = 0.82 /tm, At = (67 + 15) ps, P ~< 3 W 1000 MeV) hght pulses * For this purpose, detector D1 and the 252Cf source were removed and detector D2 was illuminated via a light grade producing an IR spot of 2 - 3 mm ~ on the front s~de of the detector (see fig. 1) The TPC was started with the fast tarmng signal of the preamphfier, whereas a light pulser reference signal was

used to stop the TPC. This way. the riming pattern of the whole sensitive detector area could be scanned

2 2 Prmctple of data anal.vsls Any measured fission fragment parameters refer to fragments after neutron emxssion In pamcular, the pulse height is related to the post neutron ermsslon kinetic energies Ek,n, = ½rnv~ of the fragments (l = 1, 2), and their masses m, and velocities c, The energy cahbranon of the pulse height x is performed with the well known cahbrahon scheme of Schrmtt et al. (refs [5,6])'

wtth detector constants a, a', b and b' being derived from the pulse height response of the indwldual detector to 252Cf spontaneous fission fragments The masses m,* before neutron emission can be calculated by iteration from the provisional masses #, and the mass number ACN of the nucleus undergoing fission Ptl Ekm,l = ~ 2 Ek,n ,2,

(2)

~1 +/~2 = A C N ,

If the neutron multiplicmes u, = rn,~ - m, are known as a funcUon of rn,~ m~=#,[1

* Hamamatsu Ltd

(1)

E k.... = ( a + a ' ) m , x + b + b ' m , ,

+

Ek'n'l Ek,n A + Ekm.2

m 2 = dCN -- m~,

1 + l h / m ' 1-1 1 + P2/m2

(3)

K Paasch et a l / Ftsslon fragment spectroseop~

560

termmaUon are those of the T O F measurements for t 1 and t 2. The experimental times T, differ from t, by (a) systemahc errors of time zero, (b) the start time derived from the cyclotron R F and (c) the resolution A/mr of the stop s~gnal denved from the detector. Here we ~hall concentrate on the detector contribution (c).

Throughout tins work, the neutron multiphcmes v, are taken from hterature (fig. 2 m ref. [7]). The kinetic energies before neutron emission can now be obtained under the assumption of isotropic neutron emission. which allows the replacement of o,* by t,, E k . . . . -- ( m * / m , ) E k ,

(4)

n ,.

The main uncertainty of the preneutron quantities rn~*, E*kln,i obtained in this way comes from eq (1) and the determination of the parameters a, a', b and b' w~th the 252 Cf source Our detectors meet the quahty requirements stated in ref. [9]; for example, the (light mass) peak to valley ratio N L / N v was typically 2.84 + 0 1 The calibration in this case is expected (ref. [10]) to be accurate to witinn A E / E = 1%. For confirmation we have checked the pulse height response of our detectors in a separate heavy ion scattenng experiment performed at the Mumch MP tandem facility with recod nuclet from the elastic scattering of 100 MeV 32S projectiles m the mass range A = 92-97 and energies comparable with those of actimde fission fragments. The determination of fragment masses rn,* from the measured velocities v, = S,/t, starts from hnear m o m e n t u m conservation

m'~v¢( = m2v 2. *

3. Measurements The pulse heights for correlated fragments of the 2sZCf source, reaching D1 and D2. and their T O F difference signal were hsted in event mode. The energy loss m the N1 foil was accounted for by a run with the source facing D2 instead of D1 The energy losses z3E, for the most probable hght and heavy fragment were 6.93 and 6.49 MeV, respectively These data are w~tinn 3% m agreement with those of ref. [12]. They were extrapolated to other energles a n d / o r masses with the relation A E , - ~r kl /l n2, / of ref. [13] The T O F difference signal was off hne corrected for the time sinft At,(AE, ), too

E k.... = ½m,S~e/t, z, At, = t,[A E,]2E k....

(5)

If we subsmute v* with v, with the argument given above and neglect the sclssion neutron multiplicity (Vsc ~< 0.5 n/scission, refs. [2,11]) as before, we obtain from eq. (5)

m~ = A c N / ( 1

$1 tl

+-~2 ' ~ 2 ) '

(7) (7a)

In addmon, the pulse height spectra of both detectors were measured by irradiation of the whole sensitwe area (8.5 cm z) and analyzed as outlined in sect. 2.2 with the corrections gaven above. The results obtained for masses and energies of fragments before and after neutron emission are shown m column 2 of table 1. They are m excellent agreement with those of ref. 14 (column 4) obtained with surface barrier detectors of about half the size (4.5 cm2).

(6)

m2• = ACN -- rn~', The dominating errors entering Into this mass de-

Table 1 Mean values of fragment masses and energies before and after neutron emission Pulse height cahbratlon (eq. (1)) is performed with the parameters of ref. [6] (S) and ref. [171 (W), respecavely. Masses are m amu, energies m MeV. Tlus work

mE m~ EL E~

Et*ot

Etot

Ref [7]

Ref [5,6] (S)

108 50 143.26 184.07+ 1 3

108.55 143.45 106.2+0 7 80.3 + 0 5 186.5 +_1.2

106.19 + 0.2 142 15 +0 2 102.58 +- 1.15 78 67+0 61 181.25 +_1 3

106.0 141 9 103.37 + 0 5 79 37+_0 5 183.14

(S)

(W)

108.35 143.65 106 4 80.2 186 6 + 1.4

108.15 143.85 104 7 79.4 184.1

108.55 143.53 105.90 80.36 186.25 + 1.2

106.4 142 1 103 0 77.5 180 5

106.60 141.82 103 96 79.38 183 34

A mL mn EL EH

Ref. [14] (S)

+2.5 106.2 141.7 104.5 79.0 183 5

+22

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Fission fragment spectroscopy

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FAg 3 Plasma delay differences (PDD) as a function of fragment masses m The mass yield curve P ( m * ) of this work As shown for reference

[V]

Fig 2 Integral tnme resolutmn of a typical detector, measured for different temperatures T (upper part), and temperature and bins voltage dependence of time shift & between the center point of the detector and a point 1 5 cm off center

Investigations of the timing properties are performed. (a) with the fast IR laser pulser to study the spatial timing distribution and (b) in a fission fragment coincidence measurement devoted to the question of plasma delay differences The time resolution for single point IR Illumination of the detector surface was obtained from the corresponding TPC spectrum corrected for the intrinsic width (67 ps) of the laser pulser The average value obtained for several detectors sampled over the whole sensitive area w a s A / d = (97 + 14) ps fwhm. For each detector, the integral time resolutmn may be obtained by addition of the differential results weighted with the sensitive area they represent. The integral spectrum calculated tlus way is shown in fig. 2 for a typical detector The average integral width IS A/mn t = (228 + 58) ps, the worst case among 7 detectors 315 ps. We assume this detector dependence to be an indication for Inhomogeneities of the doping near the detector surface, in particular near the rim (ref [15]). The timing properties could be improved by detector cooling The lower part of fig. 2 shows the time shift &, that IS, the difference in IR pulse response for the detector center and a point at r = 1.5 cm, i.e far off the center Cooling to - 2 0 ° C reduced the time shift by typically 25%, whereas further cooling (to - 3 0 ° C ) and overbmsing had no substantial effect. The Improvement also shows up in the integral time resolution that reduces to typically 150-200 ps fwhm, cf. fig. 2a. At the

same time, the spectrum approaches a gausslan shape with reduced tailing on the long tame side All further measurements were performed at - 2 0 ° C Plasma delay differences (PDD) for correlated fragments were obtained as the difference between the measured time difference zltexp = T 1 - T2 and the value Atca~c = t~ -- t 2 calculated from eqn (7), with the masses m, and energnes Ek,.. , of the Schmltt analysis P D D ( m , ) = At~,jc

-

Atexp +

At 0

(8)

The value of A t o due to electronic asymmetries between D1 and D2 has been fixed by the requirement P D D ( m s ) = 0 for symmetric fission Analysis has been performed in an event-by-event evaluation of our data Fig 3 shows the resulting P D D as a function of the mass m2 detected in D2. In first order we find a linear relation between P D D and m 2 in agreement with the results of refs. [7,15], the absolute values P D D = + 288 ps obtained for the mean heavy and light fragment, respectively, exceed those of ref [15] ( + 192 ps for 400 I2 cm resistivity and typically 4 cm 2 active area), This difference, however, does not reflect a dependence on the resistivity or sensitive area (cf. fig. 7 m ref. [8]), but is due to the lower electric field strength E e applied in our work Neldel and Henschel [8] found the P D D of ref [15] to be in agreement with the relation te =

1 33rnl/6E~(~/Ee,

(9)

that gives the plasma delay tp (in ns) as a function of fragment mass (in amu), energy (in MeV) and E~ (in k V / c m ) . For the field strength E~ = 10 kV cm of our experiments, we obtain from eq. (9) with the data of table 1 values tp = 2.96 and 2 69 ns for the light and heavy fragment, respectively The difference of 258 ps is

562

K Paasch et al / Ft,sston fragment ~pectrowop~

In good agreement with the experimental result The absolute value pre&cted by eq. (9) may be replaced by the value tp determined experimentally from the dependence t p ( U ) on the bias voltage U For this purpose, we apply the well known relation E¢ - CU to eq (9). Differentiation then yields (10)

tp = - 2 U A t p / A U ,

such that lp may be calculated from the measured shift A t p ( U , A U ) For our detectors operated at T = - 2 0 ° C , we obtained lp -.~ 2.7 ns + 20% with no systematic mass dependence. We intend to correct future T O F measurements for fragment mass spectroscopy (eq. (6)) with a constant detector specific value tp plus the smooth mass dependent P D D as gwen by fig. 3 Finally it should be mentioned that the modification of the calibration coefficients entering into eq. (1) that will be discussed in the next section, almost cancels In the determmatmn of Atomic = t~ - t 2 The P D D for the mean heavy and light fragment for example changes from _+288 to _+295 ps.

4. Consistency of time and pulse height measu~ment Upon variation of the 252Cf source positron in the setup of fig. l b by a distance a towards D1, the measured time difference changes by t o -- t a = a [ ( 1 / O l ) + ( 1 / v 2 ) ] .

(11)

Inserting m~ from eq. (6) yields t o - t, = AcNa/(rn*v,

(12)

),

or

pC( = A c N a / (

to -- ta).

(13)

Eq. (12) allows a determinatmn of the hnear m o m e n t u m solely from the T O F signals t o and t a. The pulse height reformation of the same events processed according to the Schmitt procedure also allows to determine the

linear momentum, VIZ.

(14)

p~' = v/2ml - ~ Ekm * I

In order to check the consistency of tmung and pulse height signal by c o m p a n n g the results of eqs (13) and (14), the quantity t o - t a has been determined for a shift a = 7 cm as a function of the mass m* taken from the pulse height reformation The experiment has been performed with apertures of 1 cm diameter m front of D1 and D2. Altogether 20 000 events were taken The results are shown in columns 2 and 3 m table 2 for some representative mass values (ref, [16]). The hnear momenta calculated from eq. (14) are for all masses more than 1% higher than those from eq (13). We now compare the average difference S = 1 25% with the errors entering into eqs. (13) and (14). The uncertainty of a is _+0.1 mm or A a / a <0.2%. The time difference tu t d as a &fference of differences is free of additive cahbratIon errors; the main uncertainty therefore Ls introduced by the finite time resolution and yletds A(t ° - - t o ) / ( t ~ - to)~<0.8%. The linear m o m e n t u m of eq (14) has an error of about half the value for the kinetic energy, Le. 0 5%. The systematic difference z~= 1.25% between the calculalaon of linear momenta exceeds the standard deviation hmit on the average and for all masses listed The conclusmn must be that the pulse height analysm following the Schmitt calibration procedure ~s not qmte correct. Henschel et al. [7] already stated that this energy cahbratmn scheme for surface barrier detectors yields too lugh kinetic energies for fission fragments w~th a &screpancy at the 1% level Recently Weassenberger et al [17] reported new calibration constants for eq. (1) obtmned from a comparison of the pulse height response for fissmn fragments of surface barrier detectors with the mass spectrometric (" L O H E N G R I N " at ILL) energy deterrmnation. Applicauon of these new constants yields the masses and energies hsted m column 3 of table 1 The resulting difference A Et*, = - 2 5

Table 2 Comparison of linear momenta p for selected fragment masses obtained from the pulse height (eq. (14)) and from the ume-of-fllght (eq. (13)) information and difference Ap. Detector calibration constants: (W) from ref. [17], (S) from ref. [6] m*

Detector calibration constants (S)

Detector cahbratlon constants (W)

[amu]

* 1 (2m * 1 Elun. [10 -is Ns]

Ac~a/At

Ap

AcNa/A t

Ap

[10 - i s Ns]

[%]

1~ ~ m l*~'* ~km,l ~ , 1/~ [10 -Is Ns]

[10 -18 Ns]

[%]

2.381 2 482 2.554 2.542 2.478 2.425

2.347 2.446 2.513 2.518 2 451 2.351

1.43 1.45 1.14 0 94 1 09 3,05

2 366 2 460 2.531 2.523 2.458 2 365

2.354 2 446 2.514 2 519 2.456 2 361

0 51 0 57 0 67 0 16 0,08 0 17

98 108 118 134 144 154

~'p

1.25%

~p"

0,36%

K Paasch el al / Flsston fragment spectroscopy

MeV is m agreement with the result of refs [17,14] At the same t~me, the systematic difference between the hnear m o m e n t a obtained from the pulse height and velocity determination Is considerably reduced ( , ~ = 0 36%, cf table 2) and vamshes within the error hmxts stated before In conclusion, we can state that large area (A = 8.5 cm2, P = 1000-1500 $2 cm) surface barrier detectors can be used for consistent fission fragment T O F and pulse height spectroscopy, if the time reformation is corrected for the mass d e p e n d e n t plasma delays and the pulse height m f o r m a t m n is converted w~th the Schm~tt cahbratlon scheme and constants derived from an md e p e n d e n t energy or velocity deternunatmn We thank the " B e s c h l e u m g e r l a b o r a t o n u m der TU and L M U M u n c h e n " for an excellent 32S beam, Dr. H Henschel for helpful discussions, the solid state detector group of our institute for fabricatmn of the detectors and the B u n d e s m t m s t e n u m fur Forschung und Technologle for financial support

References [11 H.C Bntt and A Gavron, m: Lecture notes m physics, vol 158, Proc lnt Symp on Nuclear fission and related collectwe phenomena and propemes of heavy nuclei (Spnnger, Berhn, 1982), p 24.

563

[2] Z Fraenkel, 1 Mayk, J P Unlk, A J Gorskl and W D Loveland, Phys Rev C12 (1975) 1809 [3] P Phschke, W. Schobel and R Wlen, Nucl lnstr and Meth 203 (1982) 419 [4] H Scholermann and H Klein, Nucl lnstr and Meth 169 (1980) 25 [5] H W Schmltt, J H Neller and F J Walter, Phys Rev 141 (1966) 1146 [6] H W Schmltt and C W Wllhams, Phys Rev B137 (1965) 837 [7] H Henschel, A Kohnle, H Hlpp and F Gonnenwem, Nucl Instr and Meth 190 (1981) 125 [8] H O Neldel and H Henschel, Nucl Instr and Meth 178 (1980) 137 [9] H W Schmltt and F Pleasonton, Nucl Instr and Meth 40 (1966) 204 [10] Y Patm, S Clerjacks, J Lachkar, J Slgaud, G Haouat and F Coqu, Nucl lnstr and Meth 160 (1979) 471 [11] P Pllschke, R Langkau. W Scobel and R Wlen, Nukleonlka 26 (1981), m press [121 S Kahn and V Forgue, Phys Rev 163 (1967) 290 [13] F Plasd, R L Ferguson, F Pleasonton and H W Schmltt, Phys Rev C7 (1973) 1186 [14] R Schmldt and H Henschel, Nucl Phys A395 (1983) 15 [15] H Henschel, H Hlpp, A Kohnle and F Gonnenwem, Nucl Instr and Meth. 125 (1975) 365 [16] K Paasch, A Kamlnsky, Y Holler, R Wlen and W Scobel, Verhandlg DPG(VI) 18 (1983) 982 [17] E Welssenberger, P Geltenbort, A Oed and F Gonnenwem, Verhandlg DPG(VI) 18 (1983) 1132