A multidetector setup for (n, xnγ) studies at 14 MeV

Nuclear Instruments andMethods 206(1983) 127-134 North-Holland Publishing Company

127

A MULTIDETECTOR SETUP FOR (n, xny) STUDIES AT 14 MeV

S. HLAVÀC and P. OBLOZINSKY

Institute of Physics, Electra-Physical Research Centre of the Slovak Acaden0" of Sciences. Dtibravskd testa, 842 28 Bratislava, Czechoslovakia Received 8 January 1982 andin revised form 13 July 1982 A setup is described for investigations of (n, xny),x=0, 1, 2, reactions at 14 MeV neutron incident energy . It makes use of the time correlated associated particle method and its basic version comprises 5 detectors. The system is developed for y ray spectmuopy in the discrete as well as the continuous spectral energy region from 0.2 MeV up to around 20 MeV, neutron time-of-flight spectroscopy andyray multiplicity measurements.

1. Introduction

In (n, xny) reactions induced by 14 MeV neutrons

generally several y rays are emitted, which usually follow the emission of 1-2 neutrons. Basic information about these reactions can be obtained from simple neutron and y ray spectra and their angular distributions. During the lastdecade an advance in the neutron time-of-flight spectrometry has filled many gaps in our knowledge of neutron spectra Il] but the situation with y production spectra is less satisfactory . Often, for example, there is a complete lack of experimental data in the intermediate spectral energy region Et - 8-14 MeV (ref. 2 ). More complicated forms of spectroscopy which can supply important additional information have practically not been used so far . The only exception to this is the work of the Vienna group[3,4], which, however, has not been systematic in character. Of special importance is y and neutron spectroscopy combined with the detection of another simultaneously emitted y ray . Direct physical information, which can be obtained in this way refers to, e.g., average y multiplicities of cascades including specific discrete y transition in a final nucleus or cascades following neutrons emitted with a given energy . Technically such measurements can be best performed with multidetector systems. Mullidetector systems, developed for recording an important portion of the information concentrated in y rays accompanying various nuclear reactions, are not uncommon nowadays. 4 detector used in the pioneeringwork of Hagemann et al. [5] in 1975 initiated arapid development of multidetector systems with has culminated in the Darmstadt-Heidelberg crystal ball consisting of 162 detectors ISO . As a rule,` however, these systems aredesigned for thedetection of extremely long 0167-5087/83/0000-000o/ :b03 .00 Oc 1983 North-Holland

y cascades populated in heavy ion collisions. In 14 MeV neutron physics the situation is very much different. Firstly, one can reasonably limit oneself to several detectors and, secondly, one has to solve specific background problems connected with spectroscopy in a neutron beam. The first point implies relative simplicity of the system, while the second point can he effectively solved by the time-correlated-associated-particle method 17] based on the kinematics of the 'Md. n)a reaction. Our system combin±s spectrometry of continuuous y rays with discrete y rays and neutrons. In the present work we describe a basic setup which consists of 5 detectors. For discrete y ray spectrometry we use a Ge(Li) detector, for y spectrometry in a continuous spectral energy region up to about 20 MeV we use a Nal(TI) and a liquid seintillator NE 213 serves in the neutron time-of-flight spectrometer. The setup is eoNnpleted by a plastic detector for associated a particles and a stilbene monitor of neutrons. In chapter 2, the full setup is described, chapter3is devoted to individual detectors andchapter 4 deals with data acquisititvn and analysis. 2. Experimental setup The associated a particle detector is used to produce reference timing signals for all other detectors. It plays the role of master detector since coincidence requirements are set to reduce significantly the background due to time uncorrelated events. In this way, neutrons are selected electronically in a given solid angle; which is further defined by a collimator of conic shape. Sptxtrometers, protected against the neutron source by the collimator, are located freely outside the neutron cone.

128

S. Mlavdé. P. Oblobnskt' / Multideteworsetup

neutrons are detected by a plastic scintillator of 0 20 mm x 0.1 mm. Neutrons enter the collimator through the entrance hole of 0 10 mm, which is twice the diameter of the deuteron beam . The collimator is 155 cm long. The latter figure is given by a fixed building disposition rather than an optimum choice, which should lead to a somewhat shorter length. The collimator is constructed with its inner part changeable to allow easy modification . Usually we use here iron segments and at the sample end lead segments. The size of the inner cone of the collimator is a result of a compromise between the acceptable dimensions of a sample and the effective neutron flux. The self-absorption of typical 1.0 MeV y rays in the sample should not exceed some 15`.6 ; the thickness of the metallic samples of medium light nuclei in that case could be about 2-3 mm. The typical sample weight should be 100-200 g in order to limit the irradiation time to a few tens of hours. These requirements can be met with the collimator exit hole having 0 100 mm, defining thus its solid angle as 41r x 2 .1 x 10 -4 sr. The solid angle of the associated particle detector is by one half larger than the solid angle of the collimator. This is in order to take into account realistic dimensions for the deuteron beam, otherwise one should decrease the electronically accepted neutron flux in the collimator. Tho irradiated sample usually has the shape of a hollow cylinder of 0 80 mm with a height of 80 mm. For precise absolute measurements, however, when one

This allows easy access to them and flexible manipulation, which is important when a variety of experimental requirements has to be met. For this solution, however, one has to pay by having relatively heavy samples. In spite of this it seems to be one of the most efficient designs, which moreover does not require a pulsed neutron source. The kinematics of the 3 H(d, n)a reaction imposes certain limitations on the geometrical arrangement of the setup . Even at low energies of deuterons the mutual angle of the emitted neutron and the associated a particle differs from the 180° in the laboratory system and the difference is of practical importance. For 120 keV deuterons this angle is 177°, provided the neutrons are emitted at 26° with respect to the deuteron beam. The latter angle has a 'direct influence on the timing resolution and it turns out that its ideal value is 98° (ref. 8). At this angle the energy spread of the neutrons is negligible around 14,0 MeV while at 26° one has E. = 14.6 t 0.2 MeV with a correspondingly, opposite spread in the associated a particle energy. Calculations show that this leads to a basic time uncertainty (fwhm) of our arrangement, which is 1 .4 ns compared to the minimal 0.2 ns. It is useful to have this point in mind when one tries e g. to reach extreme timing resolution of the neutron time-of-flight spectrometer . A layout of the setup is shown in rig. 1 . The do beam of 120 keV/200 pA of separated D* ions from the compact neutron generator (9) bombards a tritium target. The a particles accompanying emission of 14 .6 MeV

!$ 213 0120-40 " XP 2041

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ten

Fig. 1 . A schematic presentation

LEAD of the

BORATED POLYETH.

(n, xny) setup. Components

of

PON

CONCRETE

the collimator are distinguished graphically.

S. Hlaudé, P. Oblotiwky / Meltidetector saup

tries to reduce all corrections, a relatively large cylindrical sample should be avoided. In this case it is better to use a smaller planar sample even at the price of a several times lowerweight. The spectrometers are shownin fig. I in their typical positions. For y spectrometry we use a 70 cm3 Ge(Li) detector mounted in a horizontal cryostat and a m 160 mm x 100 mm Nal(TI) crystal . The latter needs additional shielding because of its considerable volume. The distance between the Nal(TI) crystal and the sample should be sufficient to allow a time-of-flight based discrimination of neutrons. The typical distance between the front face of thecrystal and thecentre of the sample is 30 cm. The neutron time-of-flight spectrometer is based on a liquid scintillator NE 213 with the dimensions m 120 mm x 40 mm . Atypical neutron flight path is 0.5-1 m. The neutron monitor based on the stilbene crystal of m 40 mm x 40 mm is located some 2 m behind the sample. Precise location of the associated particle detector as well as the tritium target with respect to the collimator axis are important for good performance of the setup. Both thetarget and the detector were carefully put into position by meansof a geodetic laser . It is useful to have information about the neutron cone, which can be obtained in the form of a beam profile measured around the collimator axis in coincidence with correlated a particles. Fig. 2 shows experimental velues measured in the horizontal plane for a deuteron beam off diameter of ¢ 5 mm. The neutron flux is symmetric' around the collimator axis and it is fairly constant in the region of the inner cone of the collimator . This means that the a detector is optimally placed with ~espect to the collimator axis and the tritium target is appropriately distant from the collimator entrance. This is important especially when absolute monitoring of neutrons is required.

-2

-t D t ANGLE (DEG)

Fig. 2. Horizontal cut of the neutron cone obtained as a neutron beam profile measured around the collimator axis in coincidence with the associated partic'e detector. Recommended position of the sample with respect to the collimator axis is indicated.

129

0

Fig . 3. A simplified block scheme of electronics . ,Abbrevtations denote standard electronic tmxlules : PSD is the pulse shape discriminator, TFA liming filter amplifier, CF"f comatara tru, tion timing. LA linear amplifier. TAC tire-to-analog converter. SCA single channel analyser output of the TAC, ('C co-, deuce unit, LG linear gate and ADC analog-to-digital amverter .

A somewhat simplified block scheme of electronics of the whole system is shown in fig. 3. Theessential part of the electronic modules are the commercial products of the firms Canberra, ZfK Rossendorf and Polon. Signals from detectors, if necessary, are processed first by fast amplifiers. Afterwards there follow fast discriminators of the constant fraction type. Forheavily loaded a detector we use the constant fraction timing unit, Canberra model 1428 A, designed for the maximum load of 50 MHz. For the neutron time-of-flight spectrometer we use the model 1428 in coperation with the neutron-gamma pulse shape discriminator of the so-called Munich type, Canberra model 2160. In the stilbene neutron monitor the interfering y component can be feasilby suppressed by means of a simple pulse height discrimination. Fast signals from timing units are led to time-to-analog converters to determine time intervals between events recorded in detectors and associated a particles. Outputs from the time converters arc used to produce required time spectra or to generate gating signals for linear branches of y spectrometers . Also shown is a typical variant of data acquisition . Singles as well as coincidence spectra are rewarded for the Ge(Li) and the neutron time-of-flight spectrometer using multichannel analysers with singles/coincidence routing. The continuous character of energy spectrum from theNal(TI) detector prevents acorrect subtraction of time uncorrelated background events unless a matrix energy x time is available . For this purpose we use the two~parametric analyser Nuclear Data ND 4420.

130

S. MaWe. P. Obloltmk~' / Multide-tor setup

3. Detectors In this chapterwe first discuss the associated particle detector, which plays in the whole system a role of master detector. Afterwards we proceed to y spectrometers and neutron detectors where we concentrate on procedures for the determination of detection efficiencies. Finally we discuss theimportant timing resolution of detectors. 3.1 . Marter detector A schematic arrangement of the a detector and the target chamber is shown in fig. 4. Adeutaron beam of a diameter of 0 5 mm is defined by two off-centre diaphragms which are close to the 0 45 mm TiT target. The target can be easily turned into another position, which is useful for long irradiations. The detector is located 268 mm from the tritium target and its solid angle is definedby a diaphragm of diameter 0 20 mm. In the vicinity of the detector, a weak 2"Am source is mounted on a movable support. Thedetector itself consists of aplastic scintillator of a thickness of 100 lam, which is essentially the range of a particles with theenergy of several MeV. The scintillator is protected against light as well as scattered deuterons by a I pm evaporated Al layer. A plexiglass light pipe which is part of the vacuum flange ensures mechanical stability of the plastic layer as well as optical contact between the scintillator and the fast photomultiplier FEU-30. The photomultiplier is surrounded by a permalloy shield to suppress the influence of the magnetic field from the accelerator.

^" D' ~ 103 0 U

--

Î

beam Am . beam

off

ihres

102

50

100 150 CHANNEL

200

Fig. 5 . Associated particle spectrum obtained with a plastic scintillator . A well pronounced peak of a particles is followed by apile-up bump andproton peak from the 2 H(d, p) reaction . The spectrum of 5.4 MeV a particles from the 2atAm source is shown for comparison.

The spectrum of associated particles shown in fig. 5 displays a broad peak of a particles followed by a pile-up bump and a weak proton contribution from the 2 H(d, p) reaction. It is seen that the a particles are quite distinctly separated from the background, the pulse height threshold of the fast discriminator is indicated by -MOVABLE SUPPORT

H,O Îin

10-

Fig. 4. A schematic arrangement of the associated particle detector and the target chamber.

13t

S. Hfuudé, P. Ohlo:insky / Malttdetector setup

an arrow. Also shown is the spectrum of 5.4 MeV a particles from 2°1Am. 3.2 . y spec7ronteters For y spectrometry in the discrete region we used a 70 em' Ge(Li) detector mounted in a horizontal cryostat . Its energy resolution at 1332 keV was 2.5 keV. A relative efficiency curve was measured in a geometry close to real experimental conditions. We used a liquid solution of t52.t5^Eu filled in an envelope of a thin walled hollow cylinder with the dimensions 080/076 x 80. The centre of the cylinder was 15 cm from the detector front face. Absolute efficiency was determined by absolutely calibrated point sources. A correction for thevolume effect calculated by theMonteCarlo method was applied to thepoint source absolute efficiency . For y spectrometry in the wide and mostly continuous spectral energy region Ev -0.2-20 MeV we used a 0 160 x 100 Nal(TI) crystal coupled to a photomultiplier 65 PK 423 (Monokrystaly Turnov and Tesla Prague). A lead collimator with an aperture of 0 110 mm wasplaced in front of the Na(TI) crystal in order to suppress the Compton component in the lineshapes. Responses of the spectrometer, i.e . the total efficiency curve and the lineshapes were determined experimentally. The total efficiency fl of the NaI(Tp was measured by the coincidence method. The method requires a set of sourceseach heaving two y rays, calibration E,"' and reference EY` r, in a cascade. The coincidence and singles spectra should be measured with a Ge(Li) detector and the respective photopeak counts N, and N, should be compared . there holds t

~~Ev

N( Err)

= N(EY er

)

W(ft) "

where W(,9) is thecorrection for the angular correlation which should account forthe realistic dimensions of the detectors. A convenient set of sources included "Eu, z° Bi, 'Co and "Na, which provided us with calibration transitions E.."t = 0.344, 0.570, 1 .173, 1 .332 and 2.754 MeV. The total efficiency curveof our Nal(TI)detector for a point source on a symmetry axis 30.4 cm from the crystal front face is shown in fig. 6. The efficiencies in the low energy region are influenced by the electronic threshold which was set to 180 keV. The experimental curve was extrapolated for y ray energies E3 >_ 3 MeV. In the extrapolation we considered calculated efficiencies of crystals close to ours [11,12], an influence of the electronic threshold and the penetration of y rays through the collimator. Estimated relative uncertainties of the extrapolated values are fairly much below 10% even at a 20 MeV y ray energy . This is acceptable since

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Fig. 6. Total efficiency curve for the 0 160 in- e: IM mm N.I(TI) detector. The detector is shielded with lead except of the 0 110 mm aperture in the front face collinnttor . The distance between the point source and the front face of the crystal is 30.4 cm. Full circles refer to experimental values. the dashed part of the curve represents extrapolation down to the electronic threshold and the dash-dotted part of the cure is the extrapolation discuW in the text.

the standard errors caused by low statistics of the weak 14 MeV neutron capture are usually much higher. Line shapes were measured for a set of energies . In the region of E, :5 3 MeV we exploited the calibration y transitions listed above. To produce high energy y lines we used 3 reactions accessible by a low energy accelerator. The reaction t=C(n, n'y) induced Uy 14.6 MeV neutrons gives 4.44 MeV y line. the reaction "Mp. y) has a resonance at the proton energy 0.163 MeV and produces acascade of 11 .67 and 4.44 MeV y lines with a weak admixture of a 16.11 MeV y line and 3 H(p, y) at about 0.1 MeV proton energy produces a 19.8 MeV y line . From expenmental line shapes we extracted parameters needed for the lineshape generator in the energy region ET = 0.2-20 MeV. Generated line shapes were supposed to be composed of theusual components with an increasingly dominating bremsstrahlung tail which prevails at higher y ray energies . The details of the procedure aredescribed elsewhere 1101 . 3.3. Neutron detection

Detection of neutrons in the time-of-flight neutron spectrometer was performed by means of a liquid scintillator NE 213. The scintillator, filled in a thin-walled aluminium vessel 0 120 x 40, was optically coupled to a fast photomuleiplier Valvo XP-2041 . Pulse shape discrimination was applied to distinguish neutrons from y rays . The electronic threshold was typically set between 0.5 and 1.0 MeV in the neutron energy equivalent. As concerns intrinsic detection efficiencies one relies often on Monte Carlo calculations which should be compared with experimental quantities whenever possible . Relative efficiencies were determined experimentally by

132

S. Hiatufï:

P. Ohlo:inske / Alultidetertor setup

means of "'Cf. presumably at lower neutron energies [13). Absolute efficiencies were measured in a narrow energy region around 14.6 MeV accesssible by the neutron generator. 252Cf We used a planar source (Amersham Intemational) with an activity of 3.7 x 10 ° Bq, which was placed directly on a plastic scintillation detector. The scintillator detected fission fragments and specified thus the time of theneutron emission. Neutrons flew through the collimator to reach NE 213 and their total flight path was about 2 m. Their spectrum should have the Maxwellian form E,t,/= exp(-Ea/T) with the temperature T- 1 .427 MeV. This was compared with the experimental spectrum to obtain the relative detection efficiencies. The associated particle method was used at 14.6 MeV neutron energy. The neutron spectrometer was located on the collimator axis. The precise location of the associated a particle detector, which is strictly required here, was checked by the neutron beam profile measurement . The intrinsic detection efficiency En is given as

where N. and N, denote the coincidence and the singles carats of a particles, w, is the solid angle of the associated panicledetector and wn is that of the neutron Spectrometer. The intrinsic efficiencies are shown in fig. 7. Monte Carlo calculations were performed with the code TAYRA (ref, 14). Relative efficiencies obtained with the"Cfwere normalized at a 3 MeVneutron energy to the Monte Carlo value. We see that theTAYRA values

are in good accord with the experimental quantities. Therefore the calculated efficiencies can be safely used in the whole region of neutron en .-rgies from 1 to 15 MeV. T,te detector for the neutron Gax monitoring consisted of a stilbene crystal 0 40 x 40 coupled to the photomultiplier FEU-85 . The threshold was typically equivalent to the y ray energy Et = 1 .8 MeV (88 Y) and the intrinsic efficiency for 14.6 MeV neutrons was around 10`x . 3.4. Timing resolution Important parameters of the whole system are timing resolutions of the spectrometers as measured versus the associated a particle detector. Typical time resolution functions of they spectrometers were obtained with the Fe sample in the geometric arrangement of fig. 1 . The time spectrum of the neutron time-of-flight spectrometer was measured in a direct beam of 14 .6 MeV neutrons without the sample. Results are shownin fig. 8. Timing resolution (fwhm) is 2.4 ns for NE 213, 9.8 ns for Ge(Li) and 7.5 ns for Nal(TI). These figures are acceptable forour purposes. We estimate that the contribution due to the kinematics of the 'H(d, n)a reaction and the time walk of the master detector represents 2 ns. Reduction of this number can therefore improve the timing resolution of the neutron spectrometer only. The time spectrum of the Nal(TI) spectrometer as shown in fig. 8 was measured with thesample placed 30 cm from the front face of the crystal and it wassummed up over all the energy pulse heights. Arather broad low amplitude tail is due to neutrons scattered on the sample and afterwards detected in the Nal(TI). It is im.

s uo

t ~07nslcnl

Fig. 7. Intrinsic efficiencies of the 0 120x40 NE 213 neutron spectrometer with a 1 MeV electronic threshold. Experimental vakxs are obtained by means of the 252Cf except for the 14.6 MeV point.The Motte Carlo values were obtained with the TAYRA code [14].

1001 120 14D 15o Is. cnl . CH

200 220 2~0 IOClnslcnl

2so

Fig . 8. Observed time resolution functions for the neutron time-of-flight, the Gc(Li) and Nal(T0 spectrometers versus the associated particle detector. Neutron time-of-flight spectrum refers to the direct 14 .6 MeV neutron beam,while Ge(Li) and Nal(TI) spectra are from Fe +n reactions. The asymmetry of the Nal(TI) time spectrum is due to neutrons scattered on the sample.

S. lllare&°, P. Oblohmke / Multideteetor settep portant to separate this neutron component for all energy pulse heights. The separation can be improved by a longer flight path with an obvious payoff in a lower detection efficiency. 4. Data acquisition and analysis Pulse height spectra from particular spectrometers were recorded more or less classically by means of several multichannel analysers . Data for monitoring and checking purposes were collected with scalersinterfaced to the minicomputer ND 812 via a CAMAC dataway extender (IS]. In the basic version of the system we record spectra containing information about y ray production cross sections in the discrete as well as thecontinuous region, the neutron production cross section and the average y ray multiplicities . From the Ge(Li) detector we record the singles as well the coincidence spectrum with an open Nal(TI) detector . The coincidence spectrum is needed to extract averagey multiplicities M for cascades identified via a specific transition E° . The averaging should be performed over the rest of the transitions E, i = 1, 2, . . ., which compose these cascades. The corresponding relation is N( E,o

N( Eo~

_ ~fl Er .o

=(M-1)fl~É,~,

where Nc and N are the photopeak areas in the coincidence and the singles spectrum, respectively. S2 is the Nal(TI) total detection efficiency including the solid angle and : denotes the average energy of emitted y rays to be dettrmined experimentally . For similar reesons as those above we record timeof-flight neutron spectra it the mode singles and the mode coincidence with .n open Nal(TI) detector. The average multiplicity of the cascades following the neutrons emitted with the energy G- is given as

N(E~)

N(E )

dl

(lo,

E7 = Mfl

where N and N refer now to specified parts of coincidence and singles neutron spectra. Averaging in eqs. (4) and (5) can be refined when additional knowledgeaboutcascades is available. When, e.g., the same y ray transition is Fresent in a dominating number of cascades, this transition should be excluded from the averaging procedure. Often, however, these refinements can be neglected because the total efficinecy of the Nal(TI) is fairly constant over a wide range of y ray energies . From the Nal(TI) spectrometer we expect to obtain essentially a simple energy spectrum . The background of the apparatus spectrum consists of a neutron compo-

133

nent and time uncorrelated events (for an illustrative example see the right-hand spectrum of fig . 8). Since these components vary with the energy pulseheight tine has to record the two-parametric spectrum energy x time . For monitoring purposes we record the number of associated a particles, N( a). the number of neu: :on, detected by the stilbene monitor. N(n) and the coincidences between the two, ,(n) .These data provide two independent ways to determine the total number of neutrons incident on the sample. Provided the stilbene detection efficiency and neutron ahsorption in the sample are known one can use N(nh otherwise one can use N(a) corrected for the ratio N,(n)/.`;(n) . This ratio, which can theoretically reach unity, represents the figureof meritof the neutron collimation . Variation in this should be avoided since it indicates the change of the initial distribution in the neutron cone . During long time runs it is useful to record numbers of events from every spectrometer and to check their ratios to the number of the associated a particles . In data analysis one has to consider several corrections which are essentially due to the dimensions of .t sample and the geometry of the experimental setup. These corrections are physically related to se1fabsorption and detection efficiency. It is undersitod that because of generally nonuniform angular distributions. the single particle production cross section does depend on an angle of thespectrometer with respect to theneutron beam . In y ray multiplicity measurements triple correlations should play a role. They are, however, generally very weak for (n, xny) reactions at 14 MeV because of the predominant statistical character of the reaction mechanism with many overlapping states involved . The corrections for selfabsorption and detection efficiency can be obtained by Monte Carlo calculations. The results show that typical corrections for simple detection efficiency represent a few percent only, selfabsorption of neutrons is more pronounced and y ray selfabsorption is the most important. Total corrections for singles spectra as well +.s y multiplicity measurements are demonstrated in fig, 9 for acylindrical Fe sample. Throughout these calculations we neglected the multiple processes, the average y ray energy was assumed to be E, = 1 .5 MeV. The correction for singles follow the energy dependence of y ray absorption coefficients as well as neutron total cross sections. The corrections for y ray multiplicities are energy dependent because of the selfabsorption of the process identifying particle (a discrete y ray or a neutron). This dependence is now reversed since the selfabsorption of this particle in average cancels in the ratio N./N, but it effectively shifts thesample closer to thespectrometer with increasing selfabsorption . This effect is rather strong due to correlation between twoefficiencies in coincidence spectra. It is seen that the total corrections increase in a

S. Hlraw è. P. ObWinske / Mulridereeror setup

134

expansion should include an additional NE 213 neutron time-of-flight spectrometer as well as additional 2-3 identical Nal(Tl) spectrometers . Thevalueof the system should be increased by complete event-by-event date acquisition possibilities.

PARTICLE ENERGY (MeV) Fig. 9, Total corrections to be applied in the analysis of singles neutron and y ray spectra as well as y multiplicity measure . meats. The corrections were calculated by the Monte Carlo method (m the Fe sample 0 80 mm/0 77 mm x80 mm with G*(Li), Nal(Tp and NE 213 spectrometers located, respectively, 15, 30 and60 cm from the sample centre. The singles y ray correction curve is valid for Ge(Li) as well as Nal(TI) spectra. The multiplicity correction curves are plotted versus the energy of the process identifying particle.

given example themeasured quantities providing sometime foras much as 20% of themeasured effect. Methods for analysis of raw spectra vary with the spectrometer used. The neutron time-of-flight spectra are converted into the energy ones after a careful subtraction of the elastic peak usually without any special algorithm . TheGe(Li) spectra areanalysed by means of well established statistical methods incorporated into the flexible interactive software package SPECTRA[161 as developed for our computer TPA-70. The raw continuousy ray spectra from the Nal(TI) are unfolded to get thetrue energy spectra. Forthis purpose we use the unfolding code of Simon [17b which expects that a matrix of the response functions and total efficiencies areavailable.

âc Our multidetector setup for (n, xny) studies at 14 MeV neutron incident energies represents a basic version of the system, beingdeveloped to obtain extensive physical information abort processes of y ray emission, neutron emission and their mutual interplay. The systemis rather flexible and we assume that its reasonable

The authors are indebted to Prof. M. Blaiek for his unceasing support of and interest in this work. They gratefully acknowledge valuable advice provided by P. HorvAlh and J. Pivart. Many thanks are due to J. Kr51 for frequent technical assistance . References D. Hermsdorf, A. Meister, S. Sassonof, D. Sceliger, K. Seidel and Fyaez Shahin, Report ZfK-277 (Ü)(Dresden, 1975), also published as IAEA Report INDC (GDR) 2/L. [21 N. Kocherov andJ.J . Schmidt(eds.) IAEA Report INDC (NDS) - 122/L (Vienna, 1980). 131 G. Stengl, M. Uhl and H. Vonach, Nucl . Phys. A290 (1977) 109. 141 P. Grabmayr and P. Hille, in Prm. Inter. Conf. on Neutron Physics and Nuclear Data for Reactors (Harwell, 1978) p. 204. 151 G.B. Hagemann, R. Broda, B. Herskind, M. Ishihara, S. Ogazaand H. Ryde, Nucl. Phys. A245 (1975) 166. 16] R.S. Simon, GS( Darmstadt Reper? 80-2 (May 1980). [7] I .E. Kozyr and G.A. Prokopets, Yad. Fiz. 26 (1977) 927 (in Russian). 18] J. Csikai, in Prm. IAEA Consultants' Meeting on Neutron Source Properties, ed ., K. Okamoto . IAEA Report INDC (NDS) - 114/GT (Vienna, 1980) p. 265 . [91 §, Bederka et al., Manual to the Neutron Generator NG-2, Report IP SAS (Bratislava, 1972), in Slovak. [10] P. Obloinsky and S. Hlavttis, to be published. [1l] J.B. Marion and F.C. Young, Nuclear Reaction Analysis (North-Holland, Amsterdam, 1968) p. 51 . [121 J.H. Neiler and P.R . Bell, in Alpha-, Beta- and GammaRay Spectroscopy, ed., K. Siegbahn (North-Holland, Amsterdam, 1965) vol. 1, p. 284. (131 A. Smith, P. Guenther and R. Sjeblom, Nucl . Instr. and Meth . 140 (1977) 397. 1141 V. Fajer andL. Alvarez, Nucl. Insu. andMeth . 184 (1981) 515. 115] P. Horviith, 1. Turco and M. MorhSe, Preprint JINR P 10-12319 (Dubna, 1979) . lip 116] M. Morh56,Code SPECTRA, SAS Bratislava, 1982), unpublished. [171 R.S. Simon, Code UNFOLD (GSI Darmstadt, 1978), unpublished. 111