Cierjacks, S.

Kopsch, D.

Nebe, J.

Schmalz, G.

Voss, F.

KITopen

March 1971Institut für Angewandte KernphysikProjekt Schneller BrüterKFK 1495A Facility for the Measurement of Fast NeutronFission Cross Sections Using an Intense Cyclotron Neutron SourceS. Cierjacks, D. Kopsch, J. Nebe, G. Schmalz, F. VoßAls Manuskript vervielfältigtFür diesen Bericht behalten wir uns alle Rechte vorGESE LLSCHAFT FüR KERN FORSCHUNG M. B. H.KARLSRUHEKernforschungszentrum KarlsruheMarch 1971 KFK 1495Institut für Angewandte KernphysikProjekt Schneller BrüterA Facility for the Measurement of Fast NeutronFission Cross Section Using an Intense Cyclotron Neutron SourceS. Clerjccks, D. Kopseh; J. Nebe, G. Schmalz, F. VoßGesellschaft für Kernforschung m.b iH, , KarlsruheContribution 11. 23Third Conference on Neutron Cross Sections and Technology, Knoxville, Tennessee;March 15 - 17, 1971ABSTRACTA facility for the study of fission ratios and fission cross sections in the energyrange between 0.5 and 30 MeV is described. For the production of a pulsedneutron beam with a continuous energy spectrum the Karlsruhe isochronouscyclotron is used , The fission fragments are detected by agas scinti lIationcounter arrangement consisting of nine fission detectors in ser ies , For data oc-quisition a CDC-3100 on-line computer is used , The principle features and oper-ating characteristics of the facility are described and first results of a measure-238U .ment on are glven.ZUSAMMENFASSUNGIn diesem Bericht wird eine neu erstellte Flugzeitanordnung zur Bestimmung vonSpaltquerschnitten und Spaltquerschnittsverhältnissen im Energiebereich zwischen0.5 - 30 MeV beschrieben. Einen gepulsten Neutronenstrahl mit kontinuierlichemEnergiespektrum liefert das Karlsruher Isochron Zyklotron. Der Nachweis derSpaltprodukte erfolgte am Ende eines 58 m langen Flugwegs mit Hilfe einer Gas-szintillator-Anordnung. Diese besteht aus neun hintereinander geschaltetenDetektorkammern. Die Datenerfassung geschieht on-line mit Hilfe einer CDC-3100-Rechenanlage. Es werden die charakteristischen Eigenschaften der Meßan-ordnung beschrieben und erste Ergebnisse einer Messung an 238U gezeigt.- 1 -1. INTRODUCTIONMost of the fission ratios and fission cross sections have been measured in thepast with monoenergetic neutrons, usually using Van-de-Graaffacceleratorsand charged perfiele reactions on light nuclei , However / a systematic study ofthe energy dependenceof these quantities over as wide an energy interval asfrom several hundred keV to 10 - 15 MeV which is required for fast reactor cal-culations is very time-consuming.ln addition, most of these measurements cancover oniy part of the total energy region of interest. The use of a pulsed cyclo-tron neutron source , however / allows such measurement with high resolutionover the tota I energy ra nge ina singIe experi menta I run ,This paper describes a facility newly designed for the study of fission ratios andfission crossseetions using the Karlsruhe isochronous cyclotron, Some of the prin-ciple featuresand operating characteristicsare shown.2. DESCRIPTION OF THE TUlIE-OF-FUGHT FiSSiON DEViCEThe experimental equipment consists ofan improved gas scintillation fission detectorarrangement used in conjunction with the Karlsruhe fast neutron time-of-flightfacility described in ref , 1. A simplified top-view of the geometrie arrangementisshown in Fig. 1. The Karlsruhe lsochronous cyc1otron is used to provide a pulsedneutron beam with a continuous energy spectrum in the range from about 0.3to30 MeV and a burst width of 1 nsec. Neutrons are produced by bombardment of anatural uranium target with 45 MeV deuterons from the i nterna I beam of the cyclo-tron , With the new deflection bunching system the time-of-flight facility can beoperated with a pulse recurrence frequency variable from 0 to 200 kHz. To avoidoverlap problems in the Fission cross section measurements, a flight path of 58 mand a neutron pulse repetition rate of 100kHz are chosen ,- 2-A. Fission DetectorFor the detection of fission fragments an arrangement ofnine gas scintillationchambers in series as shown schematically in Fig. 1 is used. In its first formthe fission detector arrangement was suggested by E. pfletschinger andF. Käppeler 2• The scintillation chambers were made of bronze, 0.4 mm thick.Silver and aluminiumwere evaporated onthe walls as reflector materials. Amixture of 85 %argon and 15 %nitrogen continuously flowing through thedetector served as scintillator. Each scintillation chamber [Fig. 3 J is separatedfrom its neighbors by a metallized sampie to obtain optically decoupled systemsand is viewed by one 56 DUVP Valvo photomultiplier tube , A typical pulseheight spectrum obtained with a 252Cf source from one of the fission chambersis shown in Fig. 2. Good separation is observed between the o--penk Icttenuctedbya factor of 50 ] and the distribution of the fission fragments, so that fissionand background events can be weil distinguished. For accurate timing each de-tector was provided with a zero-crossing time mark discriminator 3. The timeresolution of the single chambers was measured using the prompt y -rays end thedl f" . •• h f" f 252Cf Thcorrespon mg ISSIon eventsoc.curnng m t e rtssron 0 • e y-rays weredetected with a plastic scintillation counter , An overall time resolution of about4 ns was measured for all fission chambers. The fission sampies used in the firstrun each consisted of a thin layer of uranium acetate, 70 mm in diameter,[7 sampies containing 99,96 %enriched 238U and 1 sampie of 99,5 % 235UJ•The sampie layers were deposited by electro-spraying on 130 /ug/ cm2 vynsfoils, ~etallized by 20 /ug/ cm2 AI as described by Verdingh and Lauer 4.The preparation of the fissile sampies was performed at the CBNM, Geel/Belgium.This laboratory will also perform an accurate mass determination after the exper i-menrs ,-.3-B. Data AcquisifionA simplified bleck diagramofthe data acquisition system is shown in Fig. 4.In each chamber of. the fission detector Flsslon fragments witha particle energyhigher than 15 MeV are detected from both adjacent Foi ls , A 5 nsec coincidencebetween the pulses from two neighboring fission chambers is registered as a fissionevent. The corresponding eight time-of-flight spectra are separately accumulatedin 2048 channels of a CDC-3100 on-line computerand are sequentially stored ona magnetic disc , The data acquisition system additionally includes a secondbranch for simultaneously measuring the neutron fl.ux. ThisIs accompl ished bytwo different systems: a neutron double time-of-fl ight systemand a proton countertelescope. Both systems are intended to normalize the fission cross section to theweil known n-p scattering cross section. In Fig.4 the double time-of-flight deviceis schematically indicated at the right. The Hme-of-flight spectrum from the scor-terer and the pulse heightspectrum from the time-to-amplitude converter are re-corded simultaneously.3. EXAMPLES OF OPERATIONA f• f h f' . • f 238U" b' f" h dirst measurement 0 t e r rssion cross section 0 IS [ust now emg rrus e •In this measurement, fission samples with o layer of about 1,2 mg/ cm2 of 238Uacetate and 235U acetate were used , Typical time-of-flight spectra for the Fissionevents and the double time-of-flight n-p scattering events are shown in Figs. 50, b ,The data shown in the upper curve result from IV 100h of singles counting in the firstseven fission chambers. These data which represent the fission yields folded withthe neutron spectrum show the well-known steps in the fission cross sectlon of 238 Uneor 25, 15, 7 and 2 MeV. The deviation of the time-of-flight spectrum for then-p scattering events from a smooth curve is mainly due to the flux depression inthe n-p scatterer and the efficiency curve of the neutron detector.Although relatively thick samples were used, it is intended to determine the fission238 • 235 • •cross sectlon of U relative to U es weil as relative to the n-p scattermgcross secfion ,-4-Summarizing, it is shown that the cyclotron technique allows the rapid accu-mulation of fission events with high energy resolution over a wide neutronenergy range. Experimental data have been taken from which fission ratios6f (238U]j 6f ~35U] as weil as absolute fission cross sections will be deter-mined.REFERENCES1• S. Cierjacks,B. Duell i, P. Fortl , D. Kopsch, L. Kropp, M. Lösei, J. Nebe,H. Schweickert, and H. UnseidRev. Sei. Instr. 39,1279 [1968]2. E. Pfletschinger and F. KäppelerNucl. Sei. Eng. 40, 375 [1970]3. H. Bruckmann, E.L. Haase, W. Kluge and L. SchänzlerNucl. lnstr , Meth. 67, 29 [1969]4. V. Verdingh and K.F. LauerNucl. Instr. Meth. 21, 161 [1963]- 5 -np DETECTOR--~SECUNDARY FLIGHT PATH'DETECTOR HOUSE'-'\"'-",----~, \" "PRIMARY FLIGHT PATH ------.I-57m IFig. 1NGeometryof the experimental equipmentFISSION SPECTRUM OF 252Cf15 Q-PARTICLESj~ 10 r\ 1o x51)u,oer:lJJmsz150 200 250 300 350CHANNEL NUMBER4 0 450 500Fig. 2 Pulse height spectrum obtained with a 252Cf sourcePROTON RECOIlFISSIONDETECTOR------LPM DISCRIMINATO~~ rryjDlSCRIMINATO~~----cEM::: l +TIME MARKER+TIME MARKER M -J =~t! ~DISCRIMINATO~h D1SCRIMINATOR jDISCRIMINATORI~~ISCRIMINATORr~ +TIME MARKER +TIME MARKER +TIME MARKER+TIME MARKER PM s~J~>---~D1SCRIMINAT~~ I I'--- +TIME MARKE---.J 4lABEN +1000:1 4096 CHANNEl~ COINCIDENCE DIVIDER PHAIDElAV GEN.OINCIDENC.,-----j DElAV GEN. STOP FROMI CYC. HFOINCIDENCE!---->":DElAV GEN. ! STARTOINCIDENCEIo--- IOR GATE1111DElAV GEN. TTTTT I lABEN .1.11TIMEDElAV GEN. ANAlVZERDElAV GEN.II CODER CLEARICDC- 3100ON- LINECOMPUTERI MAGNETIC D1SC UNIT IFISSILE FOIlcili.PHOTOMULTIPLIER: TUBE 56DUVPi FOIl BACKING/: BRACINGFRONT VIEWGAS ENTRANCE~;llEFOIl~HOTOMULT1PUER" ~~ E'~ 56 OUVP-,QUARTZ WINDOWSCINTIllATION CHAMBER" FOIl BACKINGLLAllON CHAMBERQJTOPVIEWQUARTZ WINDO-.0Fig. 3 Simplified drawing of the Fisslon detector Fig. 4 Block diagram of the data acquisition systemx1QS15FISSION EVENTS.'•300I200x32.',.... .,." -..-.'.- .....• 0 ••• • 0-.0. .0.~ .. .: :...... TIME CHANNEL.. ..e••••• --.JII- 0••- .- "'~ .. 0 11 ••• 11... • ~ •__ ._ 11_ e; .-11 11"" 11 11 11 ....... 11 ._.' ",.. 11....'..'..'.",,---.100...r..,,'Cl' " L I1.0(J)r-z:::J ,.'0U 0.5I I I I...- 20 15 10 5I ~ T0.8 0.6En(MeV)'-lInp SCATTERING EVENTSoI ..".., r' • , _' ...'"'..--'.'. /,..~,. ..-"~Y-"'- ....-'".'.-............"...."""-,........~..(J)r-Z 0.5::::>oU......'''',, .....100I200TI ME CHANNEL~-.---300Fig. 5 Time-of-fl ight spectra obtained with a] fission events, bJ n-p scattering events

A Facility for the Measurement of Fast Neutron Fission Cross Sections Using an Intense Cyclotron Neutron Source

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A Facility for the Measurement of Fast Neutron Fission Cross Sections Using an Intense Cyclotron Neutron Source

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