Audouin, L

Berthier, B

Durán, I

Ferrant, L

Isaev, S

Le Naour, C

Paradela, C

Stéphan, C

Tassan-Got, L

Trubert, D

CERN Document Server

PoS(FNDA2006)040Fission cross sections measured at nTOF with acoincidence methodL. Tassan-Got∗, L. Audouin, B. Berthier, L. Ferrant, S. Isaev, C. Le Naour,C. Stéphan, D. TrubertIPN-Orsay, FranceE-mail: tassango@ipno.in2p3.frI. Duran, C. ParadelaUniversidad de Santiago de Compostela, Spainn-TOF collaboration, CERNThe n-TOF facility at CERN delivers neutrons from 1 eV to 1 GeV. Fission events have beencharacterized by detecting the two ssion fragments in coincidence with PPACs (Parallel PlateAvalanche Counters), which allows a clear signature of ssion among other reactions occurringat high energy. Two ssion detectors 20× 20 cm2 in size were placed on each side of the 8 cmin diameter targets deposited on a thin aluminium backing. To monitor the neutron beam the wellknown ssion cross section of 235U has been used. PPACs are well suited since they can be thinenough not to absorb neutrons. They produce a very fast signal convenient to minimize pile up,and they are equipped for X and Y position determination, allowing to obtain a precise map ofthe 8 cm in diameter neutron beam. An illustration of the technique will be given for the ssioncross sections of 234U and 237Np from 1 eV to 1 GeV.International Workshop on Fast Nuclear Data and Applications - FNDA2006,April 3-6,2006,University of Cape Town, South Africa∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(FNDA2006)040Fission cross sections. . . L. Tassan-GotContents1. Introduction 22. The n-TOF facility 23. The detection system 34. Signal treatment 45. Results 56. Conclusion 61. IntroductionThe foreseen possibility of incinerating nuclear wastes requires reactors loaded with higherconcentration of minor actinides. In such conditions there is a need of a better accuracy on theirneutron cross sections than presently achieved. In addition if reactors based on the thorium cycleare to be developed, the neutronic properties of the main isotopes involved in this cycle should alsobe known at a better accuracy. We measured the fission cross section of 237Np which is a possiblecandidate for incineration, and 232Th, 233U, 234U which are major isotopes in the thorium cycle.An accelerator driven system (ADS) is a possible and efficient reactor for incineration of longlived actinides. In such reactors the fuel assembly can suffer high energy tails of the neutron flux.This implies that the fission cross sections should be measured in a large energy range up to thespallation domain.In a measurement condition, at high energy above 20 MeV, neutron collisions can produce notonly fission, but also evaporation residues from the target actinide and from lighter elements con-tained in target backings and electrodes of detectors.Those residual nuclei recoil due to momentumtransfer, and they produce signals which can be wrongly ascribed to low amplitude fission signals.Detecting the two fragments in coincidence with PPACs (parallel plate avalanche counters) allowsa clean selection of the fission reactions.2. The n-TOF facilityThe n-TOF facility at CERN is a pulsed neutron source based on spallation of 20 GeV protonson a lead target[1]. A fraction of the produced neutrons enters a long pipe and the beam is shapedwith a collimator system. The long flight path (185m) allows an accurate time of flight measure-ment due to the short spilling time of the protons: 7ns. The main characteristics of the facility isthe large energy range of the neutron flux, from thermal region to a few GeV, and the tremendousinstantaneous flux with a low duty cycle. This last aspect is fully exploited in fission by using flashanalog to digital converters (FADC) which allow pile up resolution with limitation of data storagefor radioactive targets.2PoS(FNDA2006)040Fission cross sections. . . L. Tassan-GotFigure 1: Detection of the 2 ssion fragments by2 PPACs enclosing the target. The X and Y locali-sation in each detector allows a geometrical recon-struction of the ssion event.Figure 2: Stack of detectors and targets : 10 detec-tors interleaved with 9 targets.3. The detection systemThe fission fragments are detected in coincidence by 2 PPACs enclosing the emitting target assketched in figure 1. The entire system is filled with C3F8 gas at 3 mbar which is ionized along thefission path. A bidimensional localisation is achieved on each counter by dividing the cathodes in2 mm wide strips which are connected to the intermediate points of a delay line. The localisation isderived from the delays of the signals collected from the two ends of the delay line, as respect to thefast anode signal. Using the back to back characteristic of fission at low energy the two positionsallow a full determination of the emitting point on the target and the fission angle. Although thefolding angle becomes lower than 180◦ at high energy (above 20 MeV), due to the recoil impartedby the momentum transfer, the above approximation is still accurate for the determination of thefission trajectory.The full setup is made of a stack of 9 targets interleaved with 10 detectors as shown in figure 2.Two targets are made of 235U and 238U to determine the neutron flux and its energy dependence.More precisely the setup does not give access to absolute cross sections but to fission cross sectionratios between targets. Therefore to derive absolute cross sections one has to know the cross sectionof a reference isotope. We used 235U and 238U with cross sections taken from ENDF-B6 up to20 MeV, JENDL-HE [2] between 20 and 200 MeV, and a constant extrapolation beyond this energy:1.42 b for 235U and 1.28 b for 238U.The request of detecting both fission fragments imposes severe constraints on the thickness ofmaterials. In particular the electrodes of PPACs, the target layers and target backings have to beas thin as possible. Each electrode is made of a 1.5 µm mylar coated with 50 µg/cm2 aluminiumas continuous planes for anodes and strips for cathodes. The targets consist of a layer of actinideoxide (0.3 mg/cm2 thickness of actinide) electrodeposited as a disk of 8 cm in diameter, on a thinaluminium foil, 2 µm thick [3]. Overall the total amount of material is 2.7 mg/cm2 of actinide,6 mg/cm2 of mylar and 7 mg/cm2 of aluminium. In addition two thick windows of 125 µm ofkapton are inserted for safety reasons. The total material thickness is small enough to avoid any selfshielding correction or beam attenuation, and the fraction of scattered neutrons is very small [4].For trajectories orthogonal to the targets and detectors the fragments always reach the twodetecting gaps of each surrounding detector, and the detection efficiency is 1. For large angles3PoS(FNDA2006)040Fission cross sections. . . L. Tassan-Gotcorresponding to tilted trajectories, the thickness of crossed materials increases and some fissionfragments are stopped before reaching the second detecting gap, especially when the heavy frag-ment flies through the target backing [5]. In this case the detection efficiency drops below 1. Thiseffects becomes important beyond 45 ◦. Contrary to the case of cross section measurements withionisation chambers where a full angular efficiency is assumed, we know in our case that thisefficiency is reduced, but we measure the fission angle to correct for the efficiency drop.4. Signal treatmentEach detector delivers 5 signals: 1 fast anode signal and 4 signals from the ends of the twodelay lines. The 50 signals are recorded by FADCs running at 500 Ms/s.5000 5200 5400 5600 5800 6000 6200t (ns)160180200220240AmplitudeAnodedelay line (right)delay line (left)(a) Original recorded signal5000 5200 5400 5600 5800 6000 6200t (ns)-30-20-100102030AmplitudeAnodedelay line (right)delay line (left)(b) After convolution with the derivative of aHanning’s filterFigure 3: Anode signal (black) and the corresponding signals from the horizontal delay line, right (red) andleft (green). The rst peak is the initial muon ash.Figure 3(a) shows the anode signal and the related signals at the ends of the horizontal delayline. The first negative peak visible in the anode signal is the initial flash, corresponding to ultrarel-ativistic muons travelling at a velocity very close to speed of light. This peak is used as a referencefor the calibration of the time of flight. The five other peaks of high amplitude are fission events,produced by neutrons from 150 MeV to 1 GeV. They show up also in the delay line signals. Theassignment of localisation signals to the associated anode ones is based on the sum of the delayswhich should remain constant and equal to the total delay of the line. It can be noticed that themuon flash induces a waving of the baseline. This is treated by applying a convolution with thederivative of a Hanning’s filter which is equivalent to a derivative with an attenuation of the highfrequency components. The result is displayed in figure 3(b), showing that a flat baseline is restoredand that the peak recognition and characterisation become much easier.Another advantage of this numerical treatment is the shielding against parasitic signals. Fig-ure 4 shows how the noise generated by sparkling of the detector can be suppressed and the signalenhanced, by applying the convolution which favors the frequency components of the signal.4PoS(FNDA2006)040Fission cross sections. . . L. Tassan-Got5500 6000 6500 7000 7500 8000time (ns)0100200300amplitude5500 6000 6500 7000 7500 8000time (ns)-100-50050100amplitudeFigure 4: Left: original signal where a noise is induced by detector sparkling (beyond 7000 ns). Right:when the convolution is applied the noise is suppressed whereas the signal at 7600 ns clearly shows up.5. ResultsThe trajectory reconstruction is a specific feature of our detection system, and it is essential forthe correction of the angular dependence of the efficiency. One can check its validity by lookingat the reconstructed map of emitting points as shown in figure 5 in the left part. This can becompared with the density map of the target as represented in figure 5 in the right part, as obtainedby scanning the local alpha radioactivity. The detailed geometry of the defect on the right side iswell reproduced and the disk shape as well. However it can be noticed that the neutron flux is notuniform and it is strongly depleted in the lower region of the disk.-50 0 50-50050XmmYmm50-50 050-500XmmYmmFigure 5: Left: mapping of the ssion rate for a 234U target. Right:mapping of the α-radioactivity rate for the same target.-50 0 50-50050XmmYmmFigure 6: Mapping of the s-sion rate for the large bismuthtarget. The line crossing indi-cates the geometrical center ofthe target.This feature is confirmed when looking at the bismuth target which extends well beyond theexpected beam size, as illustrated by figure 6. It can be clearly seen that the beam spot is shiftedupward, due to geometrical misalignment. This pitfall has no harmful effect because it can be takeninto account through the emitting point determination.As already mentioned the angular efficiency drops when the fission angle increases and thesetup becomes insensitive to fission occurring at angles higher than 60◦. As a consequence theangular distribution of fission fragments for the measured isotopes should be known to calibratethe angular efficiency and to extract the cross sections. For this purpose we used the known ex-perimental results up to 20 MeV. Beyond this energy where the measurements are very scarce weapplied a constant decrease of the anisotropy, down to full isotropy at 100 MeV.5PoS(FNDA2006)040Fission cross sections. . . L. Tassan-GotThe results presented hereafter are obtained from the fission rate referred to that of 235U.2.65 2.7 2.75log( E eV)0.05.010.015.0σ f (barn)nTOFENDF-B6234U6 7 8 9log( E eV)0.01.02.03.0σ f (barn)nTOFENDF-B6234UFigure 7: Fission cross section of 234U compared with the evaluateddata from ENDF-B6. Left: in a portion of the resonance region. Right:around and above the ssion threshold.1.45 1.50 1.55 1.60 1.65log( E eV)05101520σ f (barn)nTOFENDF-B6237NpFigure 8: Fission cross sectionof 237Np in a portion of the reso-nance region compared with theevaluated ENDF-B6Figure 7 is an illustration of the obtained results in the case of 234U. In the resonance regionthe nTOF measurement is in agreement with the ENDF-B6 data. This is also the case aroundthe fission threshold and above. The nTOF measurement extends to much higher energies in thespallation domain where no previous measurement exists. The accuracy of the measurement ismainly driven by the uncertainty on the angular efficiency. The latter has been estimated by thedispersion of the results for different targets of the same isotope. It amounts to about 10 % instandard deviation.Figure 8 shows the case of 237Np in the resonance domain. It can be checked that the shapeof resonances is well reproduced, in particular the shoulder on the resonance at 38.9 eV. Howevera much larger cross section is found if compared to the evaluated data. This discrepancy is specificof the resonance region, whereas a good agreement is found in the threshold region.6. ConclusionThe fission cross section of actinides has been measured by detecting both fragments in coin-cidence. This allowed to clearly disentangle fission from other mechanisms at high energy, and themeasurement covers the large energy range available at nTOF: 0.7 eV to 1 GeV. All measurementsare relative to 235U and 238U, this requires the knowledge of their cross sections. The estimation ofthe angular efficiency relies on existing data which are limited generally to 20 MeV.References[1] Y. Kadi, White neutron sources - RB02, these proceedings[2] A. Y. Konobeyev et al., JAERI Research Report 2002-28[3] L. Tassan-Got et al., Fission of actinides induced by neutrons at nTOF, ND2004 conference, Santa Fe,New Mexico USA, Sept. 27- Oct. 1, 2004, AIP Conference Proceedings, Vol 769, (2005), 1529[4] L. Ferrant, Mesures de sections efcaces de ssion induite par neutrons sur les actinides du cycle duthorium à nTOF, PhD thesis, UniversitØ Paris-Sud, Orsay, September 7, 2005[5] C. Paradela, Measurement of the 234U(n,f) cross section with PPAC detectors at the nTOF facility,PhD thesis, Santiago de Compostela, October 6, 20056

Fission Cross Sections Measured at nTOF With A Coincidence Method

Fission Cross Sections Measured at nTOF With A Coincidence Method

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