International Conference on Nuclear Data for Science and Technology 2007DOI: 10.1051/ndata:07510

The 234U neutron capture cross section measurement at the n TOF facility

C. Lampoudis1,2 ,a, U. Abbondanno3, G. Aerts2, H. Alvarez4, F. Alvarez-Velarde5, S. Andriamonje2, J. Andrzejewski6,P. Assimakopoulos†7, L. Audouin8, G. Badurek9, P. Baumann10, F. Becvar11, E. Berthoumieux2, F. Calvino12,M. Calviani13,14, D. Cano-Ott5, R. Capote15,16, C. Carrapico2,17, P. Cennini18, V. Chepel19, E. Chiaveri18, N. Colonna20,G. Cortes21, A. Couture22, J. Cox22, M. Dahlfors18, S. David8, I. Dillmann23, C. Domingo-Pardo23,24, W. Dridi2, I. Duran4,C. Eleftheriadis1, M. Embid-Segura5, L. Ferrant†8, A. Ferrari18, R. Ferreira-Marques19, K. Fujii3, W. Furman25, I. Goncalves19,E. Gonzalez-Romero5, F. Gramegna13, C. Guerrero5, F. Gunsing2, B. Haas26, R. Haight27, M. Heil23, A. Herrera-Martinez18,M. Igashira28, E. Jericha9, F. Kappeler23, Y. Kadi18, D. Karadimos7, D. Karamanis7, M. Kerveno10, P. Koehler29,E. Kossionides30, M. Krticka11, H. Leeb9, A. Lindote19, I. Lopes19, M. Lozano16, S. Lukic10, J. Marganiec6, S. Marrone20,T. Martınez5, C. Massimi31, P. Mastinu13, A. Mengoni15,18, P.M. Milazzo3, C. Moreau3, M. Mosconi23, F. Neves19,H. Oberhummer9, S. O’Brien22, J. Pancin2, C. Papachristodoulou7, C. Papadopoulos32, C. Paradela4, N. Patronis7,A. Pavlik33, P. Pavlopoulos34, L. Perrot2, M.T. Pigni9, R. Plag23, A. Plompen35, A. Plukis2, A. Poch21, J. Praena13,C. Pretel21, J. Quesada16, T. Rauscher36, R. Reifarth27, C. Rubbia37, G. Rudolf10, P. Rullhusen35, J. Salgado17, C. Santos17,L. Sarchiapone18, I. Savvidis1, C. Stephan8, G. Tagliente20, J.L. Tain24, L. Tassan-Got8, L. Tavora17, R. Terlizzi20,G. Vannini31, P. Vaz17, A. Ventura38, D. Villamarin5, M.C. Vincente5, V. Vlachoudis18, R. Vlastou32, F. Voss23,S. Walter23, M. Wiescher22, and K. Wisshak23

The n TOF Collaboration (www.cern.ch/ntof)

1Aristotle University of Thessaloniki, Greece – 2CEA/Saclay-DSM/DAPNIA, Gif-sur-Yvette, France – 3Istituto Nazionale di Fisica Nucleare,Trieste, Italy – 4Universidade de Santiago de Compostela, Spain – 5Centro de Investigaciones Energeticas Medioambientales y Tecnologicas,Madrid, Spain – 6University of Lodz, Lodz, Poland – 7University of Ioannina, Greece – 8Centre National de la Recherche Scientifique/IN2P3-IPN, Orsay, France – 9Atominstitut der Osterreichischen Universitaten, Technische Universitat Wien, Austria – 10Centre National de laRecherche Scientifique/IN2P3-IReS, Strasbourg, France – 11Charles University, Prague, Czech Republic – 12Universidad Politecnica deMadrid, Spain – 13Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy – 14Dipartimento di Fisica, Universita diPadova, Italy – 15International Atomic Energy Agency (IAEA), Nuclear Data Section, Vienna, Austria – 16Universidad de Sevilla, Spain –17Instituto Tecnologico e Nuclear (ITN), Lisbon, Portugal – 18CERN, Geneva, Switzerland – 19LIP-Coimbra & Departamento de Fisica daUniversidade de Coimbra, Portugal – 20Istituto Nazionale di Fisica Nucleare, Bari, Italy – 21Universitat Politecnica de Catalunya, Barcelona,Spain – 22University of Notre Dame, Notre Dame, USA – 23Forschungszentrum Karlsruhe GmbH (FZK), Institut fur Kernphysik, Germany– 24Instituto de Fısica Corpuscular, CSIC-Universidad de Valencia, Spain – 25Joint Institute for Nuclear Research, Frank Laboratory ofNeutron Physics, Dubna, Russia – 26Centre National de la Recherche Scientifique/IN2P3-CENBG, Bordeaux, France – 27Los Alamos NationalLaboratory, New Mexico, USA – 28Tokyo Institute of Technology, Tokyo, Japan – 29Oak Ridge National Laboratory, Physics Division, OakRidge, USA – 30NCSR, Athens, Greece – 31Dipartimento di Fisica, Universita di Bologna, and Sezione INFN di Bologna, Italy – 32NationalTechnical University of Athens, Greece – 33Institut fur Isotopenforschung und Kernphysik, Universitat Wien, Austria – 34Pole UniversitaireLeonard de Vinci, Paris La Defense, France – 35CEC-JRC-IRMM, Geel, Belgium – 36Department of Physics-University of Basel, Switzerland– 37Universita degli Studi Pavia, Pavia, Italy – 38ENEA, Bologna, Italy

Abstract. The neutron capture cross-section of 234U has been measured for energies from thermal up to the keVregion in the neutron time-of-flight facility n TOF, based on a spallation source located at CERN. A 4π BaF2 arraycomposed of 40 crystals, placed at a distance of 184.9 m from the neutron source, was employed as a total absorptioncalorimeter (TAC) for detection of the prompt γ-ray cascade from capture events in the sample. This text describes theexperimental setup, all necessary steps followed during the data analysis procedure. Results are presented in the formof R-matrix resonance parameters from fits with the SAMMY code and compared to the evaluated data of ENDF inthe relevant energy region, indicating the good performance of the n TOF facility and the TAC.

1 Introduction

At an international level several issues accompany the long-term development of nuclear science and its applications:nuclear waste management, new reactor design, increasingsafety requirements, public acceptance, etc. This has trig-gered many new R&D activities, such as Accelerator Driven

a Presenting author, e-mail: christos.lampoudis@cern.ch

Systems or next generation reactor concepts and has also en-hanced the ongoing effort to expand and improve the existingnuclear data. Among other measurements, the neutron capturecross section determination for several isotopes related to thethorium based nuclear fuel cycle is of special interest. The234U measurements were performed at the n TOF facility [1](CERN) using TAC as the detection device. The neutron beamcharacteristics [2,3] were determined after applying availabledetection techniques, in order to have an accurate knowledgeof the beam portion that interacts with the sample. The n TOF

©2008 CEA, published by EDP Sciences

596 International Conference on Nuclear Data for Science and Technology 2007

Data Acquisition System [4] was designed on the basis of highperformance flash ADCs, making it a hardware dead-time freesystem capable of recording the full detector event series.

The capture yield is the primary quantity to be determinedfrom the procedure. The accuracy of this process depends onmany parameters such as: detection efficiency, canning effect,detector’s neutron sensitivity, etc. Thus numerous additionalmeasurements had to be done in order to estimate theseparameters and all background components related to the dataanalysis procedure.

2 Experimental setup

The most important features concerning the facility, the detec-tion apparatus and the data acquisition system are listed below,giving special attention to those aspects relevant to our dataanalysis procedure.

The main characteristics of the proton beam, the leadtarget, and the neutron beam are underlined below. The protonbeam is supplied by the CERN Proton Synchrotron (PS) andis carried up to the spallation target. Features:

– 20 GeV/c momentum corresponding to the maximum at-tainable energy with a repetition frequency of 1 pulseevery 2.4 s,

– single proton bunch intensity of 7×1012 protons or 4×1012

protons depending on the PS operating status.

Several devices are used to provide additional informationabout the beam.

The spallation mechanism relies on the highly energeticproton beam hitting on the heavy element target. The neutronproduction efficiency for one 20 GeV/c proton impinging on alead target is approximately 300 neutrons. The n TOF neutronsource is a lead block with a total volume of 80 × 80 × 60 cm3

[3]. In order to reduce the neutron beam contamination fromγ-rays and high energy charged particles produced during thespallation reactions, protons hit on the target at an angle of10 degrees respect to the TOF tube axis. A 5 cm water layer issurrounding the target as a moderator and a cooling system.

The neutron beam delivers approximately 106 neutrons perpulse integrated over the whole beam surface, at the sampleposition. The generated neutrons travel nearly 186 m inside avacuum tube divided in several sections, with a progressivelyreduced diameter. The pressure in the vacuum tube is less than1 mbar. Two collimators are placed at 136 m and 175 m toshape the neutron beam that reaches the Experimental Area(EA). A “sweeping magnet” is used to cut down chargedparticles contaminating the neutron beam. The energy rangeis wide, starting from thermal up to several hundreds of MeV.

The 4π assembly is formed by 12 pentagonal and30 hexagonal shaped crystals. However, the total number ofcrystals at n TOF was reduced to 40 in order to have entranceand exit points of the neutron beam tube. Those two removedwere of hexagonal shape. Detection efficiency for captureevents is nearly 100%. The crystals’ energy calibration andresolution were monitored on a regular basis, using properγ-radioactive sources (137Cs, 60Co, 88Y and Pu+13C). Themeasured relative resolution values (FWHM) for the 662 keVand 6.2 MeV lines were 12% and 7% respectively.

Fig. 1. An hemisphere drawing of the TAC sphere as implemented inthe Monte Carlo simulation.

Fig. 2. TAC sphere open. Shown, the neutron beam tube entrance unitalong with the beam direction and the neutron absorber.

One of the main issues during neutron capture mea-surements is the detectors’ neutron sensitivity. The effect isamplified in the region where σn > σγ. In order to reduce theeffect a C12H20O4(6Li)2 neutron absorber was employed in thecenter of the TAC sphere, surrounding the sample. Figure 2shows the arrangement with the neutron absorber.

3 Data analysis

The main aim of the data analysis procedure is the captureyield determination, Y(En), given by the following formula

Y(En) =S (En) − B(En)Φ(En) · εTAC(En)

(1)

where

S (En) is the number of events detected in the TACB(En) is the background

C. Lampoudis et al.: The 234U neutron capture cross section measurement at the n TOF facility 597

Energy (eV)1 10 210 310 410

coun

ts(a

.u)

310

410

510

U234 + background

Fig. 3. Capture events (arbitrary units) of the 234U (black) along withthe sample’s activity (blue), empty Ti canning (red) and empty frame(green).

Etot (keV)0 2000 4000 6000 8000 10000 12000 14000 16000

coun

ts

310

410

510

610

710

TAC response for U234 (all conditions)

no conditiongmult>1gmult>2nclust>1nclust>2

no conditiongmult>1gmult>2nclust>1nclust>2

no conditiongmult>1gmult>2nclust>1nclust>2

no conditiongmult>1gmult>2nclust>1nclust>2

Fig. 4. TAC energy response for different conditions.

Φ(En) is the neutron beam flux andεTAC(En) is the “global” efficiency of the TAC.

All different sources of background had to be identifiedand then analyzed regarding their influence on the capturemeasurements. These are the following:

– as mentioned earlier, background due to neutrons scatteredat the samples and captured in the detector materials:mainly in the BaF2 and with lower probability, in lessabundant surrounding structural materials.

– background due to gamma rays and neutrons emitted inthe fission process at the sample. Negligible in our case(σ f � σγ).

– background due to in-beam gamma rays present in then TOF neutron beam. These are produced by neutronscaptured in the hydrogen of the moderator water of thelead spallation target.

– the sample’s radioactivity.– the ambient background.

Several measurements were performed in order to deter-mine each of the previous (fig. 3):

– measuring under the status “beam off”, having just thetarget inside TAC (sample activity)

– having everything switched off (ambient measurement)– Carbon sample measurements (TAC neutron sensitivity)– Lead target measurements (in-beam gammas effect)– having inside TAC an empty Ti canning.

0.0001

0.001

0.01

0.1

1

cap

ture

yie

ld

1.0 2.0 3.0 4.0 5.0

neutron energy (eV)

-10

0

10

resi

du

al

Fig. 5. Capture yield (black) along with n TOF SAMMY fit (red) andENDF /B-VI.8 (green) for the first resonance of 234U.

1e-05

0.0001

0.001

0.01

0.1

cap

ture

yie

ld

20.0 40.0 60.0 80.0 100.0neutron energy (eV)

-10

0

10

resi

du

al

Fig. 6. Comparison between experimental data (black), SAMMY fit(red) and ENDF (green) for the neutron energy region up to 100 eV.

Additionally during the analysis, one can apply differentselection criteria rules in order to improve signal to back-ground ratio. Defining neighboring crystals as one cluster, wedemanded to register events that were detected in two clustersor more (Ncluster > 1). This criteria proved to be the best inour case.

The neutron beam flux was estimated by means of MonteCarlo simulations [5] and measured using different availabletechniques. The neutron flux that was considered in ouranalysis is the one called n TOF standard flux [3].

The last term of equation (1), εT AC(En) is called “global”efficiency because it corresponds to the detector’s overallefficiency. It is equal to the product of three factors:

Table 1. Resonance parameters for the 1st resonance of 234U.

En (eV) Γγ (meV) Γn (meV)

ENDF/B-VI.8 5.16 40 3.92JENDL 3.3 5.16 26 3.92n TOF 5.156 38.8 3.71

598 International Conference on Nuclear Data for Science and Technology 2007

Entries 117Mean 0.0005057RMS 0.000671

/ ndf 2χ 18.69 / 114p0 2.6ℜ± 135.5 p1 0.000036ℜ± 0.001025

gGn_0 threshold (eV)-510 -410 -310 -210

Nu

mb

er o

f L

evel

s

0

20

40

60

80

100

120

Entries 117Mean 0.0005057RMS 0.000671

/ ndf 2χ 18.69 / 114p0 2.6ℜ± 135.5 p1 0.000036ℜ± 0.001025

Integrated number of levels vs Porter-Thomas prediction

Fig. 7. Number of levels along with the Porter-Thomas distributionfit.

– εγ is the detector’s efficiency, which is close to unity.– εbeam the beam portion that interacts with the target. This

was found to be equal to 0.1987 from simulations.– εcondition a proper coefficient applied, corresponding to the

selection criteria used during the analysis process (e.g.,energy threshold, multiplicity cuts, etc.). Figure 4 showsthe energy response of TAC for various conditions applied,for 234U.

4 Results

For the fitting process of our capture yield, we used theSAMMY software package [6] using ENDF/B-VI.8 resonanceparameter set. Prior to the fitting procedure, our capture yieldwas normalized in order to reproduce the first resonance of234U. The comparison between our fit (red) and ENDF/B-VI.8(green), for the first resonance is shown in figure 5, whilefigure 6 displays the fit for neutron energy up to 100 eV. Inthis second plot one can notice a resonance at 67.5 eV, notlisted in ENDF/B-VI.8.

The average radiation width was found to be equal to40.3 ± 2.3 meV, considering resonances up to the energy of200 eV. The precise number of resonances for the resolved

resonance region was determined after applying the Porter-Thomas distribution on our experimental data (fig. 7). Usingthat value (135.4 ± 2.6) we calculated the average spacinglevel, 〈D0〉 = 11.04 ± 0.2 eV while Mughabghab gives 10.92± 0.47 eV [7]. The reduced averaged neutron width obtainedfrom this fit is:

⟨Γ0

n

⟩= (1.02 ± 0.03) × 10−3 eV.

5 Conclusions

Measurements of the neutron capture in 234U were performedat the n TOF facility (CERN), using the Total AbsorptionCalorimeter as the detection system. Resonance parametersfor the resolved resonance energy region were determinedafter following the fitting process with SAMMY. Calculationsof the average radiation width and mean level spacing validatethe good performance of the experimental facility and theadequacy of the data reduction procedure.

We acknowledge the CERN participation in the n TOF project.This work has been supported by the European Commission’s 5th

Framework Program under contract FIKW-CT-2000-00107.

References

1. The n TOF Collaboration, Tech. Rep. CERN/INTC 2000-018,CERN, Geneve (2000).

2. The n TOF Collaboration, Status Report CERN/INTC 2001-021,CERN, Geneve (2001).

3. The n TOF Collaboration, Performance Report CERN/INTC2002-037, CERN, Geneve (2002).

4. U. Abbondanno, the n TOF Collaboration, Nucl. Instrum. Meth.A 538, 692 (2005).

5. The n TOF Collaboration, Tech. Rep. CERN/INTC 2000-016,CERN, Geneve (2000).

6. N. Larson, Tech. Rep. ORNL/TM-9179/R7/Draft, Oak RidgeNational Laboratory (2006).

7. S. Mughabghab, Atlas of Neutron Resonances, Vol. 1, 5th edn.(Elsevier, 2006).