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International Conference on Nuclear Data for Science and Technology 2007
DOI: 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. Álvarez4, F. Álvarez-Velarde5, S. Andriamonje2, J. Andrzejewski6,
P. Assimakopoulos†7, L. Audouin8, G. Badurek9, P. Baumann10, F. Bečvář11, E. Berthoumieux2, F. Calviño12,
M. Calviani13,14, D. Cano-Ott5, R. Capote15,16, C. Carrapiço2,17, 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. González-Romero5, F. Gramegna13, C. Guerrero5, F. Gunsing2, B. Haas26, R. Haight27, M. Heil23, A. Herrera-Martinez18,
M. Igashira28, E. Jericha9, F. Käppeler23, Y. Kadi18, D. Karadimos7, D. Karamanis7, M. Kerveno10, P. Koehler29,
E. Kossionides30, M. Krtička11, H. Leeb9, A. Lindote19, I. Lopes19, M. Lozano16, S. Lukic10, J. Marganiec6, S. Marrone20,
T. Martı́nez5, 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 Österreichischen Universitäten, Technische Universität Wien, Austria – 10Centre National de la
Recherche Scientifique/IN2P3-IReS, Strasbourg, France – 11Charles University, Prague, Czech Republic – 12Universidad Politecnica de
Madrid, Spain – 13Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy – 14Dipartimento di Fisica, Università di
Padova, Italy – 15International Atomic Energy Agency (IAEA), Nuclear Data Section, Vienna, Austria – 16Universidad de Sevilla, Spain –
17Instituto Tecnológico e Nuclear (ITN), Lisbon, Portugal – 18CERN, Geneva, Switzerland – 19LIP-Coimbra & Departamento de Fisica da
Universidade 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 für Kernphysik, Germany
– 24Instituto de Fı́sica Corpuscular, CSIC-Universidad de Valencia, Spain – 25Joint Institute for Nuclear Research, Frank Laboratory of
Neutron Physics, Dubna, Russia – 26Centre National de la Recherche Scientifique/IN2P3-CENBG, Bordeaux, France – 27Los Alamos National
Laboratory, New Mexico, USA – 28Tokyo Institute of Technology, Tokyo, Japan – 29Oak Ridge National Laboratory, Physics Division, Oak
Ridge, USA – 30NCSR, Athens, Greece – 31Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy – 32National
Technical University of Athens, Greece – 33Institut für Isotopenforschung und Kernphysik, Universität Wien, Austria – 34Pôle Universitaire
Léonard de Vinci, Paris La Défense, France – 35CEC-JRC-IRMM, Geel, Belgium – 36Department of Physics-University of Basel, Switzerland
– 37Università 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 keV
region in the neutron time-of-flight facility n TOF, based on a spallation source located at CERN. A 4π BaF2 array
composed of 40 crystals, placed at a distance of 184.9 m from the neutron source, was employed as a total absorption
calorimeter (TAC) for detection of the prompt γ-ray cascade from capture events in the sample. This text describes the
experimental setup, all necessary steps followed during the data analysis procedure. Results are presented in the form
of R-matrix resonance parameters from fits with the SAMMY code and compared to the evaluated data of ENDF in
the 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, increasing
safety 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 existing
nuclear data. Among other measurements, the neutron capture
cross section determination for several isotopes related to the
thorium based nuclear fuel cycle is of special interest. The
234U measurements were performed at the n TOF facility [1]
(CERN) using TAC as the detection device. The neutron beam
characteristics [2,3] were determined after applying available
detection techniques, in order to have an accurate knowledge
of the beam portion that interacts with the sample. The n TOF
©2008 CEA, published by EDP Sciences
Article available at http://nd2007.edpsciences.org or http://dx.doi.org/10.1051/ndata:07510
596 International Conference on Nuclear Data for Science and Technology 2007
Data Acquisition System [4] was designed on the basis of high
performance flash ADCs, making it a hardware dead-time free
system capable of recording the full detector event series.
The capture yield is the primary quantity to be determined
from the procedure. The accuracy of this process depends on
many parameters such as: detection efficiency, canning effect,
detector’s neutron sensitivity, etc. Thus numerous additional
measurements had to be done in order to estimate these
parameters and all background components related to the data
analysis 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 data
analysis procedure.
The main characteristics of the proton beam, the lead
target, and the neutron beam are underlined below. The proton
beam is supplied by the CERN Proton Synchrotron (PS) and
is carried up to the spallation target. Features:
– 20 GeV/c momentum corresponding to the maximum at-
tainable energy with a repetition frequency of 1 pulse
every 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 information
about the beam.
The spallation mechanism relies on the highly energetic
proton beam hitting on the heavy element target. The neutron
production efficiency for one 20 GeV/c proton impinging on a
lead target is approximately 300 neutrons. The n TOF neutron
source is a lead block with a total volume of 80 × 80 × 60 cm3
[3]. In order to reduce the neutron beam contamination from
γ-rays and high energy charged particles produced during the
spallation reactions, protons hit on the target at an angle of
10 degrees respect to the TOF tube axis. A 5 cm water layer is
surrounding the target as a moderator and a cooling system.
The neutron beam delivers approximately 106 neutrons per
pulse integrated over the whole beam surface, at the sample
position. The generated neutrons travel nearly 186 m inside a
vacuum tube divided in several sections, with a progressively
reduced diameter. The pressure in the vacuum tube is less than
1 mbar. Two collimators are placed at 136 m and 175 m to
shape the neutron beam that reaches the Experimental Area
(EA). A “sweeping magnet” is used to cut down charged
particles contaminating the neutron beam. The energy range
is wide, starting from thermal up to several hundreds of MeV.
The 4π assembly is formed by 12 pentagonal and
30 hexagonal shaped crystals. However, the total number of
crystals at n TOF was reduced to 40 in order to have entrance
and exit points of the neutron beam tube. Those two removed
were of hexagonal shape. Detection efficiency for capture
events is nearly 100%. The crystals’ energy calibration and
resolution were monitored on a regular basis, using proper
γ-radioactive sources (137Cs, 60Co, 88Y and Pu+13C). The
measured relative resolution values (FWHM) for the 662 keV
and 6.2 MeV lines were 12% and 7% respectively.
Fig. 1. An hemisphere drawing of the TAC sphere as implemented in
the Monte Carlo simulation.
Fig. 2. TAC sphere open. Shown, the neutron beam tube entrance unit
along 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 is
amplified in the region where σn > σγ. In order to reduce the
effect a C12H20O4(6Li)2 neutron absorber was employed in the
center of the TAC sphere, surrounding the sample. Figure 2
shows the arrangement with the neutron absorber.
3 Data analysis
The main aim of the data analysis procedure is the capture
yield 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 TAC
B(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
co
un
ts
(a
.u
)
310
410
510
U234 + background
Fig. 3. Capture events (arbitrary units) of the 234U (black) along with
the sample’s activity (blue), empty Ti canning (red) and empty frame
(green).
Etot (keV)
0 2000 4000 6000 8000 10000 12000 14000 16000
co
un
ts
310
410
510
610
710
TAC response for U234 (all conditions)
no condition
gmult>1
gmult>2
nclust>1
nclust>2
Fig. 4. TAC energy response for different conditions.
Φ(En) is the neutron beam flux and
εTAC(En) is the “global” efficiency of the TAC.
All different sources of background had to be identified
and then analyzed regarding their influence on the capture
measurements. These are the following:
– as mentioned earlier, background due to neutrons scattered
at the samples and captured in the detector materials:
mainly in the BaF2 and with lower probability, in less
abundant surrounding structural materials.
– background due to gamma rays and neutrons emitted in
the fission process at the sample. Negligible in our case
(σ f  σγ).
– background due to in-beam gamma rays present in the
n TOF neutron beam. These are produced by neutrons
captured in the hydrogen of the moderator water of the
lead 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 the
target 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
ca
p
tu
re
 y
ie
ld
1.0 2.0 3.0 4.0 5.0
neutron energy (eV)
-10
0
10
re
si
d
u
al
Fig. 5. Capture yield (black) along with n TOF SAMMY fit (red) and
ENDF /B-VI.8 (green) for the first resonance of 234U.
1e-05
0.0001
0.001
0.01
0.1
ca
p
tu
re
 y
ie
ld
20.0 40.0 60.0 80.0 100.0
neutron energy (eV)
-10
0
10
re
si
d
u
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 different
selection criteria rules in order to improve signal to back-
ground ratio. Defining neighboring crystals as one cluster, we
demanded to register events that were detected in two clusters
or more (Ncluster > 1). This criteria proved to be the best in
our case.
The neutron beam flux was estimated by means of Monte
Carlo simulations [5] and measured using different available
techniques. The neutron flux that was considered in our
analysis 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 overall
efficiency. 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 3.92
JENDL 3.3 5.16 26 3.92
n TOF 5.156 38.8 3.71
598 International Conference on Nuclear Data for Science and Technology 2007
Entries  117
Mean  0.0005057
RMS  0.000671
 / ndf 2χ  18.69 / 114
p0  2.6ℜ± 135.5 
p1  0.000036ℜ± 0.001025 
gGn_0 threshold (eV)
-510 -410 -310 -210
N
u
m
b
er
 o
f 
L
ev
el
s
0
20
40
60
80
100
120
Integrated number of levels vs Porter-Thomas prediction
Fig. 7. Number of levels along with the Porter-Thomas distribution
fit.
– 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 shows
the energy response of TAC for various conditions applied,
for 234U.
4 Results
For the fitting process of our capture yield, we used the
SAMMY software package [6] using ENDF/B-VI.8 resonance
parameter set. Prior to the fitting procedure, our capture yield
was normalized in order to reproduce the first resonance of
234U. The comparison between our fit (red) and ENDF/B-VI.8
(green), for the first resonance is shown in figure 5, while
figure 6 displays the fit for neutron energy up to 100 eV. In
this second plot one can notice a resonance at 67.5 eV, not
listed in ENDF/B-VI.8.
The average radiation width was found to be equal to
40.3 ± 2.3 meV, considering resonances up to the energy of
200 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). Using
that value (135.4 ± 2.6) we calculated the average spacing
level, 〈D0〉 = 11.04 ± 0.2 eV while Mughabghab gives 10.92
± 0.47 eV [7]. The reduced averaged neutron width obtained
from this fit is:
〈
Γ0n
〉
= (1.02 ± 0.03) × 10−3 eV.
5 Conclusions
Measurements of the neutron capture in 234U were performed
at the n TOF facility (CERN), using the Total Absorption
Calorimeter as the detection system. Resonance parameters
for the resolved resonance energy region were determined
after following the fitting process with SAMMY. Calculations
of the average radiation width and mean level spacing validate
the good performance of the experimental facility and the
adequacy 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, Genève (2000).
2. The n TOF Collaboration, Status Report CERN/INTC 2001-021,
CERN, Genève (2001).
3. The n TOF Collaboration, Performance Report CERN/INTC
2002-037, CERN, Genève (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, Genève (2000).
6. N. Larson, Tech. Rep. ORNL/TM-9179/R7/Draft, Oak Ridge
National Laboratory (2006).
7. S. Mughabghab, Atlas of Neutron Resonances, Vol. 1, 5th edn.
(Elsevier, 2006).


The
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Interactive optimization for steering machine classification

Cut locus and topology from surface point data