measurement of the u-234(n,f) cross section with ppac detectors at the n_tof facility

Measurement of 234U(n,f) cross section. Carlos Paradela. PhD dissertation

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Measurement of the U-234(n,f) cross section with PPAC detectors

at the n_TOF facility

Carlos Paradela Dobarro

Universidad de Santiago de Compostela

paradela

It is a pleasure for me to present my PhD work.


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Contents

• Motivation

• n_TOF facility and detection setup description.

• Data reduction and efficiency estimation.

• 234U fission cross sections results.

• Conclusions.

paradela

The contents of this work can be divided in the different points:- firstly an introduction to cross sections measurements and their motivation.-Then, I'll describe the facility and the detectors involved in this work.- and subsequently, I'll pass to give a few notes about how I treat the raw data and determine the efficiencies of the experimental setup.Finally, the results I got for U-234 are shown and discussed pointing out my contributions in the conclusions.


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Neutron-induced nuclear reactions

Target (AXZ nucleus)+

Projectile (1n0)

Neutron scattering

Neutron capture

(n,)

Fission (n,f)

neutron

AXZ nucleus

Cross

Sections

f

paradela

-When a neutron strikes a nucleus, several nuclear reactions are possible. If the nucleus is heavy enough, the most probable is that:- the neutron is scattered by the nucleus.- the neutron is captured by the nucleus that emits a gamma ray.-or the neutron produces the fission of the nucleus which more often splits in two fission fragment with masses around half of the initial nucleus mass and a few neutrons. This last one is the reaction which concern us.

paradela

The probability of each reaction channel to happen is governed by the partial cross sections. Cross sections depend on the neutron energy


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Why we measure these cross sections?

Measurement of cross sections are relevant for:

1. Nuclear Technologies• Waste transmutation (ADS)

• Thorium fuel reactors

2. Nuclear Astrophysics

– Heaviest element nucleosynthesis (“r” process)

3. Fundamental Nuclear Physics

– Nuclear structure

paradela

Maybe you are asking yourselves why to measure these cross sections, here we have some fields where they are important:- Iinnovative nuclear technologies such as the transmutation of the nuclear waste or new reactors based on thorium fuel are developed by means of simulations which need accurate nuclear data. But current recommended data for certain isotopes like U-234 differs depending on the evaluation library. - In Fundamental Research, neutron cross sections are demanded for explaning astrophysical scenarios where nuclei heavier than IRON are created and theycan confirm theoretical models of the nuclear structure. Buscar mas informacion.


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234U

230Th 231Th 232Th 233Th 234Th

231Pa 232Pa 233Pa 234Pa 235Pa

232U 233U 235U 236U 237U 238U

Neutron Capture

decay

Thorium-Uranium cycle

• 232Th + n 233Th 233Pa 233U (fissile)– 233U+n 234U– 233Pa+n 234Pa 234U

“La fisión del torio y la fusión de deuterio-tritio […] son nuevas energías nucleares con residuos de corta duración y sin proliferación, capaces de proporcionar energía durante los próximos milenios.” El mundo de mañana,hoy, Carlo Rubbia, El País, 2 de Octubre.

“Thorium fission and deuterium-tritium […] are new non-proliferation nuclear energies producing short-term wastes, that can provide energy for the next millenniums.”

paradela

Hablar mas de Thorium fuel cycleNowadays, because of the actual problems in the energetic scenario, the alternative use of thorium as nuclear fuel is taking relevance, leading to the development of new concepts such as the Accelerator Driven Systems (ADS).The thorium-uranium fuel cycle is based on the production in of uranium-233 which is fissile like uranium-235 (the one used in current nuclear reactors).The process is the following: The Throrium-232 is converted into U-233 after a neutron capture followed by two beta decays In the breeding process a significant amount of uranium-234 is produced by neutron capture in the uranium-233 or in the praseodinium-233 plus beta decay, so that an accurate knowledge of its neutron cross sections must be achieved to simulate how it behaves inside a reactor. U-234 fission cross section is the main topic of this work.


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The n_TOF collaborationU.Abbondanno14, G.Aerts7, H.Álvarez24, F.Alvarez-Velarde20, S.Andriamonje7, J.Andrzejewski33, P.Assimakopoulos9, L.Audouin5,

G.Badurek1, P.Baumann6, F. Bečvář 31, J.Benlliure24, E.Berthoumieux7, F.Calviño25,D.Cano-Ott20,R.Capote23,A.Carrillo de Albornoz30,P.Cennini4, V.Chepel17, E.Chiaveri4, N.Colonna13, G.Cortes25, D.Cortina24, A.Couture29, J.Cox29, S.David5, R.Dolfini15,

C.Domingo-Pardo21, W.Dridi7, I.Duran24, M.Embid-Segura20, L.Ferrant5, A.Ferrari4, R.Ferreira-Marques17, L.Fitzpatrick4, H.Frais-Koelbl3, K.Fujii13, W.Furman18, C.Guerrero20, I.Goncalves30, R.Gallino36, E.Gonzalez-Romero20, A.Goverdovski19, F.Gramegna12, E.Griesmayer3, F.Gunsing7, B.Haas32, R.Haight27, M.Heil8, A.Herrera-Martinez4, M.Igashira37, S.Isaev5, E.Jericha1, Y.Kadi4, F.Käppeler8, D.Karamanis9,

D.Karadimos9, M.Kerveno6, V.Ketlerov19, P.Koehler28, V.Konovalov18, E.Kossionides39, M.Krtička31, C.Lamboudis10, H.Leeb1, A.Lindote17, I.Lopes17, M.Lozano23, S.Lukic6, J.Marganiec33, L.Marques30, S.Marrone13, P.Mastinu12, A.Mengoni4, P.M.Milazzo14, C.Moreau14, M.Mosconi8, F.Neves17, H.Oberhummer1, S.O'Brien29, M.Oshima38, J.Pancin7, C.Papachristodoulou9, C.Papadopoulos40, N.Patronis9, A.Pavlik2, P.Pavlopoulos34, L.Perrot7, R.Plag8, A.Plompen16, A.Plukis7, A.Poch25, C.Pretel25, J.Quesada23, T.Rauscher26, R.Reifarth27, M.Rosetti11, C.Rubbia15, G.Rudolf6, P.Rullhusen16, J.Salgado30, L.Sarchiapone4, C.Stephan5, G.Tagliente13, J.L.Tain21,

L.Tassan-Got5, L.Tavora30, R.Terlizzi13, G.Vannini35, P.Vaz30, A.Ventura11, D.Villamarin20, M.C.Vincente20, V.Vlachoudis4, R.Vlastou40, F.Voss8, H.Wendler4, M.Wiescher29, K.Wisshak8

1Atominstitut der Österreichischen Universitäten,Technische Universität Wien, Austria, 2Institut für Isotopenforschung und ernphysik, Universität Wien, Austria, 3Fachhochschule Wiener Neustadt, iener Neustadt, Austria, 4CERN, Geneva, Switzerland, 5Centre National de la

echerche Scientifique/IN2P3 - IPN, Orsay, France, 6Centre National de la echerche Scientifique/IN2P3 - IReS, Strasbourg, France, 7CEA/Saclay - DSM, Gif-sur-Yvette, France, 8Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany, 9University of Ioannina, Greece, 10Aristotle University of Thessaloniki, Greece, 11ENEA, Bologna, Italy, 12Laboratori Nazionali di Legnaro, Italy, 13Istituto Nazionale di Fisica Nucleare, Bari, Italy, 1Istituto Nazionale di Fisica Nucleare, Trieste, Italy, 15Università degli Studi Pavia, Pavia, Italy, 16CEC-JRC-IRMM, Geel, Belgium, 17LIP - Coimbra & Departamento de Fisica da Universidade de Coimbra, Portugal, 18Joint Institute for

Nuclear Research, Frank Laboratory of Neutron Physics, Dubna, Russia, 19Institute of Physics and Power Engineering, Kaluga region, Obninsk, Russia, 20Centro de Investigaciones Energeticas Medioambientales y Technologicas, Madrid, Spain, 21Consejo Superior de

Investigaciones Cientificas - University of Valencia, Spain, 22Universidad Politecnica de Madrid, Spain, 23Universidad de Sevilla, Spain,, 25Universitat Politecnica de Catalunya, Barcelona, Spain, 26Department of Physics and

Astronomy - University of Basel, Basel, Switzerland, 27Los Alamos National Laboratory, New Mexico, USA, 28Oak Ridge National Laboratory, Physics Division, Oak Ridge, USA, 29University of Notre Dame, Notre Dame, USA, 30Instituto Tecnológico e Nuclear, Lisbon,

Portugal, 31Charles University, Prague, Czech Republic, 32Centre National de la Recherche Scientifique/IN2P3 - CENBG, Bordeaux, France, 33University of Lodz, Lodz, Poland, 34Pôle Universitaire Léonard de Vinci, Paris La Défense, France, 35Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy, 36Dipartimento di Fisica Generale, Università di Torino and Sezione INFN di Torino, I-10125 Torino, Italy, 37Tokyo Institute of Technology, Tokyo, Japan, 38Japan Atomic Energy Research Institute, Tokai-mura, Japan, 39NCSR, Athens,

Greece, 40National Technical University of Athens, Greece

C. Paradela24 ,

24Universidade de Santiago de Compostela, Spain,

paradela

This work have been done in the framework of the nTOF collaboration (R) where my USC group is involved.( Activar FOTO CERN)


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paradela

Most of the collaboration activities are done the nTOF facility at CERN


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n_TOF facility (I)

paradela

At the nTOF facility the neutrons are produced by spallation reactions in a lead target and their energies are determined by time-of-fligh techniques.


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7ns

Lead target

n_TOF facility (II)4 - 7 x 1012 protons per pulse

FTN transfer line

paradela

The facility takes advantage of the CERN-Proton Synchroton that provides a pulsed 20 GeV proton beam with high intensity (between 4 and 7 times ten to the seventeen protons per pulse) and seven nanoseconds width. The proton beam (T) hit the lead target producing a few hundreds of neutrons per proton.That are moderated by the cooling water surrounding the target..... (cambiar)

paradela


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n_TOF facility (III)

TOF TUBE

Second Collimator

paradela

... and fly around 185 meters through the TOF tube (F) up to the Experimental Area, traversing the collimating system (F) (that determines the beam profile) and the sweeping magnet (F) (that reduce the charged particle backgrounds).


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Neutron beam monitors

Micromegas SiMon

n_TOF facility (IV)

paradela

In the Experimental Area (T), several detectors are used to monitor the neutron beam (T) or to perform the cross section experiments (T) for capture such as C6D6's and for fission, PPAC's. PPACs are the detectors involved in this work.


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Escape Line

PPAC gas regulation

DAQ

n_TOF facility (V)

paradela

after the experimental area, remaining neutrons enter the scape line where there are located the PPAC gas regulation system and data acqusition system based on versatile Acquiris Flash ADC.


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n_TOF beam characteristics

• Neutron spectrum

• Energy resolution

• Beam profile

En/En < 10-3

@ En < 104 eV

X (mm)

Y (

mm

)

paradela

The neutron beam of the nTOF facility is characterised by:-the energy-dependent neutron spectrum which is marked by the MeV neutron peak coming from spallation reactions and an isoletargic behaviour at lower energies due to the water moderator.-The energy resolution dominated at low energy by the different neutron paths in the target-moderator assembly and at high energies by the proton burst width of 7 ns.-the beam profile, here I show the X-profile from fission campaign.(- and the background in the Exp. Area, particularly the promtp photon flash which produces the first signal in the detectors correlated with the beam and is used for as T0 reference.)


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Fission detection setup


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Fission Detection Setup (I)

paradela

Next step is the description of the fission setup which was designed and constructed by the IPN Orsay group.


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• Fissile target in a thin backing sandwiched by two detectors Detection of both fission fragments in coincidence.

• Fission event reconstruction: target position and emission angle. Efficiency limited by the cut at large angles.

Fission Detection Setup (II)

paradela

The idea for the fission detection is to put the target in-between two detectors, so that both fission fragments emitted in a fission reaction are detected in coincidence, In addition if the we can measure the position of the FF in the detectors, we can reconstruct their trajectories obtaining the origin in the target and the emission angle.This detection setup consists basicly of target and detectors.


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Uranium target80 mm Ø

300 µg/cm2

2 µm Al backing

Epoxy frame

Targets (I)

paradela

Very thin targets have been used with a samples of 8cm diameter of and an approximate thickness of300 micrograms per square centimeter which are deposited in a 2 microns aluminum backing.Let's point here that the backing is traversed by one of the two Fission fragments.


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• Measurement of thickness and homogeneity by alpha counting.

• High purity samples (> 99 % for U-234).

234U

activity

Targets (II)

X (mm)

Y

(mm

)

paradela

The thickness of the deposited layer in the target and its inhomogeneities have been accurately measure by alpha counting. The presence of impurities have been also measured resulting high purity samples.


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PPAC (I)

• Very thin detectors.

• High FF efficiency

• Fast timing (0.5 ns de resolution using anode signal).

• FF position by using cathode signals.

Parallel Plate Avalanche Counter

paradela

The detectors we have used are the Parallel Plate Avalanche Counter (PPAC) that are very thin gas detectors with parallel plate electrodes working in semiproportional avalanche mode. They have almost 100% efficiency for crossing FP whereas are quite insensitive to gamma radiation. PPACs have a good time resolution (about 500 ps) by using the fast anode signal andalso provide the position of the Fragment from the cathode signal information.


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Discrimination with coincidences

U-234: coincidencesU-234: singles

paradela

paradela06/10/2005The use of the coincidence method allows us to improve the rejection of background produced by alpha particles and highly energetic reactions in detectors.This can be shown by comparing the signals measured by only one detector (T) against the results obtained demanding coincidence with the second detector. In this amplitude-energy plot, the signals due to alpha particles and high energy reactions that are detected...... are very suppressed when looking for coincidences and only fission events remain.


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• Selection of fission events from 234U (target 0)

Time & Amplitude selection

Time difference (1/10 ns)

Det

0 A

mpl

itud

(a.u

.)

Time coincidence window

paradela

If in addition to the time coincidence condition we use the amplitude informacion of the detectors, the fission events are succesfully selected.A esta parte le dedicado mucho trabajo que qued descrito en el script.


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PPACs @ n_TOF

10 detectors 9 targets

• U-234(2) and Th-232(5)

• Two reference targets: U-235 y U-238

• Less than 1 % of flux attenuation in the full setup.

paradela

Taking advantage of the very thin targets and detectors used, in the nTOF PPAC setup it is possible to include up to nine targets sandwiched by 10 detectors. This allow to study several nuclei simultaneously, increasing the statistic available, and using targets with known XS as references U-235 and U-238 in our case.Nevertheless, This makes the recognition of the fission events more complicate because the targets share the detectors and also because it is possible ...


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Three detectors in coincidence

Detector 0 Detector 1 Detector 2

Detector 1 Detector 2Detector 0

FF1 FF2

n

nFF2

FF1

LEFT TARGET

RIGHT TARGET

paradela

it is possible that a FF crosses two PPAC detectors so that 3 detectors are in coincidence. This produces certain ambiguity to distinguish from which target comes the fission fragments. The ambiguity...


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Three detectors in coincidence

Correlation between time differences of detectors 0,1 and 2

Fissions from target on the left Fission from target on the right

paradela

The ambiguity is solved by applying suitable coincidence windows between detector signals and comparing their amplitudes so that we know if the fission fragments comes from the target on the left or from the target on the right.


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Cathode Positioning (I)

• Positioning by using stripped cathodes and delay line readout.

• The cathode signal is split in the delay line and transmitted to both ends

Delay Line

Stripped Cathode

paradela

As explained before, PPAC detectors also provide the position of the crossing FF by using the two stripped cathodes, each one connected to a delay line. The signal produced in a few strips by the particle is transmitted through the delay line two the preamplifiers in both ends.


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cathode active surface 200 mm

Tc1 Tc2

Tc1-Tc2

Cathode positioning (II)

Diagonal condition:

(Tch1-Tanode)+(Tch2-Tanode)=DLT DLT: Total delay line length (~320 ns)

The time difference between both cathode ends provides the position of the signal.

paradela

Both cathode signals fulfil the condition that we call diagonal condition: that the sum of the travelling times of the split signals up to both cathode ends must be equal to the total length of the Delay Line (around 320 ns)The time difference between these signals tell us what is the position of the signal in the direction perpendicular to the strips. The two cathodes perpendicularly oriented provides the two-dimensional position of the FF in the PPAC.


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Cross section analysis

paradela

Slide para descansarThe data reduction which has been briefly described is explained in more detail in the Chapter 4 of the thesis memory.


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Cross Section Analysis

(E): fission cross section• n (x,y,E): fission rate obtained from raw data (x,y): surface density of the target (E): detection setup efficiency

a (x,y,E)/ b (x,y,E) ≈ 1 ± 0.01 (1%)

paradela

We determine the fission cross section with respect to a reference cross section such as U-235.After fission event selection, we got the fission rates measured for each target. This ratio gives the different number of atoms in the targets that is independent of the neutron energy. It's included as a normalisation constant. Finally, efficiencies have a relevant impact in the XS calculation so that I have dedicated an important effort in order to characterise them for each target. The flux term is not appearing in this expression because the PPAC setup practically doesn't attenuate the flux and the ratio is asssumed as 1.


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Efficiency estimation

Factors determining the efficiency:

1. Setup angular acceptance.

2. Hardware threshold cut.

3. Fission fragment angular distribution

paradela

Following factors have been found that determine the efficiency in our setup.


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Expected angular acceptance

70º

100% efficiency

paradela

The first factor is the angular acceptance of the detector, because PPAC is not a 4 pi detector, part of the fission events go outside the angle covered by the setup. Simulation including geometry says the detection efficiency should decrease below 100 % for angles larger than 70 degrees. but we found...


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Angular acceptance

50º

70º 58º

50º

Simulations Measurements

paradela

..but we found is that the cosine distribution falls from an angle around 50 degrees. This can be explained if we consider the energy losses of the fission fragments in the dead layers of the setup (backing and detectors). This implies that the heavier FFs begin to be stopped at angles around 58 degrees. Even more, if we demand that FF mush have a minimun residual energy to produce signal large enough, the full efficiency is missed from 50 degrees in agreement with data. Let's point here that most suppressed fission events are those in which the heaviest fission fragment crosses the backing.


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Hardware threshold cut

Cathode signals for En < 100 keV Cathode signals for En > 1 MeV

paradela

Another effect, that I call HTC, that reduces our efficiency is the existance of cathode channels in the detector facing the backing with a too high threshold, so that fission event signals with small amplitude are not registered by the adquisition. I should note that this effect is not affecting high energy range (above MeV) because for short time-of-flights (around the first 10 microseconds) zero-suppression method is not active by the high counting rate,and DAQ register signals even below the set threshold.In order to correct such effect...


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Target 0 Energy < 10 MeV (Assymetric fission)

Hardware threshold cut (II)

HFF LFF

HFF LFF

paradela

In order to estimate what are the efficiency reduction because of the hardware threshold cut, I have used the capability of the PPAC detectors to distinguish between the two types of the fission events: 50 % with the heavy fission fragment crossing the backing and 50 % in which the light fission fragment crosses the backing. The separation is achieved whatever is the emision angle.


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Fit to a double Gaussian with the areas below the peaks: AHFF and ALFF

Efficiency estimation obtained from the ratio:Detected events/Expected events

where the expected events are assumed to be 2 x ALFF.

Hardware threshold cut (IV)

ALFFAHFF

AHFFALFF

E ~ 5 MeV

E < 200 keV

paradela

Studying these double-peaked histograms, we realise that for MeV ENERGIES - WHERE THERE IS NOT SIGNIFICANT Hardware threshold cut- the areas below both peaks were almost equal, whereas for energies lower than 500 keV, HFF peak is much smaller than because HTC suppress this type of events.Our idea is calibrate the effect of the hardware threshold cut by means of the ratio between Light and Heavy peak areas, so that an estimation of the efficiency loss is given by the ratio:fission Events detected (total area below the histogram) divided by twice the Area below the Ligh peak (that we assumed is the expected number of fission eventsStudying ...


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High Energies

U-234 (Target 1)Low Energies

Hardware threshold cut (V)

Low Energies

High Energies

U-235High Energies

Low Energies

U-234 (Target 0)

Detected/Expected

paradela

each target for different energy and cosine intervals, we got the reduction in the efficiency due to the hardware threshold cut.For target 0 30%, no reduction for target 1 and 20% for u-235 target.


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U-234 FFAD for neutron energies near the fission threshold

Fission Fragment Angular Distribution

Cos ()Cos ()Cos ()

Log E =5.6 Log E =5.4

Log E =5.8

Log E =5.5

Log E =6.0 Log E =5.9

paradela

The last factor studied was the FFAD. Up to now we have supossed that the fission fragment were emitted isotropically, so that a flat behaviour in the angle cosine was expected in the full efficiency angular range.But this is not the case for some energy regions where the fission fragment angular distribution is clearly anisotropic. And the anisotropy changes rapidly with the neutron energy from a peaked-forward distribution to a sideways distribution.As our fission setup has a reduced angular acceptance the variation of the FFAD with the energy has a direct effect in the detection efficiency.


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Fission Fragment Angular Distribution (II)

W()1+Bcos2 , B Anisotropy parameter

W()= C(P0+P1cos )(1+Bcos2), P0 and P1 intrinsic parameters

paradela

For this reason I took care of the characteristics of the angular distributions, implementing a method to measure the anisotropy for different energy intervals that must have enough counts. The behaviour of an anisotropy distribution can be approximated by this expression used by several authors:where B is the parameter defining the anisotropy.Other people use an expansion of Legendre polynomials, but we are ok with this simplified version.and in the case of our setup I introduced this additional linear term in the cosine to reproduce the intrinsic performance, where Pzero an Pone do not depend on the enery.


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U-238 this workLeachman +Tutin

U-238 anisotropy

Fission Fragment Angular Distribution (III)

B

paradela

Applying this method for different intervals with enough statistics we found this result forthe anisotropy of the U-238, which is in agreement with previous measurements done by different authors (Leachman or Tutin). Our larger error bar are mainly motivated by the limited angular range available from ppac (up to 50 degree)


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U-234 anisotropy

Fission Fragment Angular Distribution (III)

B

paradela

For both targets of Uranium-234 the same procedure have been done with good agreement with previous data but at large anisotropy where our results overestimate the anisotropy. No previous data above 14 MeV are available.


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Efficiency estimation

U-234 (target 1)

U-234 (target 0)

U-235

W()= CE (P0+P1cos )

IN THE LOW ENERGY RANGE B supposed as 0

paradela

At low energies, isotropy is supposed for all the energies and the extrapolation in the cosine behaviour is based on the expression :...where the area below the fit is assumed as the total number of fission event expected in case of 100 % efficiency in the full range.So that the ratio detected ev/expected events provides the detection efficiency. For U-235 and target 0 of U-234 we need to correct the efficiency histogram by the Hard ThCut before the fit.


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E = 800 keV

5 5.5 6 6.5 7 7.5 8 8.5 9 9.50.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75U-234 target (0)

log E (eV)

Cal

cula

ted

effic

ienc

y

5 5.5 6 6.5 7 7.5 8 8.5 9 9.50.45

0.5

0.55

0.6

0.65

0.7

0.75U-234 target(1)

log E (eV)C

alcu

late

d ef

ficie

ncy

Efficiencies for both U-234 targets

Efficiency estimation (II)IN THE HIGH ENERGY RANGE

paradela

For higher energies anisotropy should be included in the fit as the cuadratic term but the procedure is the same. The efficiencies values calculated for the different energy intervals have been aproximated by a smooth spline function.Because of larger uncertainties in our anisotropy results, values from literature are used where they are available, that results into the different size of the error bars I have assigned and shown in the plots.


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Efficiency estimation (III)

-1 0 1 2 3 4 5 6 7 8 90.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

log E (eV)

U-235 target

Calc

ula

ted e

ffic

iency

U-235 efficiencies in the full energy range

paradela

I also show the efficiency calculated for the u-234 in the full energy range. Let's point here that this behaviour at highest energies is common for all the targets studies.


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• The cosine distribution at very high energies is disturbed because of wrong trajectory reconstruction.

Efficiency estimation (IV)

paradela

and it is related with the fact that at these energies the method to reconstruct the trajectories works worse and wrong angles disturb the cosine distribution and then the efficiency estimation.


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Cross section results

paradela

Once I have described how we obtained our cross section, the results can be shown.


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U-234(n,f) cross section

Normalised to ENDF/B-VI in the 1-4 MeV interval

Present workENDF/B-VI

paradela

In this figure the u-234 neutron induced fission cross section is shown in the whole energy range of nTOF (from eV to GeV). Our data in black are compared to the evaluated data provided by the ENDF library.To show the results with more detail we have divided it into three energy regions, first is the resonance region, then the fission threshold region up to 20 MeV (maximun energy reache d by the evaluated files) and a region above 20 MeV where no evaluated data is available


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Resolved Resonance Region

• 234U(n,f) cross section presents important subthreshold resonances.

• Resolved Resonance Region extends to 1.5 keV.

paradela

U-234 present a fission threshold coming from the need for the impiging neutron to transfer enough energy to the nucleus to overpass the fission barrier.Contrary to other even-even actinides such as U-238 or Th-232, U-234 present important fission resonances in the subthreshold region. In the evaluated files RRR extends up to 1 point 5 keV


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Previous data

Resolved Resonance Region (II)

paradela

A complete resonance parameter analysis is out of the scope of this work, but a comparison with some of them which have been reported in detail before is interesenting to show the resolution achieved in nTOF for the resonance region. For instance, around 570 eV, ORNL data have been reported and more recently data from GEEL .


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f

(b)

Resolved Resonance Region (III)

------ ORNL 1977

------ n_TOF PPAC 2003

E (eV)

paradela

Comparing with our data it can be observed that our experimental resolution is improved with respect to previous data.


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Resolved Resonance Region (IV)

ENDF/B-VI

n_TOF PPAC 2003

JEFF-3.1

n_TOF PPAC 2003

paradela

Going to the to evaluated shapes, our resonance widths are comparable with theirs.


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Narrow intermediate structure shown by James and Rae1

1. G. D James and E. R. Rae,, Nucl. Phys. A118, 313 (1968)

f

(b)

Resolved Resonance Region (V)

------ ORNL 1977

------ n_TOF PPAC 2003

paradela

The 680 region was reported as the first narrow intermediate structure observed in the U-234 fission cross section. The resonance resolution of our data improves by large previous data.


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N_TOF PPAC 2003

ENDF/B-VI

JEFF-3.1

Region up to 20 MeV

f (

b)

paradela

This region present the largest number of experimental works, but evaluations still present up to 3% of discrepancies. Our data follow evaluationsin quite agreement.


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n_TOF PPAC 2003

ENDF/B-VI

JEFF-3.1

f (

b)Region up to 20 MeV


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f (

b)

Fission threshold

310 keV

paradela

The fission threshold region has received special attention because some resonance-like structure have been recognised in them and related to beta vibrational states.


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f (

b)

f (b

)

paradela

For instance the structure at 310 region was assigned for James to beta-vibrational states and used to calculate heights of the double fission barrier. Our data structure at this energy looks in good agreement with James.


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f (

b)

450 keV

Fission threshold (I)

paradela

At 450 keV no vibrational structure is expected and we have a more straight behaviour in the slope that James' data. In fact resonance-like fluctuations in James data appear over the whole energy range and they are equally visible inside and outside of the vibrational resonances.


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f (b

)


57

f (

b)

550 keV

Fission threshold (II)

paradela

At 550 keV a plateu is observed with some structure...


58

f (b

)

paradela

but it seems that resolution is not enough to distinguish clearly the individual resonance shapes.


59

f (

b)

780 keV

Fission threshold (III)

paradela

At the top of the fission barrier we find the largest discrepancy with James data. Our data are nearer to the pronounced peak structure found by Meadows around 780 keV.


60

f (b

)

paradela

We find the peak value around 750 or760 keV, but probably resolution is again not enough to resolve individual resonances that could be related to hyperdeformed nuclei states.


61

f (

b)

paradela

I wanted to point here that these energy region is very delicated because of the rapid change in the anisotropy so that if the setup depends on the angular distribution it must be considered in the efficiency estimation as we have done. Results of the other authors are not fully described to respect this.


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Region up to 20 MeV

f (b)


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n_TOF PPAC 2003

ENDF/B-VI

f (

b)

Prokofiev*

* A.V.Prokofiev, Nucl. Instr. Meth. A 463 (2001) 557

Region above 20 MeV


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Conclusions

• A new neutron facility (n_TOF) where a detection setup based on PPACs have been used for measuring fission cross sections by using coincidence technique and trajectory reconstruction.

• An original data reduction has been developed including specific features of the DAQ, detection setup and method.

• Efficiencies for each target have been studied in detail including its dependency of the fission fragment angular distribution.

• New U-234(n,f) cross section experimental data in an extended energy range that benefit from the good resolution of n_TOF and detector.

paradela

In this work has been presented the new nTOF facility that present very good features to measure neutron cross sections such as an intense, instantaneous flux and excellent time-of-flight energy resolution and where the fission detection setup based on PPAC detectors used in this facility to study different nuclei fission cross section.The dectection is accomplished searching for coincidences due to fission fragments and their trajectories. - In order to do this, I have developed an original data reduction method which deals with the particular characteristics of the acquisition system, detectors and the coincidence and positioning procedements.-I have done a special effort in the estimation of the target efficiencies in which the different angular distribution of the fission fragments have been included.- Finally the largest contribution of this work consisted in providing a new result for the u-234 neutron induced fission cross section in an extended energy range from electron-volt to gigaelectron-volt.


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Cross Section


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The n_TOF Data Acquisition System (I)

•The n_TOF DAQ consists of 54 flash ADC channels with 8 bit amplitude resolution and sampling of 500 MSample/s.

•The full history of EVERY detector (BaF2 crystals and monitors) is digitised during a period of 16 ms (0.7 eV < En < 20 GeV) and recorded permanently on tape. Very useful feature since the raw data can be always re-investigated.

•The system has nearly zero dead time.

•7.5 TB disk space for temporary storage.

•Typical data rate of 2-3 TB/day on tape after compression.

•Pulse shape analysis is performed on the fly at the LXBATCH Linux Batch Farm at CERN (30 CPUs exclusively dedicated) and stored in highly compressed Data Summary Tapes.

•Quasi on-line analysis of the data with full statistics.

One of the big successes of n_TOF. Many TOF facilities are following the n_TOF example and moving to digital electronics!


68

The n_TOF facility

The n_TOF facility is a spallation source built in 1999 – 2000, driven by the CERN PS and coupled to a Time Of Flight (TOF) beam line of 200 m. Typical pulses of 7·1012 protons at 20 GeV/c and a time width of 6 ns are used for producing a spallation neutron beam with a spot of 4 cm diameter and 6·105 n/cm2/pulse at a 200 m flight path.

•High instantaneous neutron fluence with low repetition rate at a VERY long flight path of 200 m and VERY favourable duty cycle (key point for measuring radioactive samples).

•Excellent energy resolution En/En < 10-4 for En < 105 eV

•Highly advanced detection systems (TAC) and monitors.

•Innovative and pioneering Data Acquisition System based on flash ADCs.

Unique features worldwide for measuring

highly radioactive samples!


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70

Resolved Resonance Region (IV)


71

Hardware threshold cut (II)

paradela

Aquí explicar como funciona el threshold respecto a la baseline y que a altas energias, cerca del gamma flash tenemos un shift en la baseline y esta sube por encima del umbral dejando sin funcionar la zero-supresionBuscar RAW data cerca de flash y señales aisladas.files A_ ...


72

n_TOF beam characteristics

• Neutron spectrum

• Energy resolution

• Beam profile

• Background (-Flash)

paradela

The neutron beam of the nTOF facility is characterised by:-the energy-dependent neutron spectrum which is marked by the MeV neutron peak coming from spallation reactions and an isoletargic behaviour at lower energies due to the water moderator.-The energy resolution dominated at low energy by the different neutron paths in the target-moderator assembly and at high energies by the proton burst width of 7 ns.-the beam profile, here I show the X-profile from fission campaign.(- and the background in the Exp. Area, particularly the promtp photon flash which produces the first signal in the detectors correlated with the beam and is used for as T0 reference.)


73

Narrow intermediate structure shown by James

f

(b)

Resolved Resonance Region (V)

paradela

The region about 700 eV also have been discussed in the literature and


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1 10 100 1000 10000 100000 1000000 1E7 1E80.00.10.20.3

1

10

n_TOF James Lestone ENDF-B/VI

f, b

arn

s

En, eV


75

En (log scale)

f

(b)

En (log scale)

f (

b)Cross Sections

Elastic scattering Capture (n,)

Fission Total cross section

f (

b)

f (

b)

paradela

paradela03/10/2005, so that they usually are expressed as differential cross sections as it is shown for the different reactions.For thermal and epithermal neutron energies, cross sections follow the inverse velocitiy law, at intermediate energies, they present resonances and for higher energies (around MeV), when the resonance width is wider than the spacing, the XS becomes continuous.The sum of all the partial cross sections gives theprobability for the reaction neutron-nuclei to happen.


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measurement of the u-234(n,f) cross section with ppac detectors at the n_tof facility

Documents

neutron cross sections

measurement of cross

fission n

cross sections measurements

partial cross sections

neutron capture n

nuclear waste

accurate nuclear data