The n_TOF Collaboration , cern.ch/nTOF

C. Guerrero @ CERN EN/STI Student's Coffee (April 17th 2012)“An n_TOF experiment: the complete process from the data taken to the published results”

The n_TOF Collaboration, www.cern.ch/nTOF

CERN EN/STI Student’s Coffee (17th April 2012)

An n_TOF experiment: the complete process from the data taken to the published results

Carlos Guerrero CERN Physics Dpt. Fellow

http://www.cern.ch/nTOF


The complete process from the data taken to the published results

PHYSICS MOTIVATION BEHIND THE N_TOF EXPERIMENT AT CERN

ONE PARTICULAR EXPERIMENT:MotivationHow we make such an experiment: - Neutron source- Detectors

Data takingAnalysisWhat we do after!


Physics Motivation behind the n_TOF Experiment at CERN

NUCLEAR ASTROPHYSICS: stellar nucleosynthesisNeutron capture and (n,a) cross section of stable (& radioactive) medium mass isotopes playing a role in the s- and r-processes responsible of the creation of elements heavier than Fe in the starts.

NUCLEAR TECHNOLOGIES: ADS, Gen-IV and Th fuel cycle, Nuclear MedicineNeutron capture and fission cross sections of Actinides and Fission Fragments in the thermal, epithermal and fast energy range. Also reactions of interest for Nuclear Medicine (for instance 33Sn,a)

BASIC NUCLEAR PHYSICS: levels densities, g-ray strength funct. and angular distributionsTime-of-flight measurements with dedicated detectors provide very valuable information on basic nuclear physics quantities.

DETECTOR DEVELOPMENT AND TESTING, IRRADIATION WITH NEUTRONS:CVD diamonds, Fiber Bragg Gratings, Optical Fiber dosimeters, GEMs, MediPix, …

The n_TOF Collaboration38 Research Institutions from Europe, Asia and USA.

Based at CERN: 5 FTE + 3-5 short term visitors

E. Chiaveri (Spokesperson), E. Berthoumieux (Techn. Coord.) , C. Guerrero (Run Coord.), C. Weiss (PhD), A. Tsinganis (PhD)(Also V. Vlachoudis and M. Calviani)


MOTIVATION


Figura Nucleosintesi (frecce che si muovono)

Foto FIC239Pu: 125 Kg/yr

237Np: 16 Kg/yr

241Am:11.6 Kg/yr 243Am: 4.8 Kg/yr

244, 245Cm 1.5 Kg/yr

FP

LLFP 76.2 Kg/yr

Quantities refer to yearly production in 1 GWe LW reactor

Neutrons and production of nuclear energy (& radioactive waste)

+Energy


How do the competing (n,g) (n,f) cross sections look like?

235U (even-odd): FISSILE 238U (even-even): NON-FISSILE

Similar behavior for all isotopes with ODD number of neutrons: 239,241Pu, …, 245Cm

Similar behavior for all isotopes with EVEN number of neutrons: 237Np, …, 240,242Pu, 241,243Am

Thermal neutrons

Thermal neutrons

Fast neutrons Fast neutrons


Existing reactors have low burn-up efficiency and produce large amount of radioactive waste.

In all cases, a large reduction of actinide inventory is achieved by means of neutron-

induced reactions with FAST NEUTRONS (subsequent captures and fission).

Generation IV reactors based on recycling of the spent fuel (in particular actinides).

Neutrons and production of nuclear energy (& radioactive waste)

Using fast neutrons it is possible to fission all actinides, thus reducing the high level waste!!

Another possibility is to use dedicated burners (Accelerator Driven Systems [ADS])


(FCA) Fast Critical Assembly (JAEA)

Soft Hard

The criticality of current and fast future reactors must be known within 0.3-0.5% for operation/safety.

Differences up to 2% in the measured and calculated criticality values for FCA (JAERI, Japan) assemblies with different hardness are due to 235U s(n,g) below 2.5 keV.

Open Problems: Uncertainties in the capture 235U cross section

Sensitive mainly to the 235U(n,g) below 2.25 keV (RRR)GOAL: Measure 235 U s(n,g) below 2.5 keV

(First Goal: demonstrate feasibility)


GOAL: Measure 235U s(n,g) below 2.5 keV

nIR

I (neutrons/cm2/s)

n (atoms/cm2)

Reaction products

Measuring the neutron cross sections requires:- A facility providing a neutron beam (The n_TOF facility).- A highly pure sample.- A detection system for counting the reactions (The TAC+MGAS set-up).-The analysis tools to determine the measured cross section with the required accuracy.

The concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus.


The n_TOF facility


PS 20 GeV

Linac50 MeV

Booster1.4 GeV

Proton Beam20 GeV/c7x1012 ppp

Pb Spallation Target

Neutron Beam10o prod. angle

n_TOF 185 m flight path

The n_TOF Facility at CERN: a view

neutronstart

neutronstop

neutron time-of-flight experiment

time185 meters


The n_TOF Facility at CERN

Access Area & Escape line & DAQExperimental Area


Detection system for (n,g) reactions [fissile isotopes]

Total Absorption Calorimeter (TAC) for (n,g) 40 BaF2 crystals

4p geometry (95% coverage)16% energy resolution at 662 keVUsed for s(n,g) of actinides since 2004

C. Guerrero et al., NIM-A 608 (2009) 424-433

ng-rays

Results: distributions Esum , mcr & En

BUT…

(n,el) reaction: neutrons

(n,g) reaction: g-rays

(n,f) reaction: Fission fragments g-rays [similar to (n,g)!!!] neutrons


Total Absorption Calorimeter (TAC) for (n,g) 40 BaF2 crystals

4p geometry (95% coverage)16% energy resolution at 662 keVUsed for s(n,g) of actinides since 2004

MicroMegas (MGAS) for (n,f) Based on Bulk technologyDouble stage GAS detector:

conversion +amplification~90% efficiency for Fission Fragments (FF) Used for neutron monitoring since 2009

C. Guerrero et al., NIM-A 608 (2009) 424-433 S. Andriamonje et al., NIM-A 481 (2002) 120–129

n

n

g-raysFF1

FF2

Results: distributions Esum , mcr & En Results: distributions Amp. & En

We need to detect capture and fission reactions simultaneously!


Use of TAC and MGAS in “anticoincidence”!!


Data taking at n_TOF

n_TOF DAQ

n_TO

F R

un C

ontr

oln_

TOF

Eve

nt

Dis

play

Sounds familiar to n_TOF shifters?


Analysis and Results


1. Time and energy calibration of the TAC.

2. Coincidence algorithm among the TAC crystals.

3. Time calibration of the MGAS detectors with respect to the TAC

4. Loop runs over all MGAS events (ordered in time for each neutron pulse)

5. Loop over the TAC selecting coincident candidates (with TOFTACЄ[TOFMGAS-300 ns, TOFMGAS+50])

6. Select TAC fission event. If more than one candidate, select the highest energy and multiplicity

Analysis in coincidence of TAC and MGAS detectors

FWHM~20 ns


Information from (n,g) [and fission g-rays] with the TAC

Neutron energy MultiplicityDeposited energy

Protons (20 GeV)

Neutrons (meV-GeV)

Sweeping magnet

Collimators

gg

g


RESULTS: deposited energy and multiplicity distributions

Deposited energy (mcr>2) and multiplicity (Esum>3)distributions corresponding to resonances:

- 32% of fission events fall above mcr>9- 98% of capture events fall below mcr<9

- 33% of fission events fall above Esum>7.5 MeV- 100% of capture events fall below Esum<7.5 MeV

98%

68%

98%

67%

Sn(236U)~6.5 MeV

All – fission BackgroundFission

All – fission – backg.Fission


RESULTS: neutron energy distributions

Under the selected conditions, the counts/pulse information can be directly related to the corresponding cross sections

xNeutronFluEfficiencyitySampleDens

BackgroundCounts


With two different detectors and two different types of reactions to detect, it is important to define clearly the different efficiencies that play a role in the measurement and their interrelations.

When a neutron capture occurs, it can only be detected in the TAC → eTAC(n,g)

When a fission reaction occurs, it can be detected:

a) in both detectors, → eMGAS(n,f) ∙ eTAC(n,f)

b) in none of them, → (1- eMGAS (n,f)) (1-∙ eTAC(n,f))

c) only in the MGAS → eMGAS (n,f) (1-∙ eTAC(n,f))

d) only in the TAC. → (1- eMGAS (n,f)) ∙ eTAC(n,f)

RESULTS: Detection efficiencies: eMGAS(n,f), eTAC(n,f) and eTAC(n,g)

The efficiency for detecting fission reactions in each detector is independent from the other, but the calculation from experimental data requires that these four probabilities are properly taken into account.

eMGAS(n,f), eTAC(n,f) and eTAC(n,g)


Esum>7 MeVmcr>3

eexp(n,f)~0.90

MC simulations Samples are 318 mg/cm2, nearly identical to those of the 235U samples (316 mg/cm2) used in FIC, forwhich simulations with FLIKA give eMC(n,f)~0.94 (6% losses due to absorption in the sample).

Experimentally: Fission events produce high-energy, high-multiplicity TAC events. Assumption → eTAC ~100% for such events. Then, the detection efficiency of the FTMGs can be calculated as the ratio of tagged to all events for multiplicities higher than ~10 (no capture events).

eMC(n,f)~0.94 & eexp(n,f)~0.90 eFTMG(n,f)~0.92

RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g) Calculation of eFTMG(n,f)


mcr>0 mcr>1 mcr>2 mcr>3 0<mcr<9 1<mcr<9 2<mcr<9 3<mcr<90.1<Esum 96.9 94.3 90.1 83.5 71.0 68.3 64.1 57.51<Esum 93.3 92.7 89.7 83.4 67.3 66.7 63.7 57.42<Esum 87.9 87.7 86.0 81.8 62.0 61.8 60.2 55.83<Esum 79.9 79.8 79.0 76.5 53.9 53.8 53.1 50.5

0.1<Esum<7 64.8 62.2 57.9 51.5 58.3 55.7 51.5 45.01<Esum<7 61.1 60.6 57.6 51.4 54.7 54.1 51.1 45.02<Esum<7 55.8 55.6 54.0 49.8 49.3 49.1 47.6 43.33<Esum<7 47.7 47.6 46.9 44.5 41.2 41.2 40.4 38.0

A coincident event in the TAC is found for 97% of the FTMG events (MGASamp>20 channels). This value represents the TAC efficiency for fission events, eTAC(n,f), and is very similar to the efficiency of eTAC(n,g)=0.974(4) for capture events in 197Au (from GEANT4 Monte Carlo simulations).

The efficiency eTAC(n,f) depends on the analysis conditions for the deposited energy and multiplicity values.

Efficiency of the TAC for detecting fission events under different conditions in deposited energy and crystal multiplicity.

RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g) Calculation of eTAC(n,f)


eTAC(n,g)=0.70(3) [Esum>3 MeV and mcr>2]

The detection efficiency eTAC(n,g) can be calculated accurately by means of Monte Carlo simulations when both the experimental set-up and the details of the capture cascades are properly considered.

RESULTS: Detection efficiencies: eFTMG(n,f), eTAC(n,f) and eTAC(n,g)

Calculation of eTAC(n,g)Geant4 simulation of the TAC

C. Guerrero et al., NIM-A 671 (2012) 108-117


RESULTS: Capture and Fission Cross Sections of 235U


RESULTS: Capture and Fission Cross Sections of 235U

C. Guerrero et al.

(The n_TOF Collabo

ration), Simultaneous measurement of neutron-

induced capture and fission reactions at CERN, European Physical Journal A 48 (2012) 29


① December 2010 / April 2011: Results presented to the n_TOF Collaboration

② June 2011: Results presented to the scientific community in the ANIMMA Int. Conf.

③ October 2011: New proposal submitted (and accepted) to the CERN INTC committee:“Measurements of neutron-induced capture and fission reactions on U-235: cross sections and alpha ratios, photon strength functions and prompt gamma-ray from fission”

⑥ June 2012: New measurement (see ③) scheduled at n_TOF

④ December 2011: Paper submitted to the European Physical Journal A

⑤ February 2012: Paper accepted for publication in the Eur. Phys. J. A 48 (2012) 29

The work is not over after finalizing the analysis! ⓪ Summer 2010: Data taken at n_TOF

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ny time!!!

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