measurement of be(n,cp) at n tof: a big challenge for ...€¦ · ear2 the much higher flux in ear2...

Measurement of 7Be(n,cp) at n_TOF:a big challenge for sample preparation and experimental setups

M. Barbagallo, A. Musumarra, L. Cosentino, N. Colonna, P. Finocchiaro, A. Pappalardo,

L. Damone, O. Aberle, B. Langhans, D. Schumann, E. Maugeri, S. Heinitz, R. Dressler, J.

Andrzejewsky, J. Perkowski and the n_TOF Collaboration

CHANDA Workshop on target preparation – the needs and the possibilities, PSI, 23-25 November 2015

Outline

M. Barbagallo, ‘’Measurement of 7Be(n,cp) at n_TOF’’, CHANDA Workshop on target preparation, PSI, 23-25 November 2015

• Physics Case: The Cosmological Lithium Problem

• 7Be(n,p) cross-section measurement at n_TOF-EAR2 (2016)

• 7Be(n,a) cross-section measurement at n_TOF-EAR2 (Aug-Oct 2015)

- the experimental setup and the facility- the procedures- preliminary results

Big Bang Nucleosynthesis

Big Bang Nucleosynthesis (BBN), together with Hubble expansion and Cosmic Microwave Background Radiation is one of the cornerstones for Big Bang Theory.

BBN gives the sequence of nuclearreactions leading to the synthesis oflight elements up to Na* in the earlystage of Universe (0.01-1000 sec)

At his first formulation, it depended on3 parameters:

-the baryon-to-photon ratio h,-the number of species of neutrino n,-the lifetime of neutron t.

Nowadays BBN is a parameter freetheory**, being the cross-sections ofreactions involved the only input tothe theory.

* A.Coc et al., The Astrophysical Journal, 744:158 (2012)

**D.N. Schramm and T.S Turner, Rev. Mod. Phys 70 (1998) 303


The Cosmological Lithium Problem

BBN successfully predicts the abundances

of primordial elements such as 4He, D and 3He.

* R.H.Cyburt et al., Journal of Cosmology and Astroparticle Physics 11 (2008) 012

*

** A.Coc et al., The Astrophysical Journal, 744:158 (2012)

A serious discrepancy (factor 2-4) between

the predicted abundance of 7Li and the value

inferred by measurements (Spite et al, many

others.)

Cosmological Lithium Problem

(CLiP)


Solution to CLiP?

Approximately 95% of primordial 7Liis produced from the electron capture decay of 7Be (T1/2=53.2 d).

A higher destruction rate of 7Becan solve or at least partiallyexplain the CLiP.

n + 7Be a + a Q= 19 MeVn + 7Be p + 7Li Q= 1.644 MeV

7Be is destroyed via (n,p) (~97%) and (n,a) (~2.5%) reactions


Status of evaluations and experimental data: 7Be(n,p)

BBN calculations rely on extrapolation to higher energies of Koehler data or on indirect measurements

Direct measurements Indirect measurements

Only one direct measurement (Koehler et al., 1988, 0.025 eV - 13.5 keV),covering partially the range of BBN interest.


Only one direct measurement (P. Bassi et al., 1963) at thermal energy

Bassi et al.


Status of evaluations and experimental data: 7Be(n,a)

n_TOF program on 7Be(n,cp)


185 m

20 GeV/c

protons

20 m

Spallation

Target

EAR1

EAR2

Two beam lines and experimental areas available at n_TOF, EAR1 and EAR2



185 m

20 GeV/c

protons

20 m

Spallation

Target

EAR1

EAR2

The much higher flux in EAR2 allows to:• measure samples of very small mass (<1 mg)

• measure short-lived radioisotopes (down to a few weeks)

• collect data on a much shorter time

• measure (n,charged particle) reactions with thin samples




185 m

20 GeV/c

protons

20 m

Spallation

Target

EAR1

EAR2

The much higher flux in EAR2 allows to:• measure samples of very small mass (<1 mg)

• measure short-lived radioisotopes (down to a few weeks)

• collect data on a much shorter time

• measure (n,charged particle) reactions with thin samples


Two different measurements at n_TOF

i) n+7Be -> a+a - Coincidences technique

ii)n+7Be -> p+7Li - Telescope technique


7Be(n,p)Silicon detectors (5x5 cm2, 20 and 300 mm thickness)outside of the beam

Detection of protons (E~ 1 MeV or 1.4 MeV)neutrons

7Be sample

Si detectorPurification of sample needed: ISOLDE

Sample:

• 100 ng of 7Be (material from water cooling of SINQ spallation source at PSI)

• Offline mass separation required at ISOLDE ≤1 day (starting from 100 GBq)

• Implantation on backing


105 mg/cm2 thick LiF converter (LNS)2.05 MeV a-particles are stopped in DE detector2.73 MeV tritons loose ~1 MeV

Hosted in vacuum chamber


Commissioning of the setup for 7Be(n,p) measurement

Inside SiMon2 chamber in EAR2



Amplitude of signals vs neutron energy for thin and thick detectors

at

t



E-DE matrix for coincidence eventsTritons region clearly visible

Measurement scheduled for 2016 campaign


neutrons

Silicon 1

Silicon 2

sampleaa

Silicon detectors directly inserted in the beam(3x3 cm2 active area, 140 mm thickness)

Detection of high energy a-particles

Strong rejection of background (sample preparation)


7Be(n,a)


neutrons

Silicon 1

Silicon 2

sampleaa

Silicon detectors directly inserted in the beam(3x3 cm2 active area, 140 mm thickness)

Detection of high energy a-particles

Strong rejection of background (sample preparation)

1

2

3

4

neutrons

Two different sandwiches of silicon detectors


7Be(n,a)


Det 1

Det 2

Possible to detect coincidences

7Be(n,a): Commissioning of the experimental setup

- LiF converter on mylar foil in the sandwich- Sandwich placed in the lower part of EAR2 beam dump

In-beam test


Upper energy limit (selecting one particle) 10 keV



In-beam test


Inverse current increasing with protons on target, but energy resolution still satisfactory



In-beam test


a - no g source

a - g source

ch

co

un

ts

Test with a-source (241Am) and g-source (137Cs, 35 GBq) at INFN-LNS

Detector (and electronic) can stand to high flux of g with minor degradation of the signal

7Be(n,a): Commissioing of the experimental setup

Samples


2 different samples:(3.5 mg total mass)

Electrodeposited

Evaporated

Evaporated Electrodeposited

Backing Stretched PE (0.6 mm)

Aluminum(5 mm)

Activity 20 GBq 19 GBq

Diameter 30 mm 31.6 mm

Prepared in hot-cell at PSI

Mechanic and Procedure


1-bare scattering chamber

2-detector and sample support

3-shielded chamber

4-dose rate

1 2

4

3

Mechanic and Procedure


Insertion of the samples and shipment

Insertion of the samples and shipment

7Be(n,a) chamber

Installation at n_TOF-EAR2


First signal


7Be(n,a) cross-section measurement



Two different sandwiches of silicon detectors

1

2

3

4

neutrons

Possible to evaluate random coincidences comparing uncorrelated couples of detectors



Coincidences as a function of time after last separation should show a constant behaviour if normalized to sample mass.

Need to compensate for worsening of charge collection



Neutron energy

Co

un

ts

First evidences of 7Be(n,a) reactions above 0.025 eV

Conclusions

• Uncertainties in nuclear data strongly affect the Big Bang Nucleosynthesis calculations for theabundance of 7Li and could probably explain (at least partially) the Cosmological LithiumProblem.

• n_TOF at CERN presented an experimental program to measure the two neutron inducedreactions responsible for the destruction of 7Be.

• 7Be(n,p) cross-section measurement is planned for 2016 at n_TOF-EAR2.- For the (n,p) measurement, isotopic purity of the sample will be done at ISOLDE (final mass 100 ng)

starting from 7Be collected at PSI.- Telescope technique will be used to detect protons coming out from the reaction.

• 7Be(n,a) cross-section measurement has been already performed, using n_TOF-EAR2 neutronbeam and two samples prepared at PSI.

• Coincidences of two a particles coming from 7Be(n,a) reaction have been observed for thefirst time above 0.025 eV.


measurement of be(n,cp) at n tof: a big challenge for ...€¦ · ear2 the much higher flux in ear2...

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