Scintillating bolometers for fast neutron spectroscopy in ... · Scintillating bolometers for fast...

Scintillating bolometers for fast neutron spectroscopy in

rare events searchesM. Martínez, N. Coron, C. Ginestra, J. Gironnet, V. Gressier, J. Leblanc, P. de Marcillac, T. Redon, P. di

Stefano, L. Torres, P. Veber, M, Velazquez, O. Viraphong

1M. Martínez - TAUP 2011 - Munich , 5th – 9th September 2011

Outline

• Introduction• Scintillating bolometers for neutron

detection• 6LiF: Calibration and performances • Other 6Li and 10B based materials• Conclusions and prospects

2M. Martínez – IAS (Orsay) TAUP 2011 - Munich , 5th – 9th September 2011

Neutron background in DM experiments

3

Fast neutrons produce nuclear recoils that can mimic the WIMP signal Fast neutrons sources: • Spontaneous fission (238U) and (α,n) in the laboratory rocks

• Typical flux undergournd: ~10-6 n/cm2/s, distribution peaks at 1-2 MeV• Strongly reduced with passive shieldings (Pb+CH2) (50 cm CH2→ 6 orders of magnitude reduction)

• µ-induced neutrons• Typical flux underground ~10-9/cm2/s , energies up to several 100 MeV• Can be tagged with active µ vetoes (but efficiency < 100%)

• (α,n) reactions in the shielding and detector itself• Use only ultrapure materials!

NucleusMinimal En (keV)

to produce a recoil Er> 10 keV

Ge 187Si 75Ca 105W 465O 45

Na 63I 323

Xe 333F 53Li 23

Estimations of neutron flux inside the detectors are usually based on MC simulations. There is a lack of experimental data

Why not to use as target a material that can detect neutrons?

n

INTR

OD

UCT

ION

M. Martínez – IAS (Orsay) TAUP 2011 - Munich , 5th – 9th September 2011

Fast neutron detection

4

• Methods based on nuclear reactions. The most used are:

6Li resonance (240 keV)

• Methods based on moderation:Bonner spheres The neutron is thermalized in a moderating material before

being detected as a thermal neutron

Fast neutrons: the neutron energy is shared between the reaction products:

Edetected = Qvalue + En Direct spectroscopy!

INTR

OD

UCT

ION


Neutron detection @ IAS

5

A program is ongoing between IAS and the ICMCB (Institut de Chimie de la Matière Condensée de Bordeaux) to produce 10B and 6Li enriched targets (6LiF, 6Li6Gd(10BO3)3, 6Li6Eu(10BO3)3 ...)

• Measurement of the thermal neutron flux at LSC:

Rare Objects SEarch with Bolometers UndergrounDROSEBUD is a collaboration between IAS and Universidad de Zaragoza to characterize scintillating bolometers as DM targets at the Canfranc Underground Laboratory (LSC)

One night @ IAS (sea level)

Scintillating bolometers for neutron spectroscopy for general applications

One night @ LSC (2500 m.w.e.)

(See Puimedon’s poster)

=> Φnth≲ (3.4 ± 2.8)x 10-6 n/(cm2 s)

• Measurement of the 252Cf neutron flux inside a lead shielding

In the last campaign, a 31 gr LiF (not enriched) have been tested:

241Am α sourcethermal neutrons

Fast neutrons

INTR

OD

UCT

ION


Scintillating bolometers

6

HEATCHANNEL

LIGHTCHANNEL

Reflecting cavity(Cu + Ag coat)

Scintillating crystal

(absorber)

Ge-NTDThermometer Ge-NTD

Thermometer

The light yielddepend on the

ionizing power of the incident particle

Particle discrimination by

the ratio HEAT/LIGHT

T ~20 mK

Very good energy resolution

The energy deposition is measured as a

temperature increment∆T = E/C

Optical bolometerGe disk (~ 25 µm thick)

Ultimate energy resolution: internal energy statistical fluctuation

CTkU Brms2=∆

Target SrF2

241Am α source

SCIN

TILL

ATIN

G B

OLO

MET

ERS

FOR

NEU

TRO

N D

ETEC

TIO

N


Scintillating bolomters for neutron detection

7

3H (2.73 MeV)

α (2.05 MeV)

α (1.47 MeV)

7Li (0.84 MeV)

γ (0.48 MeV)

6Li

10B

Slightly higher light yield than α’s

Slightly lower light yield than α’s

High light yield (when absorbed)

Thermal neutron

Thermal neutron

With 6Li and 10B based scintillating crystals, µ/β/γ and recoils(1) background can be discriminated. Also α background can be (partially) discriminated, depending on the light yield

µ/β/γ

16 gr LiF(not enriched)

241Am α

10B(n,7Li)α

µ/β/γ

120 mg Li6Eu(BO3)3

241Am α

(1) For Li, the recoiling nucleus can get up to 0.5 x the energy of the incident neutron, so we can have recoils with energies > 5 MeV

SCIN

TILL

ATIN

G B

OLO

MET

ERS

FOR

NEU

TRO

N D

ETEC

TIO

N


6LiF Efficiency

8

100% efficiency for thermal neutronsFor a ~400 gr 6LiF enriched at 95%:• 18 % efficiency @ 6Li resonance (240 keV)• 4% efficiency @ 1-2 MeV• 1% efficiency @ 6 MeV

6Li resonance (240 keV)

19F elastic scattering resonances

NOTE: Multiple scattering inside the crystal is not considered here, so the calculated efficiency is a lower limit

6 LiF


6 cm

6 cm

Portable neutron detector

9

6LiF (95%) 534 mg φ = 10 mm, thick= 2,6 mm

A portable neutron detector has been constructed that allows insitu thermal & fast neutron detection• 534 mg 6LiF (95%) in a 3He cryostat (active charcoal pumping)• Operating temperature 300-400 mK• Autonomy (without pumping): > 30 h• No optical bolometer (only heat channel)

Thesis Johann Gironnet (IAS, September 2010)

500

cm

Thermal neutrons

241Am α (5.485 MeV)

Fast neutrons (6Li resonance)

Measure with not thermalized 252Cf neutron source:

Energy resolutionFWHM = 28.7 keV@ thermal neutrons peak

6 LiF


Calibration in monoenergetic neutron source

10

The fast neutron energy recovery principle has been proved in a dedicated calibration in AMANDE (l’Institut de Radioprotection et de Sureté Nucléaire (IRSN), Cadarache, France)

Good linearity (error <3% from 50 keV to 17 MeV)

Thesis Johann Gironnet (IAS, September 2010)

Selected neutron energies

50 keV

241.5 keV

1.2 MeV

4.316 MeV

17 MeV

En=1.2 MeV

6 LiF


32 gr 6LiF (95% enriched)

11

•[111] oriented natural LiF seed 3.5mm×3.5mm×2cm• mfinal∼213g• original 6LiF powder purchased by IPNL (ANR SciCryo; 30€/g)• C graphite crucible (Φint=58mm)• Kyropoulos grown (Ar gaz; duration∼1.5 month)

• Grown @ ICMCB-Bordeaux

• Mounted and tested @ IAS

A first prototype scintillating bolometer 32 gr 6LiF enriched at 95% has been constructed:

• A 32 gr cylindrical crystal was extracted(φ=25.1 mm, h=25.2 mm)

6 LiF


Prototype 6LiF (collected) light yield

12

Light ratio

γ/α 3.5

γ/(α+3H) 2.8

α/(α+3H) 0.8

Collected Light yield

η(γ) 1.2 x 10-5

η(α) 3.3 x 10-5

η(α+3H) 4.1 x 10-5

The thermal/fast neutrons band is well separated from the µ/γ/β band down to 2000 keV (but not from the α band)

µ/β/γ

α 241Am

n+6Li->α+3H

Poor light yield (3 times lower than a 16gr LiF not enriched tested at IAS)

6 LiF


Prototype 6LiF fast neutron spectrum

13

• Energy resolution @ thermal peak ≈ 50 keV (FWHM)• The thermal neutron peak has a low energy tail due to surface escapes (~ 40%)• Fast neutron spectrum up to 12000 keV (→ En ~ 7 MeV)

Surface escapes6Li resonance

α 241Am

Selecting only events in the α+3H band:

Pile-up n/α not fully rejected

6 LiF


Other 6Li / 10B based materials

14

Li6Gd(BO3)3 Li6Eu(BO3)3

By now only small samples have been tested (120 mg, 5x5x5 mm3): Sample

Ge NTDBottom of the reflectingcavity

6 Li/

10B

mat

eria

ls


6Li6Gd(10BO3)3 (95% enriched)

15

157Gd + n → 158Gd + γ Qval = 7.94 MeV (255000 barns!!)

Gd “eats” thermal neutrons, reducing the overall efficiency at the thermal peak (cleaner fast spectrum near the peak)

3 nucleus with high cross section for thermal neutrons!

Compared with 6LiF, no 19F resonancesGood material for neutron

spectroscopy in the range 10-200 keV

6 Li/

10B

mat

eria

ls


6Li6Gd(10BO3)3 (95% enriched)

16

n+10B->α+7Li + γE=2.3 MeV (the γ escapes)(b.r. 94 %)

n+10B->α+7LiE=2.79 MeV(b.r. 6%)

n+10B->α+7Li + γE=2.3 MeV + γ compton(b.r. 94 %)

αγ

n+6Li->α+3H

n+10B->α+7Li

Closer look at the n+10B->α+7Li reaction:

• Poor light yield: η(α) = 7.3 x 10 -6 (5 times lower than 6LiF, but very different cavities) • Poor energy resolution (~8%)

Measure with not thermalized 252Cf neutron source + 241Am α source:

A new crystal has been re-grown and will be tested in the next months

6 Li/

10B

mat

eria

ls


Li6Eu(BO3)3 (not enriched)

17

γ

n+10B → α + 7Lin+10B → α + 7Li + γ(the γ escapes)

n+7Li → α + 3H

Coincidences α 241Am in LEuBO + compton of γ 60 keV or e- from 241Am in the optical bolometer

α 241Am

n+10B → α + 7Li + γFWHM = 13 keV

α 241Am (5485 keV)FWHM=32.5 keV

• Light yield: η(α) = 2.2 x 10-4 (x10 better than 6LiF)• Good energy resolution (13 keV FWHM @ 10B reaction peak)

2 nucleus for neutron detection (6Li, 10B)

Very promising material!

Measure with thermalized 252Cf neutron source + 241Am α source:

6 Li/

10B

mat

eria

ls


Prospects for EURECA

18

MASS: 1 ton of cryogenic detectorsGOAL: 10-10 pb (SI)KEY: Multi-target for WIMP identification

(Ge and solid state scintillators)

MonotargetMultisite

MonositeMultitarget

PRO

SPEC

TS


Prospects for EURECA

19

• Thermal neutron detection @ detectors location -> Valuable input/check for MC simulations • Fast neutron detection during neutron calibration• Cross-check neutron shielding efficiency in background runs• Fast neutron detection during background runs:

For example, to clarify the origin of the current background in bolometric Ge experiments:Current limits(1) ~ 3 x 10-8 pb ( ~0.01 c/kg/d) (threshold: 10-20 keV) (~10-20% attributed to neutrons)Rescaling with Ge neutron elastic scattering cross section & 6LiF efficiency (@ 1MeV) -> ~0.001 c/kg/d in 6LiF(2)

~ 10 kg 6LiF (95% enriched) could allow to monitor the fast neutron flux inside the experiment

MASS: 1 ton of cryogenic detectorsGOAL: 10-10 pb (SI)KEY: Multi-target for WIMP identification

(Ge and solid state scintillators)

MonotargetMultisite

MonositeMultitarget

6LiF

6LiF

(1): “Combined Limits on WIMPs from the CDMS and EDELWEISS Experiments” Arxiv:1105.3377(2): Neutrons producing nuclear recoils under 10-20 keV threshold can be seen by LiF

… utilizable also as DM target ?

Use also

6LiF as target!

PRO

SPEC

TS


Conclusions

• Neutrons are a relevant background for DM experiments and other rare searches.• Neutron detectors based in neutron-induced nuclear reactions allow direct fast neutron

spectroscopy, essential to measure low fluxes.• 6Li and 10B based scintillating bolometers are ideal detectors for thermal and fast

neutron detection, thanks to its good resolution (<1% FWHM) and the possibility of α/β/γ background discrimination. With enriched materials, efficiencies of ~ 5 % at 1 Mev and ~ 20 % at 240 keV are expected.

• A portable 0.5 g 6LiF (95% enriched) bolometer have been constructed and calibrated in a monoenergetic neutron source, proving the recovery energy principle.

• A 32 gr 6LiF 95% enriched scintillating bolometer have been constructed and successfully tested. 50 keV FWHM resolution is attained at the thermal neutrons peak

• 6LiF light yield is quite poor, allowing β/γ discrimination, but not fully α discrimination. Alternatives are being investigated:

– Other 6Li/10B based materials: 6Li6Gd(10BO3)3 , Li6Eu(BO3)3 (Very promising!) already tested– Doping LiF?

• Using 6LiF (or other 6Li/10B based materials) as target in a 1-ton cryogenic experience could allow to monitor the thermal/fast neutron flux inside the detector and would provide very valuable information for MC simulations.

20

CON

CLU

SIO

NS


Spare

21M. Martínez – IAS (Orsay) TAUP 2011 - Munich , 5th – 9th September 2011

6LiF surface escape probability

22

(If neutrons are thermalized at 4.22 K, their mean free path can be reduced down to ~20 µ)

Thermal neutrons mean free path in 6LiF(95%) = 170 µm

There is a high probability for the 3H or α to escape without depositing all its energy

α(2.05 MeV) in LiFRange = 6 µmFor m.f.p. = 170 µm, α escape probability ~4%

3H(2.73 MeV) in LiFRange = 35 µm

For m.f.p. = 170 µm, 3H escape probability ~20%

Combined escape probability: ~12%


LiF radiopurity

23

32 gr LiFOne night at LSC

1 α event

Thermal neutrons peak

PRO

SPEC

TS


LiF luminiscence spectrum

24

77K

4 K


Irradied LiF (300000 rad) luminiscece spectrum measured @ IAS with a fiber spectrometer:

NOTE: Spectrum not corrected by cavity reflectivity & spectrometer sensitivity

Scintillating bolometers for fast neutron spectroscopy in ... · Scintillating bolometers for fast...

Documents

Transcript of Scintillating bolometers for fast neutron spectroscopy in ... · Scintillating bolometers for fast...