Low background radioactivity measurements

Pia LoaizaLaboratoire Souterrain de Modane CNRS/CEA,

France

Why do we need ultra-low radioactivity measurements? Low background gamma spectrometry: How to achieve high sensitivity ? Material selection in astroparticle experiments Geochemistry applications Environmental control Other applications

Pia Loaiza ANDES Workshop 11-15 April 20112

U,Th,K(n,,,)

Shielding

close mat.

e-

n

Det.

Rn

• External gamma radiation, neutrons

• Rn and its progenies

• Radioimpurities in shielding materials

• Radioimpurities in materials close to detectors

• Contaminants in detector itself

materialscreening

Source Reduction

Shielding

Injection of Radon-reducedair

Why do we need measurements of ultra-low radioactivity levels ?

Dark matter and 0 experiments : natural radioactivity induces background in rare event searches experiments need to reduce drastically the radioactive background by material selection

Pia Loaiza ANDES Workshop 11-15 April 20113

1,4 kg of cables ~10 mBq/kg will cause 0.7 neutron/ kg Ge/ year [20-200] keV,(gamma background shielded by Pb) on the limit of acceptable levels

Edelweiss II, NEMO3 materials screened with a sensitivity about 1 mBq/kg

‘normal’ levels ~10 Bq/kg

How low is ‘low’ ?

ROCK in the Laboratoire Souterrain de Modane:238U : (10.4 2.5 ) Bq/kg232Th : (9.96 0.82) Bq/kg

CABLES in Edelweiss II:226Ra : (10 7) mBq/kg 228Th < 6 mBq/kg

COPPER in Edelweiss II:226Ra : < 40 Bq/kg228Th : (24 +/- 12) Bq/kg

~10-100 Bq/kg is OK

(1 Bq= 1 disintegration/s)

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Gamma background in EDWII:200 evts/kg Ge/day [20-3000] keV80 evts/kg Ge/day [20-200] keV

EURECA will need Cu with 226Ra, 228Th ~ 20 Bq/kg to reach dark matter sensitivity goal

SuperNEMO needs 0 sources : 208Tl < 2 Bq/kg & 214Bi < 10 Bq/kg

Present and future

Pia Loaiza ANDES Workshop 11-15 April 20115

238U decay chain

Mass spectrometry, Neutron Activation Analysis, Alpha-spectrometry

Sub-chains

HOW TO MEASURE?

ICP-MS ~ 0.01 ppb U/Th (about 0.1 mBq/kg)

• Mass spectrometry• Neutron Activation Analysis• Alpha-spectrometry• Gamma spectrometry

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232Th decay chain

Gamma emitters

Sub-chains

Mass spectrometry, Neutron Activation Analysis, Alpha spectrometry

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Low background Ge detectors for gamma-ray spectrometry

Backg. R . M. I. t

Detection Limit

R = resolution = efficiencyI = intensity of the lineM = sample masst = time of measurement

To improve sensitivity BACKGROUND REDUCTION

Cosmic rays reduction: go underground Environmental gamma reduction: shielding Intrinsic background reduction: material selection

Pia Loaiza ANDES Workshop 11-15 April 20118

Background sources in Ge gamma-ray spectrometry

• Muons on surface GO UNDERGROUND!

(Applied Rad and Isotopes 53 (2000) 191)

Background of HPGe spectrometer:

3300 m.w.e + shielding

15 m.w.e + shielding

2 106 muons/m2 day on surface

26 muons/m2 day at 3300 m.w.e

15 m.w.e

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DET.

e-

neutron

210Bi (210Pb)-

511 keV

Rn e-e+

Pb

Background sources in Ge gamma spectrometry deep underground

SOURCE REDUCTION

External gamma radiation (up to 2.6 MeV 208Tl) Shielding

Radioimpurities in cryostat Material selection

Rn and its progenies Rn reduced air

Radioimpurities in the shielding materials Material selection

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Ge detector types

COAXIAL WELL PLANAR

high sample mass high efficiencyhigh resolutionat low energies

The choice depends on what we want to measure

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Gamma-ray spectrometry at LSM

13 HPGe detectors for:

• Material screening for SuperNEMO, Edelweiss and ultra low background instrumentation(coaxial, planar)

• Environmental studies(well type, planar)

• Environmental monitoring (well type)

Developpement of low background Ge for -spectromety: Planar, P. Loaiza et al, NIM A 634 (2011) 64–70 Coaxial (arrived 2011)

Pia Loaiza ANDES Workshop 11-15 April 201112

Selected results of radioactivity measurements for material selection:

Where do we stand in terms of sensibility?

Material Detector Mass(g)

Time(h)

210Pb(mBq/kg)

234Th(238U)(mBq/kg)

226Ra(mBq/kg)

228Ra(mBq/kg)

228Th(mBq/kg)

Aluminium Mafalda

(Planar)

1025 132 < 9 <3 <0.9 <1 1.00.3

Epoxy Mafalda

(Planar)

47 384 357 143 92 < 6 103

Glue Iris

(Coaxial)

2500 768 < 0.135 < 0.274 < 0.174

Copper GeMPI2

(coaxial)

42500 564 <0.04 <0.06 0.020.01

Low energies:46 keV, 63 keV, 92 keV

Higher energies:200 keV < E < 3000 keV

Pia Loaiza ANDES Workshop 11-15 April 201113

210Pb P2 y = 186e-0,075x

R2 = 0,95

1

10

100

1000

0 5 10 15 20 25 30 35 40

210P

bex

c

V= 3,9 mm/an

Site B,core 2 Cs

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Prof (cm)

Act

ivit

é (B

q/k

g)

0

2

4

6

8

137Cs

241Am

Studies in lake sediments use radionucleide profiles to date and obtain the sedimentation rates. The requirement on sensitivity is less stringent than those for material selection, but still need low background detectors.

Geodynamic studies

Lac du Bourget

Dating using artificial radionucleides 137Cs and 241Am

1986Tchernobyl

1963Nuclear weapons tests

210Pb excess is used to determinethe sedimentation rate ( in this case 3.9 mm/year)

Pia Loaiza ANDES Workshop 11-15 April 201114

Lago del Desierto

• Lakes Puyehue and Icalma : F. Arnaud et al, Science of the Total Environment 366 (2006) 837-850• Chile and Peru: Muñoz et al, Deep-Sea Research II 51(2004) 2523-2541

Geodynamic studies in the southern hemisphere

Kastner et al, Global and Planetary Change 72 (2010) 201-214

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7Be and 137Cs concentration in the atmosphere

Monitoring of radioactive contamination in the atmosphereMeasurements of artificial radionuclides in certain samples require very low backgrounds.

Environmental monitoring

Pia Loaiza ANDES Workshop 11-15 April 201116

Bordeaux wine datingThe concentration of 137Cs provides a simple method to estimate the wine age.

Material selection for integrated circuits

Other applications

Atmospheric neutronsand on-chip radioactiveimpurities ( -particle emitters),induce soft-errorsin the semiconductors

Material selection usinghigh sensitive gamma-spectrometres is beingexploredG. Warot, P. LoaizaREE 3, Mars 2010, 51

Philippe Hubert, Europhysics News (2005) Vol. 36 No. 1

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SUMMARY

Ultra-low radioactivity measurements are needed for material selection in rare event searches, like 0 experiments and dark matter Required levels today ~ 1 - 0.1 mBq/kgRequired levels in future ~ 10 Bq/kg

Method of measurement depends on the radioelement: Mass spectrometry (ICP-MS) for long-lived isotopes, gamma-ray spectrometry for gamma-ray emitters

To improve sensitivity in Ge for -ray spectrometry background must be reduced: - Cosmic rays reduction: go underground - Environmental gamma reduction: shielding - Intrinsic background reduction: material selection

Low-background gamma-ray spectrometry used in several fields: astroparticle physics, geodynamic studies, environmental monitoring, …

Low background Ge for gamma-spectrometry is an ideal tool to be placed in a young underground laboratory: needed for rare-event searches but also may be usedfor several applications. Low cost.

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Costs

Detector with dedicated low-background developpement: between 100 kEuro and 200 kEuro, depending on crystal mass, cooling system,…

Shielding: Archeological lead: about 200-250 Euros/kg, Low activity lead: about 2 Euros/kg

Lead casting: around 20 kEuros

Commercial acquisition system (hardware + software) : about 10 kEuro

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Which sensitivities for the future experiments?

EURECA: • Present rejection factor ~ 105

• According to simulations: ~105 evts/year in 10 keV<E<50 keV in 1000 kg of Ge from Cu 226Ra, 228Th : 20 Bq/kg

The necessary sensitivity levels are reached,but time-consuming measurements needed

need more detectors

SuperNEMO 40 mBq/kg in 214Bi 3 mBq/kg in 228Th needed for PMTS

further reduce background?

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Performances Planar

Resolution: 850 eV at 122 keVIntegral count rate 20 keV <E < 1500 keV : 150 cpd

All peak-rates < 1 c/day, except 210Pb

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Bruit de fond intégral de quelques détecteurs Ge pour la spectrométrie gamma, divisé par la masse du cristal, en fonction de la profondeur des différents sites souterrains. Le détecteur ‘LSCE’ est de type puits et installé au Laboratoire Souterrain de Modane, ‘JRC-IRMM’ correspond à un détecteur de type coaxial installé au Laboratoire Hades, en Belgique, ‘LNGS’ correspond au taux d’un détecteur coaxial appartenant au groupe du Max Planck Institute de Heidelberg, installé au Gran Sasso.

La composante cosmique ne contribue pas au bruit de fond au IRMM : le taux intégraldes détecteurs IRMM est comparable à ceux de sites plus profonds

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Radionuclides in the U and Th decay series are useful chronometers for the determination of many processes in the environment. The low natural radioactivity encountered necessitate instrumentation capable of measuring very low radionuclide concentrations.Some applications :

Quantitative evaluation of both horizontal and vertical mixing rates in the open ocean.

Determination of the rate of particle deposition on the marine sediment layer (originated by both biological and physical processes).

The decay of 210Pb provides a dating method which has been applied to lake sediments.