The Bi-Po detector for ultralow radioactivity measurements - LRT2006

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The Bi-Po detector for ultralow radioactivity measurements - LRT2006

Low Radioactivity Techniques 2006

Aussois

The BiPo detector for ultralow

radioactivity measurements

Mathieu Bongrand

LAL – Paris XI / Orsay University

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 1


Outlines

BiPo detector : principle and designs

● Background

BiPo prototype and status

● Summary

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 2


BiPo Detector : principle and design

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 3


238 U

232 Th

BiPo detector : goal and principle

Measure the radiopurity in 208 Tl and 214 Bi of SuperNEMO source foils before installation

Goal : 5 kg of foils (12 m 2 , 40 mg/cm 2 ) in 1 month with a sensitivity of :

Bi­Po Process

β

214 Bi

(19.9 mn)

0.021%

210 Tl

(1.3 mn)

β

212 Bi

(60.5 mn)

36%

208 Tl

(3.1 mn)

214 Po

(164 μs)

α

210 Pb

22.3 y

212 Po

(300 ns)

α

208 Pb

(stable)

208 Tl < 2 μBq/kg and 214 Bi < 10 μBq/kg

scintillators

A possible design

prompt e ­ (Q β = 3.3 MeV)

low background 5'' PMTs

wire chamber

delayed α (T = 164 μs)

1/2

source foil

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 4


238 U

232 Th

BiPo detector : goal and principle

Measure the radiopurity in 208 Tl and 214 Bi of SuperNEMO source foils before installation

Goal : 5 kg of foils (12 m 2 , 40 mg/cm 2 ) in 1 month with a sensitivity of :

Bi­Po Process

β

214 Bi

(19.9 mn)

0.021%

210 Tl

(1.3 mn)

β

212 Bi

(60.5 mn)

36%

208 Tl

(3.1 mn)

214 Po

(164 μs)

α

210 Pb

22.3 y

212 Po

(300 ns)

α

208 Pb

(stable)

208 Tl < 2 μBq/kg and 214 Bi < 10 μBq/kg

scintillators

prompt e ­ (Q β = 2.3 MeV)

low background 5'' PMTs

delayed α (T 1/2 = 300 ns)

wire chamber

source foil

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 5


GEANT 4 simulations :

Alpha tagging efficiency

25 % α escaping the foil with

energy > 1 MeV

(Due to quenching 40 keV are

deposited in the scintillator)

For 5 kg of 82 Se source foil (~ 12 m 2 and 40 mg/cm 2 ) :

Efficiency

– 2 μBq/kg in 208 Tl = 52 BiPo events / month (e ­ , delayed α)

– ε ~ 25 % ⇉ 12 events / month

Thickness of the foil (mg/cm 2 )

Initial energy of the α : E = 8.78 MeV ( 212 Po)

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 6


Measurement of alpha quenching

Alphas energy is reduced by pilling­up 6 μm mylar foils

GEANT 4

α

241 Am source

Q α = 5.6 MeV

n times 6 μm

mylar foils

Quenching factor for

α @ 1 MeV ~ 25

α @ 1 MeV→ ~ 40 keV

deposited in the scintillator

(NEMO 3 threshold)

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 7


2 possible designs

● Scintillators calorimeter + tracking detector

(NEMO 3 technique) :

– ε ~ 6 %

– 212 Bi and 214 Bi measurements

● Multilayer scintillator plates (MOON 1 technique) :

– ε ~ 25 %

– no 214 Bi measurements

prompt e ­

delayed α

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 8

212 Bi

212 Bi

delayed α

prompt e ­ delayed α

This 2 designs will be tested during the R&D phase

214 Bi


Background

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 9


2000

Shielding Test Facility : phase I

The 2 designs will be tested in a shielding facility in the Canfranc

Underground Laboratory

1050

Wood

300

1450 x 1450

2300 x 2300

Up to 25 capsules can be installed

Radon­tight tank

(pure iron)

Radon free air

Lead shield (13 tons)

Water shield

Shielding Test Facility :

external : 2.3 m x 2.3 m x 2 m

internal : 1.45 m x 1.45 m x 1.05 m

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 10


Shielding Test Facility : phase II

70 cm

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 11


212 Bi bulk contamination

Prompt e , T 0

e −

α

Prompt e ­ , T 0

Main background

212 Bi surface contamination

Prompt e , T 0

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 12

e −

α

Delayed α, T 1/2 ~ 300 ns

Bkg event rejected Bkg event NOT rejected

An electron from 212 Bi deposits ~ 50 keV in average in 100 μm of scintillator


NEMO 3: 212 Bi bulk contamination

Very simple measurement :

Oscilloscope

E α = 8.8 MeV

No events observed in 3 days :

A( 212 Bi) < 3 μBq/kg

e ­

~ 1 MeV deposited

α

300 keV

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 13

e −

700 ns


NEMO 3 : 212 Bi surface contamination

Trigger

e −

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 14

α

Delayed alpha

from 40 to 130 ns

NEMO 3 scintillator wrapped with aluminised mylar

(standard aluminium)

212 Bi surface activity : 150 μBq/m 2

Our hypothesis : aluminised mylar contamination

(not scintillators surface contamination)


BiPo prototype

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 15


BiPo prototype : background

measurements

Surface contamination and random coincidences measurements

α

e ­

● First capsule :

– 2 scintillators 20 x 20 x 1 cm 3 from NEMO 3

production

– scintillators spottered with ultrapure

aluminium

– installation in Canfranc Underground

Laboratory in October, 17 th

If 212 Bi surface contamination 150 μBq/m 2 comes from scintillators :

8 events / month / capsule

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 16


α

e ­

BiPo prototype : soon

Surface and bulk contamination measurements

● Second capsule :

– 2 scintillators 20 x 20 x 1 cm 3

– scintillators wrapped with aluminised mylar

(standard Al ­ NEMO 3 like) for surface

contamination measurements

● Third capsule :

– 1 scintillator 20 x 20 x 20 cm 3 for bulk

contamination measurements

● Later : serial production of capsules

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 17


Summary

BiPo detector is needed to measure the radiopurity

of SuperNEMO source foils

( 208 Tl < 2 μBq/kg and 214 Bi < 10 μB/kg)

BiPo prototype is dedicated to background

measurements

● A first capsule will be installed October, 17 th in

Canfranc

● A Shielding Test Facility is being constructed

The 2 BiPo possible designs will be tested in this

shielding in Canfranc

Mathieu Bongrand Low Radoactivity Techniques 2006 ­ Aussois 18

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