Surface sensitive bolometers (SSB) for the rejection of continuous

alpha background in CUORE

MARISA PEDRETTIINFN - Milano

OutlineOutline

• The radioactivity background problem

• Basic idea of Surface Sensitive Bolometers

• Test Runs @ Insubria University (2004-2005)

• Run I @ LNGS (July – August 2005)

• New Small SSB Test @ LNGS (Sept – Oct 2005)

• The Future Run II @ LNGS

130Te

CUORE and CuoricinoCUORE and Cuoricino

CUORE will use bolometer technique for the searching for 0DBD of 130Te

CUORE sensitivity will depend on the radioactivity background in the 0DBD region

Cuoricino is not only a self consistent already running experiment but it is also a test for CUORE experiment.

The Cuoricino background in 0DBD

region is 0.18±0.01 c/(keV kg y)

MC: the background in CUORICINO is due to

degraded alpha’swhich release only a part of their energy in the detector

(surface contamination)

214Bi

60Co p.u.

208Tl

TeO2 TeO2

Cu

Cuoricino background

CUORE and CuoricinoCUORE and Cuoricino

Heat sink

Thermal coupling

particle

Thermometer

Basic principle of bolometric techniqueBasic principle of bolometric technique

Crystal absorber

Cu holder

NTDsensor

5 cm

TeO2

cristalSignal: T = E/C

The Surface Sensitive BolometersThe Surface Sensitive Bolometers

The idea is to use auxiliary thin bolometers to control surface contamination

Classical pulse

Classical pulse

Classical pulse

Fast high and saturated pulse

TeO2 thermistor

VIf we make a scatter plot of coincident pulse amplitude (pulse amplitude obtained with thermistor on secondary crystal versus those obtained with thermistor on TeO2 crystal) we obtain two well separated behaviours

background discrimination

An other possibility is to discriminate by using the shape of coincident pulses

SSB @ Insubria UniversitySSB @ Insubria University

Various materials for active shields were tested on small size detectors (main absorbers 2×2×2 - 2×2×0.5 cm3)

TeO2 shields

Pros

•Low cost

Cons

•Low purity

Pros

•Thermal contractions with main absorber

•Known material

Cons

•Fragilty

•Thermal coupling with thermistors

Ge shields Si shields

Pros

•Excellent results

•High purity

Cons

•High cost

Germanium

Example of experimental Scatter Plot

alfa peaks

Events in the main bulk224Ra

220Ra

216Po

212Po

SSB: discrimination by using pulse time SSB: discrimination by using pulse time

• ~ 6.4 ms: events on Ge bolometer

Rise time on Ge_slab [ms]

Counts

~ 9.6 ms: events on TeO2 bolometer

There are 2 well separated families: fast

pulses corrispond to surface events whereas slow pulses correspond

to bulk events A selection of the events by their rise time clearly identifies the two event bands

SSB @ LNGS: descriptionSSB @ LNGS: description

Above ground tests are difficult due to cosmic raysIn summer 2005 we test large SSB bolometers at LNGS

Si shields Absorber TeO2 5x5x5 cm3

Parallel read-out of signals in order to reduce the read-out channels

SSB @ LNGS: picturesSSB @ LNGS: pictures

Test in NOT clean condition!

Many wiring and construction problems (very complicated

assembly)

SSB @ LNGS: resultsSSB @ LNGS: results

Surface events

Bulk events

Mixed events

Am

plit

ude o

n s

labs

(a.u

.)

Amplitude on TeO2 (a.u.)

The scatter plot is coherent with the expected behaviour.

However, there are some problems. For example…

• There is a considerable contamination ( in the slabs or close to them.

• While surface events may be identified, the parallel configuration seems to confuse the physical understanding of each element in the plot.

Surface Sensitive BolometerSurface Sensitive Bolometer

τr vs amplitude

It is possible to identify all the families in the scatter plot by using rise time on slabs.

Amplitude on slabs (mV)

Ris

e t

ime (

ms)

Scatter plot

SSB: decay time on main bolometerSSB: decay time on main bolometer

Cuoricino- like detector without SSB

Amplitude on main bolometer

detector with slabs(SSB)

Amplitude on main bolometer

Deca

y t

ime o

n m

ain

bolo

mete

r

Deca

y t

ime o

n m

ain

bolo

mete

r

anomalous events

Decay time vs Energyfor pulses read by the main TeO2 absorber

SSB @ LNGS: resultsSSB @ LNGS: results

Good efficiency in the active backgroud discrimination by the decay time read on the main absorber

E [MeV] 3.2-3.4 3.4-3.92.9-3.2

0.58±

0.08

0.51±

0.04

0.18±

0.08

0.51±

0.16

0.29±

0.08

bkg(no shields)

[c/kg keV y]

bkg (shields)

[c/kg keV y]

0.44±

0.06

Important result: the bkg obtained with high contamination is similar to the one obtained after a careful cleaning

Good active efficiency

Cutting on the DT distribution allows to isolate a great deal of

unwanted events.

Surface Sensitive BolometerSurface Sensitive Bolometer

D3

GLUE

VACUUM GREASE

URANIUM SOURCE

0.5 cm 0.5 mm0.5 mm

2 cmSLAB 1 SLAB 2MAIN

TEST RUN: we studied in which way contamination in different point of the detector contribute in a scatter plots

SLAB 1 (implanted)

234U alphas ( ≈ 4.7 MeV)

238U alphas ( ≈ 4.2 MeV)

bulk events

surface events mixed events

Surface Sensitive BolometerSurface Sensitive Bolometer

MC simulations: contamination in slabMC simulations: contamination in slab

MC - Simulation

Surface Sensitive BolometerSurface Sensitive Bolometer

SLAB 2 (see the implanted side of the crystal )

surface events

bulk events

mixed events

SSB @ LNGS : future testSSB @ LNGS : future test

Goal: see if it is possible to reduce the background level clean condition.

• 4 crystals with SSB • 0.9 mm thickness TeO2 SSB• connection between slabs and

main crystal made by 12 glue spots

• 2 crystals with ntd on all the slabs read independently

• 2 crystals with 5 slabs without ntd and with only a read out slab.

• Coupling between ntd’s and slabs: epoxy glue

SSB @ LNGS : future testSSB @ LNGS : future test