Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE
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Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE. MARISA PEDRETTI INFN - Milano. Outline. The radioactivity background problem Basic idea of Surface Sensitive Bolometers Test Runs @ Insubria University (2004-2005)

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Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE

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Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Surface sensitive bolometers (SSB) for the rejection of continuous alpha background in CUORE

MARISA PEDRETTI

INFN - Milano


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Outline

  • 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


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

bb (0n) 130Te

CUORE and Cuoricino

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

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

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

The Cuoricino background in 0nDBD region is

0.18±0.01 c/(keV kg y)


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

TeO2

TeO2

208Tl

214Bi

Cu

60Co

p.u.

CUORE and Cuoricino

MC: the background in CUORICINO is due to degraded alpha’s

which release only a part of their energy in the detector

(surface contamination)

Cuoricino background


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Heat sink

5 cm

Cu holder

Thermal coupling

Thermometer

NTD

sensor

particle

DT= E/C

Signal:

Crystal absorber

TeO2

cristal

Basic principle of bolometric technique


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Classical pulse

Classical pulse

If 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

DV

Classical pulse

Fast high and saturated pulse

TeO2 thermistor

The Surface Sensitive Bolometers

background discrimination

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

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


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

  • Pros

  • Low cost

  • Pros

  • Thermal contractions with main absorber

  • Known material

  • Cons

  • Low purity

  • Cons

  • Fragilty

  • Thermal coupling with thermistors

SSB @ Insubria University

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

Ge shields

Si shields

TeO2 shields

  • Pros

  • Excellent results

  • High purity

  • Cons

  • High cost


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Germanium

alfa peaks

Events in the main bulk

Example of experimental Scatter Plot

212Po

216Po

220Ra

224Ra


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB: discrimination by using pulse time

  •  ~ 6.4 ms: events on Ge bolometer

  •  ~ 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

Counts

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

Rise time on Ge_slab [ms]


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB @ LNGS: description

Above ground tests are difficult due to cosmic rays

In summer 2005 we test large SSB bolometers at LNGS

Absorber TeO2 5x5x5 cm3

Si shields

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


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB @ LNGS: pictures

Test in NOT clean condition!

Many wiring and construction problems (very complicated assembly)


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB @ LNGS: results

The scatter plot is coherent with the expected behaviour.

However, there are some problems. For example…

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

Surface events

Mixed events

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

Amplitude on slabs (a.u.)

Bulk events

Amplitude on TeO2 (a.u.)


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Surface Sensitive Bolometer

Scatter plot

τrvs amplitude

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

Rise time (ms)

Amplitude on slabs (mV)


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Cuoricino- like detector

without SSB

detector with slabs

(SSB)

anomalous events

SSB: decay time on main bolometer

Decay time vs Energy

for pulses read by the main TeO2 absorber

Decay time on main bolometer

Amplitude on main bolometer

Decay time on main bolometer

Amplitude on main bolometer


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

E [MeV]

2.9-3.2

3.2-3.4

3.4-3.9

bkg

(no shields)

[c/kg keV y]

0.44

±

0.06

0.58

±

0.08

0.51

±

0.04

bkg (shields)

[c/kg keV y]

0.18

±

0.08

0.51

±

0.16

0.29

±

0.08

SSB @ LNGS: results

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

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

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

Good active efficiency


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

Surface Sensitive Bolometer

SLAB 1

MAIN

SLAB 2

D3

URANIUM

SOURCE

2 cm

VACUUM

GREASE

GLUE

0.5 cm

0.5 mm

0.5 mm

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


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

238U alphas

( ≈ 4.2 MeV)

234U alphas

( ≈ 4.7 MeV)

surface events

mixed events

bulk events

Surface Sensitive Bolometer

SLAB 1 (implanted)


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

MC simulations: contamination in slab

MC - Simulation


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

surface events

bulk events

Surface Sensitive Bolometer

SLAB 2

(see the implanted side of the crystal )

mixed events


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB @ 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


Surface sensitive bolometers ssb for the rejection of continuous alpha background in cuore

SSB @ LNGS : future test


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