Status of Surface Sensitive Bolometers University of Insubria – Como, Italy INFN – Milano, Italy...

Status of SurfaceSensitive Bolometers

University of Insubria – Como, Italy

INFN – Milano, Italy

Prague, 20.04.2006Chiara Salvioni

Outline of the presentation (1)Outline of the presentation (1)

Topics to be discussed

Brief summary of Surface Sensitive Bolometers activity

Latest experimental tests

Detector simulations

Next SSB test @ LNGS

Surface background in CUORESurface background in CUORE

Goal: () of 130Te

~ 1000 TeO2 bolometers

Q = 2530.3 keV

Predictions on the future background expected for CUORE from Cuoricino background analysis and Monte Carlo simulations...

Experimental data and simulations suggest one major contribute for CUORE background in the DBD region:

and degraded particles emitted by 238U and 232Th surface contaminations on the Cu frame and

on the crystal surface.

BKG = 0.18 ± 0.01 c/(keV kg y)

T1/2 > 2×1024 y @ 90% C.L.

Cuoricino

130Te

TeO2

CuTeO2

Surface Sensitive BolometersSurface Sensitive Bolometers

Background reduction may be achieved through both passive

and active methods

Creation of a new kind of detectors able to recognize surface events

Identification of background events

Surface

Sensitive

Bolometers

Auxiliary bolometer Main bolometer

SSB

Classic pulse

Classic pulse

Classic pulse

High and fast pulse

Dynamic behavior:

Event originating inside the main bolometer (DBD event)

Event originating outside the main bolometer (degraded )

The difference between heat capacities generates a difference

in pulse height and shape...

Idea: cover each face of a classic bolometer by gluing an active layer, in order to provide a 4 shielding

First SSB experimental results (Como)First SSB experimental results (Como)

Amplitude comparison

According to the described dynamic behavior, various pulse parameters proved to be effective in discriminating surface events.

(Scatter plot)

-Individual thermistor read-out-Parallel thermistors read-out

r on auxiliary thermistor

Bulk events

Surface events

Pul

se a

mp

litu

de

on

aux

ilia

ry N

TD

[m

V]

Pulse amplitude on main NTD [mV]

d on main thermistor

Surface events

Bulk events


Pul

se a

mp

litu

de

on

aux

ilia

ry N

TD

[m

V] “Slow” bulk

events

“Fast” surface events

(To be investigated)


Pul

se d

ecay

tim

e on

mai

n N

TD

[ms]


Brief resume of Surface Sensitive Bolometers activity




Recent LNGS tests – Run 1Recent LNGS tests – Run 1

SSB Run 1

Tests performed in Como had all small main TeO2 absorbers (2 cm3); moreover, various shield materials and different techniques to couple layers and crystals had already been tried.

Features:

Main absorber 5×5×5 cm3

Full coverage of 4 TeO2 crystals

Si active shields

Parallel thermistors read-out

Run 1: scatter plotRun 1: scatter plot

Surface events

Bulk events

Mixed events

Recap of the results:

Parallelread-out

Run 1: decay time on main thermistorRun 1: decay time on main thermistor

We also found a structure in the decay time distribution vs amplitude for pulses read by the thermistor on the main TeO2 crystal:

Pu

lse

de

cay

time

on

ma

in N

TD

[m

s]


SSB

Pul

se d

eca

y tim

e o

n m

ain

NT

D [

ms]


Not shielded

3D plot3D plot

Note

Recent LNGS tests – Run 2Recent LNGS tests – Run 2

SSB Run 2

Aims

Features:

Main absorber (small) 2×2×0.5 cm3

One TeO2 crystal with two shields (no full coverage)

TeO2 active shields

Independent thermistors read-out

Alpha source implanted in two differentpoints of the detector

TeO2

TeO2

shield 1

TeO2

shield 2

Vacuum grease

Glue

Alpha source

-verify how contaminations in different points of the detector contribute to scatter plots

-read shield thermistors independentlyVery important to understand

if we can identify contaminations that are not

just external, but also internal to the detector itself

Run 2: scatter plots (1)Run 2: scatter plots (1)

Shield 1 (implanted)

Surface events

Bulk events

Mixed events

3D plot3D plot

238U ’s (~4.2 MeV) 234U ’s

(~4.7 MeV)

Surface events

Bulk events

Mixed events

Shield 2 (facing the implanted side of the main crystal)

Run 2: scatter plots (2)Run 2: scatter plots (2)


Surface contaminations of the thin TeO2 layer

Origin: due to nuclide recoil, there is a fixed maximum energy that can be released in the

main absorber(dependence on

contamination depth)

Run 2: rise time on shield thermistorsRun 2: rise time on shield thermistors


Shield 2 (facing the implanted side of the main crystal)

Ris

e tim

e on

shi

eld

2

Amp. on main Amp. on shield 2

Surface events – fast signals: r ~ 2 ms

Run 2: decay time on main thermistorRun 2: decay time on main thermistor

Bulk events

Surface events –shield 1

Surface events –shield 2

3D plot3D plot

Thermal model of SSBsThermal model of SSBs

C1 9.9x10-9·T C6 4.3x10-8·T3 g50 1.8x10-5·T2.4

C2 2.7x10-8·T3 g20 9.6x10-5·T2.4 g54 1.3x10-1·T4.37

C3 2.3x10-3·T3 g21 7.0x10-1·T4.37 g63 5.2x10-4·T3

C4 1.9x10-9·T g30 4.0x10-5·T2 g65 2.6x10-4·T3

C5 5.1x10-9·T3 g32 2.3x10-3·T3 T0 9 mK

6-node model

Shield/crystal, NTD/shield and NTD/crystal thermal couplings realized with glue

“Big” TeO2 crystal (CUORE size) with one Si shield

Thermistor/heat bath: gold wires

Main crystal/heat bath: PTFE

Parameters for static & dynamic simulations

Initial values

Test 0: decay time on main thermistorTest 0: decay time on main thermistor

Focus on: pulse decay time in the main thermistor read-out

Test #0

Tests performed varying one parameter at a time (or a group of connected parameters)

Energy releasedIn the shield

Energy releasedIn the main absorber

The decay time growth vs amplitude reflects the behavior observed in

experimental tests

Tests 1 & 2 [links to the heat bath]Tests 1 & 2 [links to the heat bath]

Test #1 Test #2

g50 x 6 (shield thermistor

gold wires)

g30 x 10 (main/bath

PTFE)

Shield energy rel.

Bulk energy rel.

Shield energy dep.

Bulk energy dep.

Shield energy

dep.

Bulk energy dep.

@ 3.1 MeV

Tests 3 & 4 [passive auxiliary NTD]Tests 3 & 4 [passive auxiliary NTD]

Test #3 Test #4

g50 = 0 (shield thermistor not

polarized –passive node)

g54

C4

C5

x 6(shield thermistor as

a passive nodeand with larger

volume)

Shield dep.

Bulk energy dep.

Shield dep.

Bulk energy dep.

Shield dep.

Bulk dep.

@ 3.1 MeV

This decay time vs amplitude trend corresponds to

experimental results

g50 = 0

Tests 5 [shield heat capacity]Tests 5 [shield heat capacity]

Test #5

C6 x 100

g50 = 0(larger shield heat capacity

and passive aux. thermistor)

Shield energy dep.

Bulk energy dep.

Shield energy dep.

Bulk energy dep.

@ 3.1 MeV

A thermal add-on to the slab?

Next LNGS test: Run 3Next LNGS test: Run 3

SSB Run 3 Features:

“Big” main absorbers: 5×5×5 cm3

4 SSBs featuring total coverage (6 shields each)

TeO2 active shields (reasons: -thermal contractions with main absorber -known material)

2 SSBs have thermistors on each layer and independent read-outs

2 SSBs have 5 “passive” slabs (no thermistor) and one readable slab

Thicker TeO2 slabs (0.9 mm) to avoid known mechanical problems

Great attention for clean working conditions in order to avoid possible contamination sources

Run 3: single SSBRun 3: single SSB

Single detector assembly

Run 3: assemblyRun 3: assembly

4-SSB module

Run 3: remarksRun 3: remarks

The 4-SSB module will be cooled down with other 8 unshielded crystals (possibility to compare background results)

Presence of 5 “passive” slabs on 2 SSBs to understand if they work as pulse shape modifiers; the remaining shield thermistor will help evaluate if event discrimination by main channel read-out is possible

Test of background reduction

Status of Surface Sensitive Bolometers University of Insubria – Como, Italy INFN – Milano, Italy...

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