Results obtained withlarge mass surfacesensitive detectors

Zaragoza -November 7, 2005

University of Insubria - Como, Italy

INFN - Milano, Italy

Surface background in CUORE Surface background in CUORE

CUORE – of 130Te Q= 2528.8 keV

Cuoricino background analysis and Monte Carlo simulations allow to make predictions on the future background expected for CUORE…

From what we knowuntil now, experimental

data and simulations suggest that the main

contribute expected for CUORE background in the

DBD region could be:

and degraded particlesemitted by 238U and 232Thsurface contaminations

on the Cu frame

BKG = 0.18±0.01 c/(keV kg y)

T1/2 > 2×1024 y @ 90% C.L.

130Te

Surface background reductionSurface background reduction

There are three possible ways to reduce the background dueto surface Cu contaminations:

Minimizing the amountof Cu surface facingthe detectors

Improving the quality of surface treatment for Cu components

Creating new typesof detectors able to recognize surface events

This can be done by shielding a bolometer with layers of different materials.

Each layer is an auxiliary bolometer…

…thermally coupled to a “classical” bolometer.

This detector is a Surface Sensitive Bolometer (SSB)

Dynamic behaviour of SSBsDynamic behaviour of SSBs

TeO2 main bolometer

Active shield

Classic pulse

Classic pulse

Classic pulse

Fast and high pulse

Event originating inside the main bolometer (DBD event)

Event originating outside the main bolometer (degraded )

The origin of events can be determined. The addition of active shields alters the dynamic behaviour of the bolometer, leading to pulses different in both amplitude and shape. An -surface event gives distinct pulses on the thermistors of the SSB.

Particles releasing energy in the detector heat up each element of this composite bolometer:

Identification of surface events – Amplitude Identification of surface events – Amplitude (1)(1)

Surface events may be identified comparing pulse amplitudes

read simultaneously by the thermistor on the main absorber and on

the active shield.

The main tool is a graph reporting the pulse

height from the shield sensor vs the pulse

height from the main sensor.

SCATTER PLOT

Am

plitu

de o

n ac

tive

shie

ld s

enso

r [

mV

]

Amplitude on main absorber sensor [mV]

A DBD event occurring inside the absorber gives an equal amplitude pulse on both sensors

An particle coming from outside the detector gives a higher amplitude pulse on the shield sensor than on the main sensor

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

Surface events

Surface events

TeO2 shieldsPros

•Low cost

Cons

•Low purity

Notes

•IET Si slabs (Polish) gave good results

•IRST Si slabs (Italian) gave some unexplained results

Pros

•Thermal contractions with main absorber

•Known material

Cons

•Frailty

•Thermal coupling with thermistors

SSB prototypes @ ComoSSB prototypes @ Como

Ge shields Si shieldsPros

•Excellent results

•High purity

Cons

•High costSurface events

Parallel readout!

Identification of surface events – Rise time Identification of surface events – Rise time (2)(2)Not just pulse amplitude discrimination But also pulse shape discrimination

Cou

nts

Rise time on active shield thermistor [ms]

Surface events correspond to pulses with shorter rise times on the shield sensor

Two classes of events

Rise time discrimination is a powerful tool to isolate outer events

Preparation for the large mass testPreparation for the large mass test

After testing a great deal of SSB prototypes, a test with CUORE real size crystals was organized.

TeO2 crystals, size 5×5×5 cm3, m=750 g

Main absorber

Si slabs produced by IRST, size 5×5×0.3 cm3, resistivity 20kΩ(floating zone crystal with < 1012 atoms/cm3 of P)

Active shields

Initial idea for the active shields: TeO2. However, the producer could not deliver slabs in time for the beginning of assembly. Therefore…

Four SSBs with these characteristics were assembled.

Slab sensors connected in parallel configurationSignal readout

Moreover, in order to reduce the number of necessary channels…

Test @ LNGS – Single SSBTest @ LNGS – Single SSB

Two configurations - 3 detectors type A, 1 detector type B

Type A Type B

Very different from both a thermal and a mechanical point of view

Slabs thermally coupled to absorbers through Ge stand-offs; teflon holders keeping crystals attached to the Cu frame.

Test @ LNGS – Four detectors moduleTest @ LNGS – Four detectors module

The four SSB module (Cu frame and PTFE holders)…

…and the whole structure mounted in the cryostat (along with other eight nude crystals for an assembly test).

Test @ LNGS - ProblemsTest @ LNGS - Problems

Only two crystals were firmly held.Crystals with unequal heights

The fully covered crystal (even if its height was increased by PTFE holding it through the slabs) was not securely fixed.

The last one was not held at all.

Slabs not fully realiable Si slabs by IRST had previously lead to results whose physical origin has yet to be determined.

Difficult assembly Because of very different thermal contractions between Si and TeO2, slabs must be glued only close to the center of the TeO2 crystal face.

Many slabs detached during assembly and had to be provisionally attached with vacuum grease.

Lost signals During the test, we lost contact with a set of slab sensors in parallel: signals from one SSB could not be acquired.

A lot of problems, but in spite of them…

The two firmly held crystals worked properly and one of them had readable slabs, from which we gathered data.

Test @ LNGS – Scatter plotTest @ LNGS – Scatter plot

The scatter plot is coherent with the expected behaviour.

Surface events

Bulk events

Mixed events

However, there are some problems. For example…

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

• This is probably a slab with a bad thermal coupling to the absorber.

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

Test @ LNGS – Rise time distribution on Test @ LNGS – Rise time distribution on shieldsshieldsAgain as expected, we can recognize various classes of events from the rise time distribution on the parallel slabs channel. Rise time vs amplitude plot on slab sensors:

Surface events & decay time on main crystalSurface events & decay time on main crystal

Surprise: there is another way to distinguish surface events from bulk events.

Decay time – amplitude relation on main absorber sensor

Detector with no slabs Detector with slabs

Identification of surface events – Decay time Identification of surface events – Decay time (3)(3)

Bulk events are well isolated by a cut in the decay time – amplitude plot on the main sensor

Shields as pulse shape modifiersShields as pulse shape modifiers

Shields act as pulse shape modifiers – behaviour to be investigated in the future. We found a similar feature in the previous tests at Como.

Important feature…Is it possible to get rid of the slab sensors?In this way, slabs could be a very promising option for CUORE.

Background reductionBackground reductionCutting on the DT distribution allows to isolate a

great deal of unwanted events.Quantitatively – background

obtained region)

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 slabs)

[c/kg keV y]

Bkg (slabs)

[c/kg keV y]

0.44±

0.06

The region shows a considerable improvement: the obtained value is very similar to the Cuoricino value.

Important result: even though the test was not performed in extremely clean conditions, the background is comparable to that obtained with accurately cleaned detectors…

Rejection by slabs works

Conclusions & future investigationsConclusions & future investigations

Not complete efficiency of the SSB rejection (high contamination, not working slabs)

Next test at LNGS

Four detectors with TeO2 slabs (problem of thermistor coupling to be solved)

Careful cleaning of all the detectors

Single readout of the slab sensors of two detectors (instead of parallel)

“Passive” slabs for the other two detectors (can we use just the main absorber sensor?)

Origin of the decay time distribution on the main absorber sensor and reproducibility of surface events identification with this parameter

Topics to be cleared by analysis & future tests

Motivation for the background value (~ 0.2 c/kg keV y): SSBs are proving themselves to be very helpful in understanding the nature of the background.