surface sensitive bolometers (ssb) for the rejection of continuous alpha background in cuore
DESCRIPTION
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) - PowerPoint PPT PresentationTRANSCRIPT
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