- Laboratorio Subterráneo de Canfranc

CHARACTERIZATION AND RADIOACTIVE BACKGROUND REDUCTION
OF AN HYPERPURE GERMANIUM DETECTOR AT THE LSC
Patricia Villar
Laboratorio de Física Nuclear y Astropartículas – University of Zaragoza, Zaragoza (Spain)
and Laboratorio Subterráneo de Canfranc, Canfranc, Huesca (Spain)
ISAPP 2013
LSC, Canfranc, Spain
July 20, 2013
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Detector chacteristics
COSME is a 0.234 kg hyperpure germanium detector
kept underground for the last 23 years at the
LSC protected from cosmic radiation
Background wihout shielding
Linearity
Deviations < 0.4%
Detector efficiency
Energy resolution
Satisfactory
properties
have been
obtained
P. Villar University of Zaragoza - LSC
July 20, 2013

SHIELDING
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Noise filtering
% Events rejected
Sample week
NonADCs ratio
rejected
11%
14%
Time
17%
DATA ADQUISITION FOR COSME
OPTIMIZATION PHASE:
-
Time &
ADCs ratio
58%
HV source + Preamplifier
2 Amplifiers + 2 ADC (different shaping times)
Time & ADCs ratio
Time filter
ADCs ratio filter
Time
ADCs ratio
Combined filtering
No filter
applied
Both filters
applied
P. Villar University of Zaragoza - LSC
July 20, 2013
Non-rejected
SHIELDING
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Noise filtering
% Events
rejected
137Cs
Calibration
NonADCs ratio
rejected
11%
14%
Time
17%
DATA ADQUISITION FOR COSME
OPTIMIZATION PHASE:
-
Sample week
Time &
ADCs ratio
58%
HV source + Preamplifier
2 Amplifiers + 2 ADC (different shaping times)
Time & ADCs ratio
Time filter
ADCs ratio filter
Time
ADCs ratio
Combined filtering
No filter
applied
Both filters
applied
P. Villar University of Zaragoza - LSC
July 20, 2013
Non-rejected
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Energy threshold
Cosmogenic activation
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
for systematic error
Short half-life isotopes with the
exception of 3H
Eth = (3.68 ± 0.59) keV
<0.028 events.keV-1 kg-1 day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
July 20, 2013
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Problem
on
measuring
COSME energy threshold
Cosmogenic activation
Short half-life
exception of 3H
isotopes with the
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
Eth = (3.68 ± 0.59) keV
<0.028
events.keV-1 kg-1
day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
July 20, 2013
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Problem
on
measuring
COSME energy threshold
Cosmogenic activation
Short half-life
exception of 3H
isotopes with the
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
Eth = (3.68 ± 0.59) keV
<0.028
events.keV-1 kg-1
day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
July 20, 2013
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Problem
on
measuring
COSME energy threshold
Cosmogenic activation
Short half-life
exception of 3H
isotopes with the
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
Eth = (3.68 ± 0.59) keV
<0.028
events.keV-1 kg-1
day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
July 20, 2013
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Problem
on
measuring
COSME energy threshold
Cosmogenic activation
Short half-life
exception of 3H
isotopes with the
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
Eth = (3.68 ± 0.59) keV
<0.028
events.keV-1 kg-1
day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
July 20, 2013
Characterization and radioactive background reduction of an hyperpure germanium detector at the LSC
Borated water tanks were implemented to moderate neutrons from the rock
Low energy region spectrum
DM Sentivity
Low energy region background rate
Borated
water tanks
installation
Before
After
Significant reduction in the very low energy region is observed
Background level: (2.75 ± 0.03) events ∙ keV-1 kg-1 day –1
Problem
on
measuring
COSME energy threshold
Cosmogenic activation
Short half-life
exception of 3H
isotopes with the
Estimated using Pb k-Shell
X-Rays and old spectra
cosmogenic activation peaks
Eth = (3.68 ± 0.59) keV
<0.028
events.keV-1 kg-1
day –1
Negligible for present background level
P. Villar University of Zaragoza - LSC
Curve
Eth(keV)
Background
(events.keV-1 kg-1 day –1)
(a)
3,68
2,75
(b)
1
2,75
(c)
3,68
0,01
(d)
1
0,01
July 20, 2013
Conclusions
Conclusions
 An hyperpure germanium detector named COSME, located at the LSC, has been characterized and its radioactive background
has been reduced.
 Many properties were studied: linearity (deviations < 0.4 %), energy threshold (3.68 ± 0.59 keV) as well as its efficiency for
puntual and diffuse sources.
 Its radioactive background has been reduced implementing different techniques: passive lead and borated water tanks
shielding, radon purge with LN2 as well as time and channel ratio filters removing 85,7 % of the events registered in the low
energy region.
 Residual cosmogenic activation has been also studied concluding that 3H contribution to the present background is
negligible.
 Finally, its sentivity as a Dark Matter detector has been studied. With an inprovement in the reduction of the radioactive
background and mainly in the energy threshold, COSME would be a competitive detector for the search of light WIMPs in SI
interactions.
P. Villar University of Zaragoza - LSC
July 20, 2013
COSME
Hyperpure germanium
(PNL)
Implemented PA
44 cm3
0.234 kg
Underground since1990
Unused since 1998
P. Villar University of Zaragoza - LSC
July 20, 2013
12
1
3
2
6
4
5
7
P. Villar University of Zaragoza - LSC
9
8
July 20, 2013
Radioactive background reduction by a factor of 104
P. Villar University of Zaragoza - LSC
July 20, 2013
Efficiency ~ 90 %
137Cs

.
Calibration
P. Villar
P. Villar University of Zaragoza - LSC
July 20, 2013
15