Stefano Pirro LRT-2004 Sudbury December 13, 2004 Cleanliness backgrounds and surface contaminations...

Stefano Pirro LRT-2004 Sudbury December 13, 2004

Cleanliness backgrounds and surface contaminations in Cuore

Stefano Pirro INFN-Milano

Stefano Pirro LRT-2004 Sudbury December 13, 2004 Sum energy: 5407 keV

210Po α line

3600 keV

Bkg in DBD region

Continuum

2528 keV

Surface contaminations – bolometers


Cuoricino

Cold finger

Tower

Roman Pb shield

Mixing chamber

Total mass40.9 kg

Cuoricino: 62 detectors array.11 module of 4 detectors 2 modules of 9 detectors

9 detectors 3x3x6 cm3, 340 g each

4 detectors 5x5x5 cm3, 790 g each

New limit 1024 y

The largest DBD experiment presently operating


Direct measurements

The measured surface activity in the -region is

0.2 c/keV/kg/y 0.79 kg 150 cm2

0.003 c/cm2/day/MeV

This value is far away from the best alfa detector available ( 0.1 c/cm2/day/MeV)

The CUORE Goal implies a reduction on this value by a factor 10

The idea is therefore to do a indirect screening on materials

Perform a bolometric run deep underground


Surface radioactivity levels in Cuoricino

The amount of radioactive contaminants are estimated from the background

Unfortunately this is not a simple task because it depends on several items

Roughness of the surfaces

Profile of the contamination

Statistics

Breaks in the secular equilibrium

Using the coincidences we estimate that 80 % of the background arise from copper

Assuming an exponential profile of the contaminations (λ=5 m), and matching low energy -peaks together with the continuum, we find an estimation of the surface radioactivity.

238U 1.6 10-10 g/g

232Th 7 10-10 g/g In the first 1 m


HR-ICPMS Measurements

Aim: 238U and 232Th contamination measurement in the surface layer (05 m thickness) of copper parts.

The idea is to remove by chemical attack a thin layer from the surface of the copper and then to measure U and Th concentration in the obtained solution via Mass Spectrometry.

Due to the Mass factor 50001000sol

etchCu

M

M

In order to reach sensitivity of 10-10 - 10-11 on the concentration in Cu

Reagents, containers tools must satisfy a purity of 10-14 g/g


Measurements

Test measurements were performed at JCR-Lab - Ispra , Italy

VG-Axiom-Plus PTFE Container- pliers- thermometerSample during etchingSample draw

Etching rate 0.5 m/minute (2,5 % HNO3 at 45 °C)

5 samples + blank

Cu bar surface 100 cm2 amount of etsching solution 150 ml

ICPMS

VIS Spectrophotometric


Measurements

Sample without STD

Cu X/Blank

Sample + STD 1

Sample + STD 2

Sample + STD 3

Sample without STD

Cu X/Time1

Sample without STD

Cu X/Time3

Sample step 3

CuX/Time3

Sample without STD

Cu X/Time4

Sample step 4

CuX/Time4

Sample step 5

Cu X/Time5

Copper sample in nitric acid solution

Blank sample

nitric acid solution

Sample + STD 1

Sample + STD 2

Sample + STD 3

Sample step 1

Cu X/Time1

Sample + STD 1

Sample + STD 2

Sample + STD 3

Sample + STD 1

Sample + STD 2

Sample + STD 3

Sample + STD 1

Sample + STD 2

Sample + STD 3

Sample without STD

Cu X/Time5

Sample without STD

Cu X/Time2

Sample step 2

Cu X/Time2

Sample + STD 1

Sample + STD 2

Sample + STD 3


ICPMS-Results

Systematic effects are not well understood

A possible solution is to increase the statistics (number of pieces)

TECM sample - integral measureTC sample - integral measure

Some points are not physically compatible

Problems arise in order to quote errors bars


Direct Measurements

Hall C Cryostat Setup

Inside!More inside!

Eight 5x5x5 cm3 TeO2 crystals

10 days of LT measure to measure at 1 σ level the CUORICINO background


Results

A radioactivity Test Run was performed during September-November 2004 (222 kgxd)

We changed both the copper treatment and the crystal lapping procedure

Unfortunately the background in the continuum showed no significant decrease

The surface radioactivity of the crystals is reduced by a factor 4 (match CUORE milestone)

230ThU232Th

No other lines are present with a limit of 0.2 Bq/kg (16 ppq for 238U)

4.4 10-17 g/g


Conclusions

Copper surface radioactivity remains the only task for CUORE

Different types of Copper treatment are under development

Chemical etching

Electro-etching

Electrochemical polishing

Plasma cleaning

Ion beam cleaning“Screening” HR ICPMS measurements started this year

Bolometric technique is the only one that gives the actual answer.

Thanks are due to the LNGS chemical lab and in particular to Dr. S. Nisi

Stefano Pirro LRT-2004 Sudbury December 13, 2004 Cleanliness backgrounds and surface contaminations...

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