The comparison of the ARPANSA and IAEA-K4 graphite calorimeters for the measurement of absorbed

dose from 60Co

Ganesan Ramanathan, Duncan Butler, David Webb, Chris Oliver and Roland

Sargent

Overview1. ARPANSA calorimeter failed in 20002. IAEA calorimeter on loan since 20023. ARPANSA calorimeter repaired in 2006

- recommissioned by Ganesan Ramanathan4. Comparison between calorimeters 5. Comparison with Co-60 decay-corrected absorbed

dose rate

Two Calorimeters• The ARPANSA graphite calorimeter

– Based on Domen design– Purchased in 1991 from ARCS (Austria)– Compared with BIPM in 1997– Became the Australian national standard in 1999

• IAEA K-4 calorimeter was loaned to ARPANSA in 2002– Based on same Domen design– Built in 1983– Earlier Series I (ARPANSA calorimeter is Series II)

Calorimeter Segments:Medium, Shield, Jacket, Core

Shield

Cap

Jacket

Core

Medium

Radiation

Vacuum gaps

All segments- graphite- have heater and thermistor- thermally isolated by vacuum gaps and mylar coating

Cross-section of the graphite calorimeter

IAEA and ARPANSA calorimeters

side-by-side in front of Co-60 source

Thermistor failure in ARPANSA calorimeter and repair

• March, 2000 – Jacket heating thermistor resistance high– ~44 kΩ instead of ~10 kΩ– Higher power required for heating jacket, resistance not

stable.• Initial remediation attempted:

– Use jacket measuring thermistor as heater without measuring the jacket temperature and relying upon the earlier heater setting.

– But then second jacket thermistor failed!– Cause not determined (maybe vacuum failure?)

Thermistor replacement (2006):• The shield assembly (containing core and jacket) was removed• Two new ultra-small VECO thermistors were fixed to the wall of the jacket after drilling small dimples and gluing with epoxy.• The joint between thermistor and Evanohm lead wires was insulated with a coating of nail polish.

Rear of shield assembly

“Shield”

“Jacket”

“Shield”

Cap

Cap

Shield assembly

Jacket thermistor replacement

“Shield”

“Jacket”

“Shield”

New thermistors

Cap

Cap

Shield assembly

Attach 2 new thermistors to

outside of jacket

Jacket thermistor replacement

The IAEA Calorimeter– Apparent jacket thermistor failure in 2003

(not again!!!)– Cold solder joint identified and repaired 2006

– Faults in controller electronics repaired

Distance to the core centre:Thickness, mm

Part Material IAEA - K4 ARPANSA

Front Window Mylar® 0.13 0.138

Medium Graphite 2.0 2.0

Gap Vacuum 0.5 0.65

Shield Graphite 1.0 0.744

Gap Vacuum 0.5 0.65

Jacket Graphite 0.75 0.546

Gap Vacuum 0.5 0.55

Core Graphite 1.37 1.375

Foils and layers on medium, shield and jacket

Mylar®and Epoxy resin

0.1 not stated

Sum - 6.85 6.65

Core mass:

1562.20Buoyancy corrected

1561.171467.7Sum

0.12Resistance wire

1.24Thermistors, Copper wires

0.72Copper wires

1.39 / 2 = 0.69

0.75Polystyrene rods (for core suspension)

1.43Mylar, Epoxy

3.4Mylar, Epoxy, Thermistors

1557.691462.8Graphite

ARPANSAIAEA - K4

Mass, mgMaterial

Difference 0.20 mm Difference 94.5 mg (6.4%)

The room temperature is maintained within + 0.2° C by the air-conditioning system

Vacuum in the calorimeter is maintained at 10-5 mbar level by a 50 l/sec turbo-molecular pump backed with a rotary pump

Drift in the Temperature of the Medium:- The short-term stability (~minutes) is less than + 0.5 mK

- Long-term (~day) is less than 5 mK

ARPANSA 3 hours

IAEA 4 hours

Core drifts:The short-term temperature stability of the core is quoted to be within + 1µK

The drifts for ARPANSA and IAEA cores have been found to be within this limit.

4 minutes

Mean absorbed dose-rate measured adiabatically:

3.15376 mGy/sec

Mean absorbed dose-rate measured isothermally:

3.15628 mGy/sec

Mean absorbed dose-rate measured adiabatically:

3.14776 mGy/sec

Mean absorbed dose-rate measured isothermally:

3.15018 mGy/sec

Quasi adiabatic mode

Quasi-isothermal mode

0

1

2

3

4

5

6

7

0 500 1000 1500 2000Time (s)

Cor

e te

mpe

ratu

re (a

rb u

nits

)

Initial driftFirst electricalRadiation heatingSecond electricalFinal drift

-2-1012345678

0 500 1000 1500 2000

Time (s)

Cor

e te

mpe

ratu

re (a

rb u

nits

)

Quasi-isothermal mode

Comparison of ARPANSA and IAEA calorimeters

Quasi-isothermal modeQuasi-

adiabatic mode

Electrical CFDose dose to graphite rate*

Devi-ation#

mJ/% %sd %sdm mGy/s %sd %sdm % mGy/s

* Dose rates are normalised by applying the decay correction to the starting date 1/9/2006

ARPANSA -145.4 0.6 0.2 3.1563 0.5 0.1 0.12 3.1538

IAEA -115.4 0.3 0.1 3.1517 0.4 0.1 -0.09 3.1478

# % deviations are from the measured dose rate to the decay corrected dose rate relative to 15/3/1997

Agreement between calorimeters ~ 0.2%

Agreement with decay corrected Co rate from 1997 ~ 0.1%

Initial results from new operators

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

0 2 4 6 8 10 12 14 16 18 20

Measurement number

Diff

eren

ce fr

om h

isto

rical

(%) IAEA Isothermal

ARPANSA Isothermal

- operators learning to use calorimeter- all data shown- dose rate low (3 mGy/s) – expect better results with 30 mGy/s

Physical quantities, correction factors and relative standard uncertaintiesfor the determination of absorbed dose to graphite at ARPANSA

ARPANSA relative standard uncertainty

Quantity ARPANSA value

100 si 100 ui

P (power calculation) ---- 0.08 0.04Repeatability ---- 0.05M (core mass/gm) 1.5622 0.01kgap (calorimeter gaps) 1.0074 <0.01 0.04Kz (graphite) 0.9934 0.01 0.03Krn (radial non-uniformity) 1.0026 0.02 0.04Kan (axial non-uniformity) 1.0000 <0.01 0.05Kt (source decay) ---- 0.01Quadratic summation 0.1 0.09Combined relative standard uncertainty 0.13

Existing electronics

Future plans:Automatic calorimeter with virtual PID:- Temperature control implemented through Labview software.

Future plans:

PC based Lock-in-amplifier:- Implement in Labiew software- No hardware is required excepting the need for a signal conditioning amplifier at the input.

Conclusions

• Thermistor replacement successful• ARPANSA and IAEA calorimeters agree within 0.2%• Co-60 decay corrected rate within 0.1%(1997 – 2007)