IDOS Symposium-IAEA Benjamin Rapp 9-12 November 2010, Vienna1/12

The LNE-LNHB water calorimeter : Measurements in a 60Co beam

B. Rapp, A. Ostrowsky, J. Daures,

CEA, LIST, Laboratoire National Henri Becquerel (LNE LNHB), F-91191 Gif-sur-Yvette, France

IDOS Symposium-IAEA Benjamin Rapp 9-12 November 2010, Vienna2/12

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Graphite or water calorimeters are mainly used as reference for absorbed dose to water in most of the national metrology laboratories involved in ionizing radiations.

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LNE-LNHB has a long experience with graphite and tissue-equivalent calorimeters, so graphite calorimeter is still today the reference for photon and electron beams dosimetry.

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Associated with a transfer procedure from graphite to water, it leads to the reference of absorbed dose to water which is the reference quantity for radiotherapy.

• The LNE-LNHB today owns 3 graphite calorimeters.

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It was decided a few years ago to develop water calorimetry at LNE-LNHB, not in replacement of graphite calorimeter but to carry out joint measurements on same radiation beams as often as possible.

•Then, two new water calorimeters have been built.

Introduction : Calorimetry at LNE-LNHB

Graphite calorimeter

Water calorimeter

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1st Water Calorimeter : design

30 cm

Beam

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Water calorimeter is designed to operate at 4°C (at the maximum density of water to minimize convection).

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A cooling fluid regulated at 4°C circulates in the double PMMA wall between the water phantom and the polystyrene foam insulation (8 cm of thickness).

Cooling system producing a flow of cold air at 4°C (±

0,01°C) through the calorimeter window.

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Pressure Regulator

Compressed Air

Water Calorimeter window :

PIDControl loop

Under LabView

Voltage (V)

Temperature given by Pt100 or thermistor positioned inside window

Polystyrene layer (2 cm thickness)to avoid air condensation

Mylar sheet (0.1mm)

1st Water Calorimeter : Entrance window design

Entrance window cooling system :

• This system allows to create an air layer that is stable and regulated in temperature (± 0.01°C).

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It minimizes the amount of material in front of the beam (only 2 cm of polystyrene and 0.1 mm Mylar sheets).

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Then, the radiation perturbation factor kp

(Enc.) due to the thermal enclosure is 1.0023 (measured with an ionization chamber and confirmed by MC calculations).

Beam

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Quartz vessel and temperature probe

Temperature probe

•The temperature probe is positioned inside a quartz vessel of Ø

10 cm, which contains high purity water saturated with N2

gas (in order to have a zero heat defect of water).

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The thermistor is positioned inside the calorimeter by an optical system.

•Temperature probe in quartz is used to measure the rise of temperature. • The external diameter of the quartz capillary is 0.6 mm. It contains a GE Sensing NTC thermistor of 9500Ω

at 4°C connected to a DC Wheatstone bridge. •

The capillary is filled with epoxy resin to ensure the thermal conductivity, electrical and physical insulation from water.

0.36 mm

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2nd water calorimeter : design

Quartz vessel, can be embedded in the PMMA wall phantom to allow low depth measurements in water (for medium energy X-rays or electrons)

Extruded polystyrene(8 cm thickness)

Water phantom

Air cooling system (for enclosure and entrance window)

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Design of a new water calorimeter cooled only with cold air produced by vortex tubes.

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We obtained same thermal stability as previous calorimeter (Thermal slope < 20 μK/min).

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More compact and lighter design compared to previous calorimeter.

•Allow low depth measurements (1-2 cm) in water.

51 cm62

cm

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Water calorimetry principle and correction factors

( ) ρkkkhTCD pcpW ⋅⋅⋅−⋅Δ⋅= −11where h is the water heat defectCp

is the specific heat of waterkc

is the thermal conduction correction factorkp

is the radiation field perturbation correction factor and kρ

is the density of water correction factor.

The absorbed dose is calculated from the temperature rise by :

• The low thermal diffusivity of water allows measuring a local temperature rise and then the absorbed dose at the reference point in the water phantom.

• A balance between irradiation time -

to get a good ratio signal to noise -

and time scale of thermal diffusivity has been chosen.

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Irradiations sequences :

Zoom over an irradiation of 4 min :

Temperature measurement

• Thermal slope < 20 μK/min.

• Peak to peak noise ~ 20 μK.

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Sequences of 4 irradiations of 4 min, followed by a pause time of 1.5 hour without stirring (to allow the cone of heat produced by irradiation in water phantom to vanish).

T (°

C)

340 μK

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1st water calorimeter results in Cobalt 60 beam

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Absorbed dose rate to water measured by water calorimeter is having a dispersion between 1 and 2%.

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The automation of measurements allows to reduce the uncertainty

on the mean value (N=420) to 0.07%.

• Gaussian distribution of the absorbed dose rate to water.

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Uncertainty budget

The main sources of uncertainty are :

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The heat defect of water which vary with the organic carbon impurities content of water and dissolved oxygen concentration (depending on the quality of high purity water produced, of the saturation by N2

gas and the filling of quartz vessel).

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Positioning of temperature probe (difficulty to measure the position of thermistor inside the quartz vessel filled with water).

Relative uncertainty Source of uncertainty Value 100 si 100 uj

Temperature probe calibration - 0.1 Temperature probe positioning - 0.3 Specific heat of water (J.Kg-1.K-1) 4204.8 0.1 Thermal conduction correction factor kc * 0.1 Radiation field perturbation correction factor kp 1.0032 0.15 Heat defect of water h 0 0.3 Density of water correction factor kρ 1.00032 0.01 ∆T measurement reproducibility (N=420) - 0.07 Quadratic summation 0.19 0.45

Combined relative standard uncertainty on Dw 0.49 * Thermal conduction correction factor (tirr.= 240 s) : kc = 1.0043, 1.0012, 1.0004, 0.9997

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Comparison of graphite and water calorimetry results in Cobalt 60 beam

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Absorbed dose rate to water measured in Cobalt 60 source of LNHB/LMD.

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Reference established by ionometric

transfer, from graphite calorimetry with application of graphite to water transfer coefficient (relative uncertainty ~ 0.46%) (1).

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Measurements by graphite calorimetry with graphite to water transfer (relative uncertainty ~ 0.36%) (2-3-4).

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The absorbed dose rate to water given by water calorimeters is in agreement with previous measurements based on graphite calorimetry, with a slightly larger uncertainty (~ 0.5%) (5-6).

Both methods (graphite calorimetry + graphite-water transfer and water calorimetry) gives results in agreement and allows to obtain two independent measurements of the absorbed dose rate to water.

systematic errors (grey rectangles)statistical errors (black error bars)

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Conclusion & Perspectives

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The new water calorimeters allows measuring the absorbed dose rate to water with a relative standard uncertainty lower than 0.5% complementary to the existing references based on graphite calorimeter.

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Theses instruments permits to check the consistency of the laboratory dosimetric references by two independent methods.

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It is very valuable to operate both graphite and water calorimetry at LNE-LNHB to have a different approach in photon and electron beams for radiotherapy

purposes.

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These calorimeters will now be used to contribute to the references on accelerator high energy X-ray and electron beams; and to establish new references for medium X-ray and proton/ions beams.

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THANK YOU FOR

YOUR ATTENTION !

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Calorimetric measurements need to be corrected by the influence of thermal transfers.

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Operating at 4°C (maximum density of water) and using a confinement quartz vessel we are minimizing the effect of natural convection in water phantom.

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But the quartz vessel and the temperature probe are causing thermal perturbation by conduction since the temperature rise in quartz is about 6 times larger than in water.

•Thermal transfers by conduction have been simulated by coupling Monte-Carlo MCNPX code (for calculation of heat deposition in water by radiations) and finite elements software COMSOL (for calculation of heat transfers in the calorimeter).

Distribution of absorbed dose inside the water phantom in front of Cobalt 60 source obtained by MCNPX , used as heat source for time dependant finite element calculations, done with COMSOL software :

Thermal conduction correction factor

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3D temperature map during irradiation calculated from the Monte-Carlo dose distribution :

Thermal conduction correction factor

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The thermal conduction correction factor kc

is lower than 1.005 for an irradiation time under 4 min.

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The correction factor is calculated for each of the 4 steps of the irradiation sequence (same timing for the irradiation scheme is used for measurements).

3D mesh of the quartz vessel used for calorimetric measurements :

Comparison between temperature rise calculated by COMSOL (quartz vessel and temperature probe effect combined) and ideal temperature rise (without thermal transfer) :

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