New constant-temperature operating mode for graphite calorimeter at LNE-LNHB

Abstract

The realization of the unit of absorbed dose at LNE-LNHB is based on calorimetry with the present GR8 graphite calorimeter. For this reason the calorimetric technique must be maintained, developed and improved in the laboratory. The usual quasi-adiabatic operating mode at LNHB is based on the thermal feedback between the core (sensitive element) and the jacket (adjacent body). When a core-jacket temperature difference is detected, a commercially available analogue PID (Proportional, Integral, Derivative) controller sends to the jacket an amount of electrical power to reduce this difference. Nevertheless, the core and jacket temperatures increase with irradiations and electrical calibrations whereas the surrounding is maintained at a fixed temperature to shield against the room temperature variations. At radiotherapy dose rates, fewer than ten measurements, or electrical calibrations, per day can be performed. This paper describes the new constant-temperature operating mode which has been implemented recently to improve flexibility in use and, to some extent, accuracy. The core and the jacket temperatures are maintained at fixed temperatures. A steady state is achieved without irradiation. Then, under irradiation, the electrical power needed to maintain the assigned temperature in the core is reduced by the amount of heat generated by ionizing radiation. The difference between these electrical powers, without and with irradiation, gives the mean absorbed dose rate to the core. The quality of this electrical power substitution measurement is strongly dependent upon the quality of the core and jacket thermal control. The core temperature is maintained at the set value using a digital PID regulator developed at the laboratory with LabView software on PC for this purpose. This regulator is versatile and particularly well suited for calorimetry purposes. Measurements in a cobalt-60 beam have shown no significant difference (<0.09%) between the two operating modes, with an equivalent reproducibility (1sigma < 0.06%). These results corroborate the negligible difference of heat transfer between steady and irradiation periods when working in quasi-adiabatic mode with thermal feedback between the core and the jacket. The new constant-temperature mode allows numerous and fully automated measurements. The electrical calibration is an integral part of the measurement; no extra runs are needed. It also allows faster thermal equilibrium before starting runs. Moreover the quality of vacuum within the gaps between the bodies is less important.

Construction of a large graphite calorimeter for measurements in small fields used in radiotherapy

“…All graphite calorimeters at LNE-LNHB have the same structure described in [7,11]. They are made of 3 bodies: a core, a jacket and a shield.…”

Radiative Heat Loss Correction for 3-Body Graphite Calorimeters

“…4. The expression of heat loss correction factor for this case, F C+J can be theoretically determined using the first two equations in (9), in which all e i factors cancel, and P 2 = 0. The result is * * This coefficient is named "heat transfer coefficient between the core and jacket" and in SI it has the measurement units W K −1 .…”

“…16 in [7]), but they represent the temperature rises (K) of the core, jacket and shield, respectively, above the constant temperature of the surrounding medium † . Besides, if the graphite calorimeter works in the quasi-isothermal mode of operation (or constant-temperature operating mode -as it is named by Daures and Ostrowsky [9] from CEASaclay, DETECS -LNHB, France), then the condition in the first observation becomes less important. Indeed, the rapid temperature changes that prevent direct measurement of the temperature differences that determine heat gain would be eliminated by operating the calorimeter in quasi-isothermal mode [10].…”

La métrologie de la dose au CEA : le Laboratoire National Henri Becquerel

“…2). Même si les performances du calorimètre graphite étaient déjà excellentes, un nouveau mode de fonctionnement, dit « à température constante », a été mis au point ces dernières années au LNHB (Daures et al, 2005). Le principe consiste à maintenir l'écran à une température fixe et à dissiper dans l'absorbeur une puissance électrique par effet Joule permettant d'atteindre un équilibre thermique.…”