Calorimetric Calibration Setup for Bremsstrahlung Radiation from Electron Accelerators
Abstract Generated abstract
A calorimetric setup is described for absolute calibration of bremsstrahlung intensity from betatron electron accelerators, intended for detector calibration in flaw detection and nuclear physics under conditions of substantial background radiation. The design uses a single absorber lead calorimeter with internal shielding, vacuum insulation, thermostatting, a thermistor bridge with photoamplifier readout, and a thermal key for temperature equalization, supplemented by an absolute two-cavity ionization chamber and monitor chamber. The system measures intensities down to about 5 x 10^-6 W/cm2 with roughly 2 percent accuracy for a 4 cm irradiation field, and supports detector calibration and radiation measurements behind absorbers for bremsstrahlung endpoint energies of 6 to 30 MeV.
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CALORIMETRIC CALIBRATION SETUP FOR BREMSSTRAHLUNG FROM ELECTRON ACCELERATORS
V. A. Vorob'ev, V. I. Gorbunov, Yu. D. Gavkalov, G. M. Isakov
In a number of experimental studies using bremsstrahlung from a betatron, it is necessary to know the radiation intensity in absolute energy units. At present, methods for absolute measurements of bremsstrahlung intensity have been developed, and the measuring setup described below is proposed as an engineering solution to the problem for specific conditions.
The setup is intended for calibrating detectors used in flaw detection and nuclear physics, and consists of a calorimeter, an absolute ionization chamber, a monitor chamber, an electronic circuit for measuring the ionization current of the chambers, and an electrical measuring circuit for the calorimeter.
Under existing conditions, measurement of the betatron radiation intensity has to be carried out in the presence of substantial background radiation, which requires heavy shielding of the calorimeter. In order to reduce the weight of the shielding, it was decided to place the lead shielding inside the vacuum jacket of the calorimeter. At the same time, as calculations showed, the role of the protective shell in stabilizing the temperature in the calorimeter makes it possible to dispense with a differential-type calorimeter [1], i.e., to use a calorimeter with a single measuring absorber. To ensure measurement conditions with a sufficiently small change in the resistance of the thermistor soldered into the absorber, the design provides a thermal key for connecting the absorber to the shell, which makes it possible to achieve rapid equalization of the temperature to the required value between the absorber and the calorimeter shell. This design makes it possible to reduce the dimensions of the calorimeter in comparison with the differential type and to eliminate the laborious operation of selecting two identical thermistors.
The design of the calorimeter is shown in Fig. 1.
The calorimeter consists of a measuring absorber 1, which is a lead cylinder 70 mm in diameter and 100 mm long. A thermistor 3 and a calibration heater 2 are soldered into the absorber with Wood’s alloy. The absorber is suspended on nylon threads in a brass cylinder 4 by means of tension screws. The latter is inserted into a lead protective cylinder 5 with a wall thickness of 35 mm. The absorber is additionally surrounded by a shield 6. The lead protective cylinder is mounted on six Plexiglas posts in a vacuum steel-
Fig. 1. Calorimeter design
... cylinder 7. The calorimeter is placed in the bath of thermostat 8, in which heater 9, stirrers 10 driven by motors 11, contact thermometer 16, and cooling tube 15 are located. All internal parts of the calorimeter are polished and nickel-plated. The calorimeter was evacuated through branch pipe 12 to \(10^{-5}\) mm Hg. Units 13 and 14 are the heat key and the electrical leads from the thermistor and the calibration heater.
The thermistor is connected into one arm of a Wheatstone bridge. As the null instrument of the bridge, we used a photoamplifier constructed by us, consisting of an M-25/13 galvanometer and two photoresistors connected in a bridge circuit. The thermistor resistance \(R_T\) was selected from the condition of optimal sensitivity \(R_T \simeq 1.7 R_\Gamma\) \((R_\Gamma\)—
resistance of the galvanometer). The change in the resistance of the thermistor during measurement of the betatron radiation intensity and during calibration was compensated by means of a series-connected MCP-60 resistance box and was recorded on a self-recording instrument. The use of a photoamplifier made it possible to measure intensities down to \(5\cdot 10^{-6}\ \text{W}/\text{cm}^2\) with an accuracy of the order of \(2\%\) for an irradiation field of the absorber \(4\ \text{cm}\) in diameter.
For calibrating detectors, when an accuracy better than \(3\text{–}4\%\) is not required, an absolute ionization chamber preliminarily calibrated by means of the calorimeter may be used. This chamber is also intended for measuring radiation intensity behind absorbers of large thickness. Calculations performed for the sensitivity of chambers with aluminum-wall thicknesses of \(5\) and \(7.5\ \text{cm}\) for the spectrum of the primary radiation behind aluminum layers of \(4\) and \(60\ \text{cm}\) showed that the change in sensitivity (with increasing absorber thickness) does not exceed \(8\%\) for \(E_M = 6\ \text{MeV}\) and \(5\%\) for \(30\ \text{MeV}\). In this case the deviation of the calibration curve \(S_1\left(\dfrac{S_1}{S_2}\right)\) is of the order of \(1\%\) (\(S_1, S_2\) are the chamber sensitivities for wall thicknesses of \(5\) and \(7.5\ \text{cm}\), respectively). The foregoing served as the basis for the manufacture of thick-walled-
Calibration coefficients of the ionization chamber \((\text{coulomb}/\text{MeV}\cdot\text{cm}^2)\cdot 10^{-20}\)
(Collimator diameter \(4\ \text{cm}\))
| \(E_M\) (MeV) \(H\) (cm) |
6 | 15 | 30 |
|---|---|---|---|
| 5 | \(125 \pm 3.6\) | \(129 \pm 2.9\) | \(138 \pm 2.5\) |
| 7.5 | \(92 \pm 2.7\) | \(104 \pm 2.3\) | \(113 \pm 2\) |
Fig. 2. Calibration setup
of a two-cavity ionization chamber. The chamber consists of a housing in which there are two gas cavities, 16 mm wide. The chamber walls are electrically interconnected aluminum disks, 80 mm in diameter; the front wall is 5 cm thick, and the middle and rear walls are 25 mm thick. The collecting electrodes, made of aluminum foil 0.2 mm thick, are separated from the high-voltage electrodes by insulators and guard rings connected to the housing, and are connected directly to the unit for connecting the charged capacitors and to the external measuring unit. The chamber is shielded and placed in a protective lead casing. The potential on the charged capacitor is measured by a compensation method; an electrometer using a 232P1 tube is employed as the null instrument.
The calibration setup includes, in addition to the calorimeter, the absolute ionization chamber and the thin-walled witness ionization chamber, a calibration table, the front wall of the shield with an opening for interchangeable collimators, and a shutter. The calibration table makes it possible to move the calorimeter aside and to install in its place the absolute ionization chamber or another detector (Fig. 2). The device makes it possible to return the calorimeter quickly and accurately to its former position. The presence of the shutter allows the calorimeter to be placed along the axis of the radiation beam when adjusting the betatron.
This setup can be used for calibrating detectors by irradiating them with bremsstrahlung from a betatron with \(E_m = 6\text{—}30\) MeV, at a radiation dose \(> 1\) roentgen per minute at a distance of 1 m from the target, as well as for measuring the intensity of radiation behind absorbers.
REFERENCES
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