High-voltage Elements Research Articles

Absorption of 2-mev constant potential roentgen rays by lead and concrete.

Roentgen rays produced at constant potentials of 2 million volts are now available in compact apparatus for radiation therapy (1, 2), industrial radiography (3), and for biological and physical studies (4). I t therefore becomes of interest to determine the protective value of lead, concrete, and other materials for this penetrating radiation and to compare its quality with that of the gamma rays from radium (5) and cobalt60 (6). Since x-ray absorption measurements made with constant potential equipment are preferred for purposes of comparison and standardization, the copper absorption curve and the half-value layer in copper, lead, and concrete were measured for these supervoltage rays and compared with the closely equivalent though less intense gamma rays from radioactive sources. The transmission of 2-million-volt constant potential x-rays through barriers of lead and concrete was determined for both small and large beams. Two Mev Constant Potential Ray Source The 2-million-volt radiation was produced by a Van de Graaff electrostatic accelerator manufactured by the High Voltage Engineering Corporation and based on a prototype developed at the Massachusetts Institute of Technology. This radiation source is completely contained in a steel chamber, 3 feet in diameter and 6 feet long, the high-voltage elements being insulated in a compressed gas mixture of N2 and CO2 at 27 atmospheres pressure.. The high constant potential of the Van de Graaff electrostatic generator results from the accumulation of electric charge on a well insulated metal terminal and conveyed by a rapidly moving insulating belt. The acceleration tube is of the grounded anode type with its cathode end within the highvoltage terminal. A steady equilibrium voltage IS maintained by balancing the current transferred by the belt with the x-ray tube current. The output intensity of this Van de Graaff source at 2 million volts and 250 microamperes is about 75 r per minute, measured at 1 meter with an equivalent target filtration of 6.8 mm. of lead. This x-ray source is designed to operate routinely under these conditions for x-ray therapy. The constant voltage of the x-ray generator is measured continuously by a generating voltmeter. This device actually measures the electric field intensity at the wall of the tank opposite the high-voltage terminal and gives a continuous and linear indication at the control panel. It was calibrated by observing the photon disintegration of beryllium, which begins sharply at photon energies of 1.63 mev. In this calibrating technic the voltage of the x-ray source is raised gradually until the onset of neutrons is observed proceeding from a piece of beryllium placed immediately below the x-ray target. Measurement Technic Measurements of the x-rays transmitted through both lead and concrete barriers were made using the arrangement shown in the insert of Figures 1 and 2.