The bremsstrahlung emitted from particle accelerators, such as electron LINAC, is highly energetic and extremely intense pulse-modulated. It is experimentally verified that commercially available G. M. counters of alcohl-quenched type produce huge pulse heights which are proportional to the intensity of incident pulse modulated photons by lower applied voltages than the usual G. M. starting voltages. By the use of this characteristic, the instantaneous value of radiant intensity for intense pulse-modulated radiations can be determined by pulse height measurements and their mean value by starting voltage measurements of the counter. This method also will be applied to regulate the operation of the output from nuclear radiation sources.
The fast neutron detectors used for protection against radiation are desirable to be capable of measuring the dose in rem units, for RBE value of neutron. varies widely with the energy of neutron. For developing a new type of detector, the energy response of existing detectors should be well acquainted with in advance, but the available data are very few. By the use of a 2 MeV V. d. G. accelerator, the variation of sensitivity of detectors-various fast neutron scintillators, a Hurst-type proportional counter, BF3 proportional counters surrounded with paraffin of various thickness, and a Nal scintillator surrounded with paraffin-in the range of 5 keV_??_1.6 MeV of neutron energy is examined, the results of which are compared with the energy dependence of the absorbed dose of fast neutrons.
X-rays from an electron linear accelerator are pulsatory with the same pulse width and repeti-tion rate as those of the accelerator. Hence, from such X-rays, very high dose rate per pulse duration is obtained even when the average dose rate is low. The counting loss in dose-rating with a pulse detector such as GM counter is discussed, and the relation of counting loss to average dose rate is clarified. Example: When a GM counter of effective cross-sectional area of 20cm2 is used for pulsed X-rays of maximum 18 MeV and repetition rate of 200 pps, the average dose rate with probable counting loss of 10% will be about 0.9mr/hr. The relation of counting rate to pile-up of pulses in measuring pulsed X-ray spectrum is also discussed and a formula is derived for calculating the ratio of the number of signal pulses with no pile-up to that of total signal pulses. Example: When the counting rate is below 1/5 of repetition rate of the electron beam, the ratio becomes 0.9 or more, in other words, over 90% of signal pulses are of correct height.
Ionization efficiency and energy characteristics of ionization chamber used for radioactive gases are discussed. As to the ionization efficiency, experiments are made with a chamber, 114mm in diameter, 150mm in height with walls of stainless steel, by the use of a radioactive point source or 41A in it. The efficiency, obtained by varying the position of the source and by using sources of different energies, is found almost inversely proportional to maximum β-ray energy, and the efficiency obtained at 1.20 MeV and that obtained with 41A are in agreement within experimental error. As to the energy characteristics, the relation of ionization current us. concentration is investi-gated. In the range of 0. 167_??_1.70 MeV of β-ray energy, ionization current I is found, indepen-dently of the energy value, proportional to concentration C in μc/cc as I_??_6×10-9C. In compa-rison with ionization chambers of other dimensions, the one used by the authors is found to have the optimum energy characteristics.
An 18 MeV Betatron has been developed for service in research laboratories, industrial plants and medical institutions. the following are its main characteristics. (1) Energy: 6_??_18 MeV variable (2) Output K-ray: 30 r/min. at 1m from the target Electron: 500 rad/min. at 50cm from the exit window (3) Power consumption: 20kW (4) Weight of Betatron magnet: 1, 870kg (5) Cooling: By built-in fan (6) Rating: Continuous This machine is now being used for medical purpose at Hyogo Cancer Center.
A quick response ionization survey-meter and some problems concerned with a-contamination are described. To measure small currents a high resistor and a D. C. amplifier are used. The time response of such a device has usually been improved by the negative feed back. But there is a limitation because of the capacity parallel to the high resistor. This capacity can be reduced by means of a new device shown in a figure, and the use of a high gain D. C. amplifier at the same time makes the response extremely quick. By this method, the time constant less than 0.5 second was accomplished with a resistor of 1013 ohms, and 1 mr/h of full scale was realized with an air ionization chamber of the volume about 400 cc. The response of the device is actually limited by the statistical fluctuations of the ionization current due to the measuring γ-radiations. The theo-retical considerations about these problems are discussed and confirmed by the measurement using an experimental instrument. Another problem for quickening the response and raising the sensitivity of the survey-meter is the effect of α-contamination of the air ionization chamber. Random pulses or spikes are pro-duced by stray α-emitters found in the chamber. By the analysis of their pulse height distribution, five causes of these random pulses are traced, among which the emitters in the wall material, on the walls and in the air are in the proportion of 55: 27:18. These spikes were reduced to 60% after the chamber was left sealed for one year, and this reduced value is nearly equal to the first figure of the above mentioned ratio. When the response is sufficiently quick, the presense of α-contamination has no influence on the measurement, because the recovery to the normal state is quick and the discrimination of these pulses is very easy.
With an improved gas flow proportional counter and a low noise amplifier, X-rays of 10_??_100Å are detected and characteristic X-ray peaks of O-K, N-K, C-K, B-K and Be-K are observed. The transmission of the window film used in the counter is 51.4 percent for C-K (43.6Å) and the film lasts for longer than one month. The intensities at the mean pulse height are measured as 6, 000 cpm for Be-K (pulse height: 10 V), 38, 400 cpm for C-K (32 V) under the condition of the electron beam potential of 5 KeV and target current of 0.05 μA. The relative standard deviations of the pulse height distribution for the above X-rays are also measured. The agreement between theoretical and experimental values is satisfactorily good. When PR-gas is used as the counter gas, the O-K main and its escape peaks are clearly separated, and the energy resolution of smaller than 250 eV in the energy region of a few hundreds eV is, expected in the present experiment. Some applications of this system to the electron probe X-ray microanalyser are tried. Qualitative analysis of alumina and quartz, scanning analysis of boron nitride by N-K and graphite by C-K are successfully carried out.