Absolute calibration by helium-ion-implanted samples for helium accumulation method applied to neutron dosimetry

A He accumulation method applied to neutron dosimetry is a useful means to measure a fast-neutron fluence in a high-flux neutron field. The number of He atoms produced by (n, xα) reactions should be absolutely measured in this method. One of the methods of the calibration of a He atom measurement system is a standard He gas method which is based on the measurement of the pressure, volume and temperature of the He gas. The uncertainty in measuring the number of He atoms is largely influenced by the standard gas purity. In this paper, a He-ion-implanted sample method has been developed as a new method for the calibration. The number of implanted He ions can be determined by measuring the current of ions analyzed by a magnetic field where uncertainty of the measurement can be made reasonably small as it depends only on the ion current measurement. The two methods have been compared by using Al samples implanted with He ions. Both the methods can be applied as a mutually complementary calibration each. The He-ion-implanted sample method is particularly useful for the absolute calibration in the lower concentration range of He atoms in the sample gas than the atmospheric pressure. (author)