Correction method of photoneutron in boric acid concentration measuring instrument

摘要

PURPOSE:To easily correct the influence of photoneutron by providing a function to hold still heavy water contg. boric acid to be passed to the above instrument and measuring the output of a detector plural times upon lapse of certain time by changing the time after the flow of the heavy water comes to a halt. CONSTITUTION:Valves 6a and 6b are previously provided to the line in which the heavy water contg. the boric acid is passed usually. The water flow is stopped at certain time t0. This time t0 is set as the point of the start time and the output C(ti) of a counter tube is successively measured at every certain time interval DELTAt. Here is set ti=t0+DELTAtXi (i=1, 2... n). This C(ti) is indicated by the sum of the respective contribution components of the normal neutron source and a photoneutron source and, therefore, indicates eventually a decrease by as much as the attenuation ratio of the photoneutron source after the water flow is stopped. The half-life is as long as 2645 years in the case of, for example, 252Cf in this case and has a difference as compared with the half-life 16N: 7.35 sec, 24Na: 15 hours, 56Mn: 2.6 hours of gamma nuclide and, therefore, the count rate by the nuclide can be identified by utilizing the difference in the ratio of the attenuation. There is no need for determining the detector output in the state in which the photoneutron does not exist.