The very precise energy calibration is available in LINAC calibration, however, the setting of LINAC is a hard task and the beam direction is only downward. In addition to LINAC, we use Nickel calibration. It is so portable that we can do it easily. The direction of Nickel calibration is uniform. It is possible to reduce the several systematics using Nickel calibration in addition to LINAC.
The gamma rays emitted from the thermal neutron capture reaction on Nickel are used as a radioactive source for energy calibration of low energy event. Table 1 summarizes nuclear parameters of neutron reactions on Nickel, the natural abundances of Nickel isotopes, the capture cross sections for thermal neutrons, and the total released energy through gamma emissions.
Table 1: The nuclear parameter concerned with the Nickel calibration.
Each isotope of the reaction has many branches of transition. In the reaction, 52.7% of the decays give an energy release via a single gamma ray of 9 MeV. The intensity of this gamma ray is the maximum of all branches of any isotope.
For the neutron source, fission is used. Its half life is 2.65 years, and it decays through alpha-decay (96.9%) and spontaneous fission (3.1%). When the fission occurs, an average of 3.76 neutrons with an average energy of about 2 MeV each and 10.8 rays with energy of about 8 MeV in total are emitted per one fission. The intensity of the source used is 1.7 .
The reaction occurs essentially only for thermal neutrons. The moderator for thermalizing a neutron with 2MeV energy is water. Neutrons are thermalized by protons in water through about 18 elastic scatterings on average. It takes a few micro sec to thermalize. If the thermal neutron is captured by Nickel, a gamma ray with an energy shown in the transition diagram (see Y.Koshio's PHD thesis, page 63) is emitted. If the thermal neutron is not captured by Nickel, it is captured by a proton or oxygen in water. The cross section for these is 0.332barn and 0.178mbarn, respectively. The calculated mean capture time in water is 205sec. The energy of emitted gamma rays via is 2.2 MeV, this gamma ray is the main background in this energy calibration.
Fig 1 shows the schematic view of the Nickel calibration system. As shown in the figure, the cylindrical polyethylene container, the diameter and height of which are each 20cm, is filled with 2.84kg Nickel wire of 0.1 mm and pure water. At the center of the container, a proportional counter, in which the source is painted on an electrode, is located. The proportional counter is used for tagging fission and gives a precise trigger signal of the occurrence of a fission (``fission trigger''). The data is taken only for 500 sec after each fission trigger. Fig 2 shows the distribution of the time intervals, and the mean capture time of the emitted neutron in this setup is 85sec. For this analysis, subtraction of the late region from the early region is done in order to suppress the continuous backgrounds.
Fig 3 shows distribution of the measured and the Monte Carlo data with the Ni source placed at the center position (x,y,z) = (35.3,-70.7,0)cm and at the top position (35.3,-70.7,1200)cm. From these data, the peak value and resolution of N50 are reproduced for the measurement by the Monte Carlo data. Note that there is a bump in the low hit region in the measured data. This is caused by the 2.2MeV gamma ray from the reaction.
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