Nickel calibration
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.
Reaction |
natural abundance |
capture cross section |
|
|
67.88 | 4.4 | 9.000 |
 |
26.23 | 2.6 | 7.820 |
 |
3.66 | 15 | 6.838 |
 |
1.08 | 1.52 | 6.098 |
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.
|
Figure 1: The schematic view of Nickel calibration system. |
|
Figure 2: The distribution of the time intervals from the fission trigger. |
|
|
Figure 3: |
The distribution of the measured and the Monte Carlo data of Nickel
calibration placed at (a) center position (x,y,z) = (35.3,-70.7,0)cm, and (b) top position
(35.3,-70.7,1200)cm. |
