Description

Neutron calibration sources containing $^{252}$Cf have been made by encapsulating the material into small acrylic cylinders. These sources are discussed in [30].

We rely on the neutrons emitted following $^{252}$Cf fission to perform the neutron efficiency calibration. A distribution of neutrons result from this fission, with multiplicity detailed in Table 12.3.7 [28]

Table 12.1: The $^{252}$Cf neutron multiplicity distribution.

multiplicity	probability	error
0	0.003	0.001
1	0.021	0.005
2	0.140	0.011
3	0.264	0.017
4	0.307	0.020
5	0.191	0.018
6	0.061	0.013
7	0.008	0.007
8	0.005	0.003
9	-0.006	0.005

The neutron energy distribution is described by a Maxwell-Boltzman distribution with temperature T=1.42 MeV (Equation 12.1).

$$\mathrm{\frac{dN}{dE} = \sqrt{E} e^{-\frac{E}{T}}}$$ (12.1)

The calibration signal arises from neutron capture on deuterium, providing a 6.25-MeV $\gamma$-ray with every capture. This class of events leads to an Nhit distribution having a centroid value 47$\pm$10 Nhit when the source is placed at the center of the D$_2$O. Two sources have been prepared that produced neutrons at 1.15 and 11.5 neutrons per second on 01 June 2000.

The primary decay mode for $^{252}$Cf ($t_{1/2}$=1.97 y) is $\alpha$-decay to the ground state and first excited state ( 40 keV) of $^{248}$Cm. The sources were prepared during May 1999 and therefore are significantly converted to $^{248}$Cm, but this isotope ( $t_{1/2}=3.5\times10^5$ y) overwhelmingly decays by $\alpha$ emission. This decay does not influence the neutron emission spectrum.

There is a contribution to source backgrounds because the fission fragments are unstable against $\beta$-decay. These decays are almost completely uncorrelated with the fission event because of long half-lives (seconds or longer). The $\beta$-particles are able to produce Cerenkov light in the source acrylic and the D$_2$O. These events should reconstuct in position close to the source.

Another contribution to the source background is neutron capture on the manipulator. Neutron capture on Fe may produce 7 MeV in $\gamma$-ray energy. Steps taken to reduce this background involve increasing the distance between the source and the manipulator by approximately 45 cm in length. This is accomplished with acrylic offset and mounting pieces. The steel represents a smaller fractional volume as it is moved to greater radii with respect to the source.