36
K. Hashimoto
Fig. 2.19 Comparison of
respective prompt neutron
decay constants obtained
from present cross-power
spectral and previous
Rossi-α analyses (Ref. [21])
the previous decay constant is not too bad. The analysis for a much longer time than
10 min may be required to enhance the agreement.
2.2.3.3 Indicator of Non-poisson Character of Spallation Source
Dividing Eq. (2.38) by Eq. (2.39) of the formula for cross-power spectral density,
the following equation can be obtained:
f R
C 1
C 2
=
λ f ν (ν − 1)
m 1 α
+
m 2 − m
2
1
m
2
1
.
(2.50)
The above equation has no detection efficiency and asymptotically approaches to
the second term with an increase in the subcriticality, i.e., the prompt-neutron decay
constant α. The second term quantitatively expresses a non-Poisson character of the
spallation source and may be referred to as “the Degweker’s factor” of multiplicity
distribution of neutrons in a pulse bunch. When the multiplicity follows the Poisson
distribution, the factor becomes zero. Degweker et al. theoretically simulated the
Rossi-α analysis changing parametrically the above factor to investigate an impact
of non-Poisson source [8]. The Degweker’s factor is a useful indication characterizing
the multiplicity distribution of the spallation neutrons in a pulsed bunch.
Figure 2.20 shows a subcriticality dependence of the ratio f R C 1 /C 2 determined
from the present cross-power spectral analysis. At the more deeply subcritical system
than pattern C, the ratio seems to be an asymptotic value. Seeing the ratio within the
subcritical range from pattern C to F, no systematic dependence on the subcriticality
K. Hashimoto
Fig. 2.19 Comparison of
respective prompt neutron
decay constants obtained
from present cross-power
spectral and previous
Rossi-α analyses (Ref. [21])
the previous decay constant is not too bad. The analysis for a much longer time than
10 min may be required to enhance the agreement.
2.2.3.3 Indicator of Non-poisson Character of Spallation Source
Dividing Eq. (2.38) by Eq. (2.39) of the formula for cross-power spectral density,
the following equation can be obtained:
f R
C 1
C 2
=
λ f ν (ν − 1)
m 1 α
+
m 2 − m
2
1
m
2
1
.
(2.50)
The above equation has no detection efficiency and asymptotically approaches to
the second term with an increase in the subcriticality, i.e., the prompt-neutron decay
constant α. The second term quantitatively expresses a non-Poisson character of the
spallation source and may be referred to as “the Degweker’s factor” of multiplicity
distribution of neutrons in a pulse bunch. When the multiplicity follows the Poisson
distribution, the factor becomes zero. Degweker et al. theoretically simulated the
Rossi-α analysis changing parametrically the above factor to investigate an impact
of non-Poisson source [8]. The Degweker’s factor is a useful indication characterizing
the multiplicity distribution of the spallation neutrons in a pulsed bunch.
Figure 2.20 shows a subcriticality dependence of the ratio f R C 1 /C 2 determined
from the present cross-power spectral analysis. At the more deeply subcritical system
than pattern C, the ratio seems to be an asymptotic value. Seeing the ratio within the
subcritical range from pattern C to F, no systematic dependence on the subcriticality