5.2.4. Cut-off time, cut-off weight, and weight window

Table 5.2.49 tmax(i)

Value	Explanation
(D=1.e+9)	Cut-off time for i-th particle [nsec].
i=1-20	(i; particle id, see Section 4.7 ).

Table 5.2.50 wc1(i)

Value	Explanation
(D=-0.5)	Minimum weight for i-th particle.

Table 5.2.51 wc2(i)

Value	Explanation
(D=wc1/2)	Cut-off weight for i-th particle.

Table 5.2.52 swtm(i)

Value	Explanation
(D=1.0)	Minimum source weight for i-th particle.

Table 5.2.53 wupn

Value	Explanation
(D=5)	Maximum value of weight window is given by the product of wupn \(\times\) (minimum value in [Weight Window] section). Here, wupn \(\ge 2\) should be set.

Table 5.2.54 wsurvn

Value	Explanation
(0.6*wupn)	Survival weight value. Here, 1 \(<\) wsurvn \(<\) wupn should be set.

Table 5.2.55 mxspln

Value	Explanation
(D=5)	Maximum number of split and maximum multiple number of survival. Here, mxspln \(>\) 1 should be set.

Table 5.2.56 mwhere

Value	Explanation
(D=0)	Where the weight window takes place. -1: at nuclear reaction, 0: both, 1: at region crossing.

Table 5.2.57 iwwbias

Value	Explanation
(D=0)	Option for [ww bias].
=0	Values defined in [ww bias] are not used.
=1	Values defined in [ww bias] are used. The [weight window] parameters multiplied by inverse of the defined biases in [ww bias] are used.

Table 5.2.58 nfcseg

Value	Explanation
(D=50)	The number of target area divisions when forced collision is applied to charged particles. Smaller values shorten the calculation time, but may result in less accurate calculations for thicker targets, since detailed energy changes within the target cannot be taken into account.

Table 5.2.59 istdcut

Value	Explanation
(D=0)	Option for controlling whether tally calculations should be stopped or not when their statistical uncertainties become less than stdcut. Note that the random numbers might be duplicated when you restart the PHITS simulation after terminating some tally calculations in the previous simulation. In the restart case, you have to write the tally lastly stopped in the previous simulation should be written at the top of the input file.
=0	Continue all tally calculations until the end of the PHITS simulation.
=1	Stop each tally calculation when its statistical uncertainty becomes less than stdcut.

Table 5.2.60 istdbat

Value	Explanation
(D=0)	Inactivate stdcut until when batch number exceeds istdbat. This parameter is useful particularly for the OpenMP version because statistical uncertainties of tally results are fluctuated very much at the early stage of the PHITS simulation, due to the automatic selection of the batch variance (istdev = 1).

Table 5.2.61 ctmin(1-3)

value	explanation
(optional, D=-9999)	Lower limit for counters 1-3. Particle transport is terminated if each counter value falls below this limit. Different values cannot be set for each particle species.

Table 5.2.62 ctmax(1-3)

value	explanation
(optional, D=9999)	Upper limit for counters 1-3. Particle transport is terminated if each counter value exceeds this limit. Different values cannot be set for each particle species.

The cut-off time for each particle should be specified as tmax(i) (in units of [nsec]). After arrival at the cut-off time, the particle is killed; although this is not effective in the context of high energy particle transport, it is useful for low energy particle transport calculation.

A particle’s weight is affected by the importance, forced collisions, implicit captures, and weight window functions. When the weight is lower than the user-defined weight cut off, a Russian roulette method is applied to determine whether or not the particle is killed. This function is not available for particles defined in the weight window.

Under the Russian roulette method, when the weight wgt is lower than the product, wc2 \(\times\) R, of wc2 and the ratio R of the importances at the source and current points (i.e., when wgt \(<\) wc2 \(\times\) R), the particle survives with a probability wgt \(/\) (wc1 \(\times\) R), which is a function of the particle’s own weight, WGT. If the particle survives, its weight is changed to wgt \(=\) wc1 \(\times\) R. If wc1 and wc2 are negative, they are respectively set to |wc1| \(=\) swtm(i) and |wc2| \(=\) swtm(i).

Any particles or regions for which the user has not set importances are given default importances of 1.

In this case of iwwbias=1, the products of the multiplication are output in the input echo of [weight window], and [ww bias] with off is output. If an input file without [ww bias] is used, all values of [ww bias] in the input echo are set to 1.