Input File Parameters Additions

Additions to the XPASS input file parameters were made to allow users access to the features developed for active interrogation. These include a new AI block, additions to the physics block, additional parameters that must be specified for AI, and slightly modified definitions of certain parameters when used for AI. The AI block as well as the new physics parameters are described in tables 4.5 - 4.8. Specification of the AI block

within the input file initiates the AI mode of XPASS; XPASS will run in passive detection mode if the AI block is excluded.

When XPASS is run in AI mode, the physics parameter mactime must be set to the value “on”, and the parameter interval must be set to a value greater than zero. Active interrogation was not been implemented for a stationary vehicle, and therefore

mactime must be set and the width of a time step must be specified with the parameter interval. In addition, AI was not implemented for scenarios which lack a vehicle.

Therefore, the vehicle block must be present for vehicle type truck as it is the only option currently available within the XPASS data library.

In AI mode, setting the physics parameter background to “on” will turn on calculation of background detector count rates from natural sources (cosmic and terrestrial) only. This will not initiate an active background calculation from neutrons created within the cargo. The active background calculation as well as the option to specify a background count rate is set through the AI block. Each source of background (natural, active, user specified) is treated independently within XPASS and the total background is the sum of the three components.

The user defined time step width set through the parameter interval takes on a slightly different meaning when XPASS is used in AI mode. In passive mode, interval is simply the time step used to build the time structure for the truck traversing through the radiation portal monitor. This time structure is used to calculate the vehicle location as a function of time for transport of particles from a moving source to the detector. In AI, the value set for interval is still used to build the vehicle transit time structure, however the width of the time bins within that structure are not constant over all times.

To allow for measurements of the die-away signal between pulses, a very fine time structure on the order of milliseconds or less must be used. Simulation of the

vehicle transit through the interrogation system over multiple seconds in increments of roughly 1 millisecond leads to unnecessarily long computation time. A variable width time structure was used for the vehicle time structure, where a course binning over all time is set by the parameter interval. XPASS then calculates the time at which the SNM within the vehicle will intersect the interrogation beam, and modifies the time structure in this region. The calculation of when this intersection occurs is dependent upon the SNM size and location(s) within the cargo, vehicle velocity, source location, and the collimated width of the interrogation beam.

The time structure is modified by binning time into time bins of width , where

freq is the pulsing frequency of the interrogation source and is specified within the AI

block. This creates a single time bin for each pulse of the source. However, for each pulse there are two components, a “beam-on” time which typically lasts for several microseconds, and a “beam-off” time which fills the remainder of the time before the next pulse. This “beam-off” time is when the detector system is measuring the die-away profile of the neutron count rate. Having a single time bin for each pulse does not produce the fidelity in time required to see the die-away after the interrogation pulse has ceased. Therefore, each of the newly created time bins is then divided into nFineTime number of bins, where nFineTime is an integer value and specified within the AI block.

When specifying a neutron alarm algorithm in AI mode, the parameter nint should be set to a value of “1”. This is because the integration over multiple time bins implemented when nint is set to a value greater than 1 assumes the time bins are of equal width. In AI mode, this is not the case, and the routine has not been updated to integrate over a fixed interval of time rather than a fixed number of time bins.

A final caveat for using XPASS in AI mode is the save block is not used. An output file for each neutron detector will always be generated.

Keyword Req'd Values Description

sigmaeff yes ≥ 0 macroscopic effective attenuation XS [cm-1] Table 4.5: XPASS Input: physics block addition

Keyword Req'd Values Description

electrons yes n/a initiates electron block of input current yes ≥ 0 electron beam current [mA] pulsew yes electron pulse duration [μsec] freq yes source pulse frequency [Hz] nFineTime yes number of fine bins between pulses erad yes > 0 electron beam radius [mm]

target yes [W, Pb, Al,

Fe] bremsstrahlung target material thickness yes > 0 [mm]

sx yes [-inf,inf] source position in x direction [cm] sy yes [-inf,inf] source position in y direction [cm] sz yes > -132 source position in z direction [cm]

BW yes >0 collimated beam width at vehicle surface [cm] FBH yes 1 ≥ FBH > 0 fraction of cargo height covered by beam

bgXw no > 0 voxel size in x direction for induced AI background calculation [cm]

bgYw no > 0 voxel size in y direction for induced AI background calculation [cm]

bgZw no > 0 voxel size in z direction for induced AI background calculation [cm]

bg no n/a initiates background block of input Table 4.6: XPASS Input: AI block

Keyword Req'd Values Description

[energy] yes > 0 unnormalized amount of electrons in energy group⁺†‡

⁺ all electrons at 8 MeV ‡ 10% electrons at 8 MeV, 40% at 12 MeV, 50% at 18MeV

8 1 8 1

† all electrons at 12 MeV 12 4

12 1 18 5

Keyword Req'd Values Description

[ndet

number] yes > 0

background count rate in given neutron detector [cps]⁺†

⁺ ndet 0: 10 cps † ndet 0: 10 cps, ndet 1: 12 cps

0 10 0 10

1 12

Table 4.8: XPASS Input: bg block