ORTEC
®
Fission Meter
®
(FM-P3) Portable Neutron Source Identification System
931026D / 060118
14
When you start the next mobile search, its data are added to the previous search data. Each time you go back
to the main screen, the previous search data are cleared.
Figure 14. Neutron Source Present. Switch to
Characterization Mode.
3.3 Static Search Mode
Figure 15 shows the Static Search Mode screen. If Mobile Search Mode shows a potential fissionable source
nearby or at a distance, or if you have independent intelligence of a suspicious package, use the static mode
to collect 15
–
20 minutes of data or at least 30,000 counts to better characterize the object.
3.3.1
Nearby and Distant Fission Sources
In situations where the neutron count rate is slightly elevated above average background, it can be difficult
to determine whether the fission source you are measuring is nearby or distant. The problem lies in the
principle of operation. An SNM source is emitting correlated (approximately simultaneous) neutrons.
Detection of two or more simultaneous neutrons depends on the efficiency of the detection system, which
will fall off with distance from the source. In essence, a distant SMN source looks less and less correlated as
distance increases, and more and more “innocent.”
The Fission Meter takes the Static Search Mode data and makes two calculations based on internal constants.
One assumes the source is far away (this is tracked by the
N-Cosmic
indicator; see Figure 12) and one assumes
the source is near (this is tracked by the
N-Cosmic Near CPS
indicator). If the
N-Cosmic Near CPS
indicator
shows a non-cosmic fraction, a fission source is very likely close by and further investigation of the package
and the data are needed. The
N-Cosmic
readings should be interpreted in light of the actual situation: for
example, if the instrument is in the middle of a foot
ball stadium, the “far” result might be more appropriate.
If against a suspect package, the “near” result is favored.
When you enter Static Search Mode and start counting, you can stop and restart the count without losing
the previous data from this search session. However, when you return to the main screen, the search data
from this session are cleared.
