Population Monitoring

Initial Screening Decision Level

This is a level of contamination (determined by hovering the detector an inch over the area of interest) that is high enough to warrant early intervention actions such as changing clothing and/or decontamination.  Different references have different names, methods of measurement, and justifications for this Decision Level:


  • FEMA REP-22: “Fixed + Loose Contamination” where it is assumed that nearly all contamination will be removed by decontamination and/or normal hygiene that would occur within 36 hours of the contamination incident (essentially stopping the exposure).  Although instrument readings are provided as the criteria, it is based on 1 uCi spot contamination or 0.04 μCi/cm2  (88,800 dpm/ cm2)

  • IAEA / ICRP: “Decontamination is Warranted” if > 1 μSv/h (0.1 mrem/h) at 10 cm or > 10,000 Bq/cm2 (600,000 dpm/cm2) beta and gamma surface contamination

  • NCRP Commentary 19 Identifies this as the “screening level” above which decontamination is warranted.  1 uCi spot contamination.

  • NCRP Report 161: “Intervention Required” Prevent inadvertent ingestion and inhalation, limit spread of contamination and decontaminate. >600,000 dpm/ cm2

  • NCRP Report 165: >0.1 mR/h exposure rate at 10 cm, >600,000 dpm/cm2 beta and gamma surface contamination

  • CRCPD RDD Handbook: 1,000 cpm, If the incident is large the “release level” can be increased to 10,000 cpm.


Screening activities are meant to by “high throughput” to minimize delay of those leaving the contaminated area.  This is especially important for fallout contamination after a nuclear detonation.  The rapid decay of the fallout radiation in the first few hours and days of the incident mean that a delay in screening and/or decontamination for just a few hours could result in the victim receiving significant exposure while waiting.

 

Screening Decision Levels for fallout contamination in the first 3 days after a nuclear detonation, use the instrument specific monitoring parameters for 1.0 uCi provided by FEMA REP-22.  This results in a spot contamination reading of ~150,000 cpm for a modern pancake GM instrument. For fallout; EARLY, simple self-decontamination such as changing outer layer of clothing and washing or wiping exposure skin and hair can significantly reduce exposure and should be done without waiting for screening.If screened, decontamination is a priority for those above this level.  Those below this level should still be given instructions for self-decontamination as time allows.

Screening decision level prior to portal monitoring should look for 20 uCi to help avoid extraneous alarms when multiple portal monitors are in use.

For all other large-scale beta/gamma screening activities,

  • Severely Resource constrained: If screening wait times exceed several hours, modified survey techniques (surveying only likely contamination locations) and “walk by” surveys can be done to identify any highly contaminated individuals that are a priority for decontamination. The IAEA guidance of 10 mrem/h (100 uSv/h) at 1 meter can be used in extreme cases.

  • Resource constrained: If screening wait times exceed 1 hour, use the “fixed+loose” screening decision level of 1 uCi (spot) or (0.04 μCi/cm2) provided by FEMA REP-22.This would result in screening measurements of over 100,000 cpm for a modern pancake GM instrument. For a dose rate instrument, look for > 0.1 mR/h exposure rate at 10 cm.

  • Nominal: if the screening resources can process all arrivals (and the resources are not needed elsewhere), consider using the of 0.1 uCi (spot) or (0.004 μCi/cm2) Detailed Screening Decision Criteria. This would result in measurements of over 10,000 cpm for a modern pancake GM instrument.

For large-scale alpha screening activities, Screen for >1,000 Bq/cm2 (>60,000 dpm/ cm2) if resource constrained. Use >100 Bq/cm2 (>6,000 dpm/ cm2) if time permits.