Minimum Detectable Activity Lower Limit of Detection Specific Activity

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Minimum Detectable ActivityLower Limit of
DetectionSpecific Activity
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Definitions Specific Activity

• Specific Activity ratio of radioactivity per
  unit mass of a radionuclide or labeled compound
• Units Ci/mmol or Bq/mmol
• SA µ 1/half-life
• Theoretical Specific Activity Maximum specific
  activity for a carrier free isotope or compound

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Minimum Detectable ActivityLower Limit of
Detection

• LLD
• Smallest amount of activity that can be detected
  for comparison with limits
• Normally survey instruments
• MDA
• Smallest amount of activity that can be
  quantified for comparison with limits
• Normally counting equipment

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General Information Needed

• Normal background
• Counting efficiency
• Effective service area (portable survey
  instruments)
• Surface area monitored (i.e., 100 cm2)
• Counting time (counting equipment)
• Counting standards

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LLD

• 1.645(Rb/Tb)1/2
• LLD -----------------------
• E(A / 100 cm2)

• Rb Background count rate
• Tb Time of the background count
• E Detection efficiency of the meter
• A Effective area of the probe

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Example of LLD

• A portable survey meter is used to monitor for
  fixed P-32 contamination in an active laboratory.
  It is operated in a count rate mode and has a
  standard Pancake GM probe attached
• Data
• Rb 50 CPM (background count rate)
• Tb 1 minute
• E 25
• A 20 cm2

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Example of LLD

• A portable survey meter is used to monitor for
  fixed P-32 contamination in an active laboratory.
  It is operated in a count rate mode and has a
  standard Pancake GM probe attached
• Data
• Rb 50 CPM
• Tb 1 minute
• E 25
• A 20 cm2

1.645(Rb/Tb)1/2 LLD (DPM)
----------------------- E(A
/ 100 cm2)
Answer 233 DPM
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MDA

• ( 2.71 / TS ) 3.29 ( Rb/Tb Rb/Ts
  )1/2
• MDA --------------------------------------------
  --
• E(A / 100 cm2)

• Rb Background count rate
• Tb Time of the background count
• Ts Time of the sample count
• E Detection efficiency of the counter
• A Area monitored or wiped

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Example of MDA

• A liquid scintillation counter is used to monitor
  for tritium contamination. A wipe test was take
  over a 100 cm2 area.
• Data
• Rb 25 CPM (background count rate)
• Tb 5 minutes
• Ts 5 minutes
• E 68

10
Example of MDA

• A liquid scintillation counter is used to monitor
  for tritium contamination. A wipe test was take
  over a 100 cm2 area.
• Data
• Rb 25 CPM (background count rate)
• Tb 5 minutes
• Ts 5 minutes
• E 68

( 2.71 / TS ) 3.29 ( Rb/Tb Rb/Ts
)1/2 MDA --------------------------------------
----------------
E(A / 100 cm2)
Answer 16 DPM
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Theoretical Specific Activity Examples

• Iodine-123 (13.2 h half-life)
• SA Ci/ mol
• A N l -gt N A / l -gt N A t1/2 / ln 2
• N / 6.022 x 1023 mol
• mol (A t1/2 / ln 2) / 6.022 x 1023
• 1 Ci of I-123 3.7 x 1010 dps
• mol (3.7 1010 s-1 13.2h 3600 s/h / ln
  2) / 6.022 x 1023
• mol 4.2128 x 10-9 mol
• SA Ci/ mol 1 Ci / 4.2128 x 10-9 mol 2.3737
  x 108 Ci/mol
• SA (Iodine-123) 237370 Ci/mmol

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Theoretical Specific Activity Examples

• Iodine-123 (13.2 h half-life)

Iodine-125 (59.4d half-life)
SA Ci/ mol
A N l -gt N A / l -gt N A t1/2 / ln 2
N / 6.022 x 1023 mol
mol (A t1/2 / ln 2) / 6.022 x 1023
1 Ci of I-123 3.7 x 1010 dps
mol (3.7 1010 s-1 59.4d 24h/d 3600 s/h
/ ln 2) / 6.022 x 1023
mol 4.550 x 10-7 mol
SA Ci/ mol 1 Ci / 4.550 x 10-7 mol 2.198 x
106 Ci/mol
SA (Iodine-123) 237370 Ci/mmol
SA (Iodine-125) 2198 Ci/mmol