Minimum Detectable Activity (MDA) for Radiation Measurements

Determining the lower limit of detection (LLD) or the minimum detectable activity (MDA) requires that the following information regarding the performance of a survey meter or counting system be known:

- The normal background count rate.

- The counting efficiency of the meter or counting system for the radioisotope being measured.

- The effective surface area of the detector probe (for portable survey meters).

Note: Contamination survey limits are in units of activity per 100 cm² of area. The results must be adjusted if the area swiped or if the effective area of a detector probe is less than 100 cm².
 

The USDA uses the information found in "Accuracy and Detection Limits for Bioassay Measurements in Radiation Protection", published by the NRC as NUREG-1156 in April, 1986, and "A Handbook of Radioactivity Measurements Procedures" published by the National Council on Radiation Protection and Measurements as Report No. 58 in November, 1978, as the basis for this bulletin.

Portable survey meters used for radiation measurements must be calibrated on an annual basis, or after repair.

The meter's Lower Limit of Detection (LLD) and counting efficiency must be known for the radioisotopes routinely used in the laboratory.

Laboratory counting equipment used for radiation measurements, such as liquid scintillation counters and multi-sample gamma counters, must be calibrated on an annual basis, or after repair.

The equipment's Minimum Detectable Activity (MDA) and counting efficiency must be known for the radioisotopes routinely used in the laboratory.
 

Recommendation:

Gamma counters and liquid scintillation counters should have a set of calibrated standards that are counted annually. After correcting for radioactive decay, counting efficiencies should be calculated for each appropriate radioisotope/energy window setting and posted by the counters.

The Lower Limit of Detection is defined as the smallest amount of activity that can be detected for comparison with regulatory limits.

1.645(R_b/T_b)^1/2

LLD = -------------- (1)

E•(A / 100 cm²)

T_B = Time of the background count

E = Detection efficiency of the meter

A = Effective area of the probe

The Minimum Detectable Activity is defined as the smallest amount of activity that can be quantified for comparison with regulatory limits.
 

Formula:
 

( 2.71 / T_S ) + 3.29 ( R_b/T_b + R_b/T_s )^1/2

MDA = -------------------------------- (2)

E•(A / 100 cm²)
 

where: R_B = count rate of the natural background

T_B = time of the background count

T_S = time of the sample count

E = detection efficiency of the counter

A = area monitored or wiped
 

The MDA formula can be simplified when the counting time for the background count and the sample count are the same.
 

( 2.71 / T ) + 4.65 ( R_b/T )^1/2

MDA = -------------------------------- (3)

E•(A / 100 cm²)
 

The MDA formula can be simplified once again for counting wipe tests when the wiped area equals 100 cm².
 

( 2.71 / T ) + 3.29 ( R_b/T )^1/2

MDA = -------------------------------- (4)

E
 

A portable survey meter is used to monitor for fixed Phosphorus-32 contamination in an active laboratory. It is operated in a count rate mode and has a standard "Pancake" GM probe attached:
 

Data: R_B = 50 CPM

T_B = 1 minute

E = 25%

A = 20 cm²
 

Entering this data into Formula 1 gives:
 

LLD = 233 DPM
 

Since the LLD is less than the limits in Table 3 for Beta emitters, the instrument can be used for the survey.

A portable survey meter is used to monitor for fixed Iodine-125 contamination for a close-out survey. It is operated in a count rate mode and has a standard thin window NaI probe attached:
 

Data: R_B = 100 CPM

T_B = 1 minute

E = 25%

A = 5 cm²
 

Entering this data into Formula 1 gives:
 

LLD = 1316 DPM
 

Since the LLD is greater than the limits in Table 1 for I-125, the instrument can not be used for the survey.

A laboratory gamma counter is used to monitor for Iodine-125 contamination. A wipe test was taken over a 100 cm² area 
 

Data: R_B = 10 CPM

T_B = 10 minute

T_S = 10 minute

E = 75%
 

Since the background and sample counting times are the same and the wiped area is equal to 100 cm², Formula 4 can be used:
 

MDA = 7 DPM

This equipment is acceptable to monitor I-125 contamination.

A liquid scintillation counter is used to monitor for tritium contamination. A wipe test was taken over a 100 cm² area 
 

Data: R_B = 25 CPM

T_B = 5 minute

T_S = 5 minute

E = 68%
 

As in the previous example, Formula 4 can be used:
 

MDA = 16 DPM

This equipment is acceptable to monitor tritium contamination.

A laboratory area can not be released from radiological controls when the fixed and removable contamination is greater than the limits shown in Table 1.

Removable and fixed contamination levels for active radioisotope use laboratories and surrounding areas can not exceed the limits shown in Tables 2 and 3.
 

A separate Technical Bulletin on performing acceptable close-out surveys for radioisotope use laboratories in available from the USDA Radiation Safety Staff.

X-ray Emitters
 

( DPM/100 cm² )

( DPM/100 cm² )

Emitters
 

( DPM/100 cm² )

Instrument calibration records, including LLD and MDA calculations, must be retained by the LRPO for the facility for a period of three years.

Radiation Safety Staff 
5601 Sunnyside Avenue
Mail Stop 5510
Beltsville, MD  20705-5000
Phone: (301) 504-2440
Fax: (301) 504-2450

8/24/95 

Date

/s/ 

John T. Jensen

Director, Radiation Safety Staff