Ludlum M375 vs M375PRadioactive Waste Monitors

Alarm Level Logic is Different by Clyde Makinson

Ludlum Representativephone509-943-5288 clyde@owt.com

M375 Series

NaI(Tl) Detector M375P Series

Plastic Detector

600 VDC from instrument to detector12 VDC from instrument to detector

M375 M375P

Response: 3 seconds from 10% to 90% of final reading

Alarm Levels Set for sum of detector reading, Low Alarm and High Alarm

Display response: 60 seconds from 10% to 90% of final reading

One alarm level is set for sum of detector reading which is displayed

One alarm level for standard deviation above background for each detector set for 0.5 sec or 2 sec time constant

Background is the displayed value

M375P Advantages

• Sensitivity is increased with an alarm level set for each detector vs the sum of the detectors.

• Measured background is used to set the alarm level at a statistical confidence level.

• The sum alarm provides a warning that the background is too high.

M375P Disadvantages

• Upon a sigma alarm, there might be very little change in the displayed countrate which could be misinterpreted as a false alarm.

• Upon a sum alarm, it might take several minutes to drop below the alarm level because of the 60 second time constant.

Calculation for Sigma Alarm For example if the sigma alarm to be 4x over the background of 21 kps background

the sigma alarm is set at 434

• The SIGMA ALARM is a fast acting alarm. Depending on the fourth dipswitch setting, readings will be based on either 0.5 or 2.0 seconds. The SIGMA ALARM is based on the square root of background radiation for each detector.

• ALARM POINT = Bkg + (SIGMA ALARM × SQROOT (Bkg)). • When calculating the ALARM POINT, ensure that the Bkg number is in counts per 0.5 or 2.0

seconds (set by the fourth dipswitch).

• Example: To set the sigma alarm for 4x over the background (approximately 84 kcps).

• M375P Display shows 21.0 kcps which is the sum of two detectors. By pressing the CH1 button the display will show the 60 second average countrate for channel one which should be ½ of the normal display. If the time constant is switched to 2.0 seconds for the sigma alarm, multiply the CH1 reading by 2 and that is the number you will use for sigma calculations.

• The sigma alarm should be set for 84 kcps – 21 kcps = 63 kcps above the background of 21 kcps.

• If the time constant is switched to 2 seconds, then we can use the numbers above. If it is set to .5 seconds then all the numbers above have to be divided by 4.

• At 21,000 one sigma = square root of 21,000 = 145.• 434 sigma x 145 = 63,000

• ALARM POINT = Bkg + (SIGMA ALARM × SQROOT (Bkg)). • 83 kcps = 21 kcps + (434 sigma x 145)