Determining Detector Operating Voltage

Many of our customers inquire about the procedure we use to check the operating voltage of scintillation or proportional type detectors. We call this process “to run a plateau”. Hopefully, the following will help to answer a few of these questions.

To determine the operating high voltage (HV) for your detector, the following items will be required: a counting instrument e.g., a scaler or analog meter, a radioactive source, and detector to be tested. The radiation given off by the source should be matched for the detector. For example, use a beta source such as ¹⁴C with a Model 44-1 beta scintillation detector, or use a low energy gamma source such as ¹²⁹I with a Model 44-3 low-energy gamma scintillation detector. We use ²⁴¹Am to run a plateau for most high energy gamma sodium iodide (NaI) type detectors. This ensures the detector will detect lower energy radiation at the operating voltage.

Definitions

Figure 1 below is an example graph of a plateau that highlights the key reference points used in determining the detector operating voltage.

Figure 1 - Example Plateau Graph

Source Count Curve
A graph of data for source radiation, plotting counts (for a given time period) versus detector high voltage.

Background Count Curve
A graph of data for background radiation, plotting counts (for a given time period) versus detector high voltage.

Plateau
The level portion of the counting-rate vs. voltage curve where changes in operating voltage introduce minimal changes in the counting rate. It is the region defined at the low end by the knee of the source count curve and at the high end where the source and background count rates start to "take off" (as the voltage is increased, the background and source counts increase rapidly). The region between these two points is called a plateau because the graph is relatively flat; that is, the count rate is relatively independent of applied voltage.

Knee
The edge of the plateau where a small decrease in detector high voltage produces a sharp decrease in source counts.

Operating Voltage
The detector high voltage for normal operation. It is selected by choosing a point that is approximately in the middle of the plateau and the background reading is at or below specifications.

Buffer
A voltage region on either side of the selected operating voltage that meets stated specifications for operating voltage, that is, efficiency and background count rate. It is a “cushion” above the knee and below the “take off” point to allow for slight changes or imprecision in establishing the operating voltage and accommodates the change in operating voltage that can occur as temperatures change or detectors age. For example, a 50-volt buffer for a 750-volt operating voltage would require a plateau length of 100 volts, ranging from 700 volts to 800 volts. The “size” of the buffer region, in volts, represents a tolerance (±) on the operating voltage. The required size of the buffer varies with individual detectors. See the detector specifications or the calibration procedure for the required minimum buffer.

Efficiency
Efficiencies are typically performed with the distance of the source at the surface level of the detector.

2π (Intrinsic) Efficiency
2π efficiency is a measure of the probability that a count will be recorded when a particle or photon of ionizing radiation emitted towards the detector is incident on a detector. In other words, the efficiency for 2π geometry is calculated as the percentage of the number of counts registered by the instrumentation in cpm to the surface emission rate from the source. These are defined as counts per minute (cpm) or particles or photons per minute (ppm). This relationship is expressed mathematically as:

4π (Absolute) Efficiency
4π efficiency is a measure of the probability that a count will be recorded to the total number of photons or particles emitted by the radioactive source. In other words, the efficiency for 4π geometry is calculated as the percentage of the number of counts registered by the instrumentation in cpm to the emissions from the source in dpm. This relationship is expressed mathematically as:

Unless otherwise noted, efficiencies stated by Ludlum Measurements correspond to 4π geometry.

Running a Plateau

The following is a brief example of a plateau method for a Model 44-2 NaI scintillation detector, a Model 2200 scaler, and an ²⁴¹Am check source.

1. Set the Model 2200 high voltage to 500 V and threshold to 10 mV. Ensure the window is turned OFF.
2. Set the count time controls for a six-second count.
3. Connect the Model 44-2 to the Model 2200.
4. Be sure there are no radioactive sources nearby that will affect the local background. Take a background count. Record the high voltage and the background reading.
5. Increase the voltage by 50 V.
6. Take and record another reading.
7. Repeat steps 5 and 6 until the voltage reaches 1000 V or the background count increases dramatically.
8. Lower the Model 2200 voltage back to 500 V.
9. Place the ²⁴¹Am check source on or near the detector. Take a source count. Record the high voltage and source count reading.
10. Increase the voltage by 50 V.
11. Take and record another reading.
12. Repeat steps 10 and 11 until the voltage reaches 1000 V or the source count increases dramatically.
13. Select and document an operating voltage at or near the middle of the plateau.

Contact Ludlum Measurements for more specific instructions or for instructions for different instruments.