The E-600 can be configured and optimized in any number of ways to support a very wide range of probe types.

One of the more useful applications of this technology is to support pulse height analysis (PHA) with NaI detectors. Each probe can be configured to automatically configure the E-600 to use from 1 to 3 PHA channels. Another useful configuration is to set up a gross counting channel to find any isotope and then define 1 or 2 PHA channels to support isotopic identification of the discovered material. Pressing the channel button on the E-600 handle allows the user to change from gross counting to any defined PHA channel as often as necessary.

This application note describes the methods used to configure a gross counting channel and one or more PHA channels for specific isotope identification.

Many monitoring applications are strictly isotope specific and have no need for a gross counting channel. Unless all three channels of the E-600 have been previously defined for PHA identification, it is a good idea to set up a gross counting channel which supports monitoring all gamma energies (above a certain threshold level) which may be missed when monitoring with one or more PHA channel(s).

Begin by selecting one of the three channels and editing the parameters to agree with the following setup screen. The choice of channel type and units is strictly up to the user, however, a channel type of gamma for the gross gamma channel is our choice for a gross gamma channel.

Note that this discussion sets up the channel to use units of counts/sec with a calibration constant of 1.0. Calibration to other more useful units such as dpm can be accomplished later if desired.

Next, choose a source whose energy is at least as low as those which will be measured with the instrument. A good choice is probably ²⁴¹Am which has a 60 keV gamma. This energy is lower than most which will be encountered in field measuring operations.

The example voltage plateau graphs on the following page illustrate that ²⁴¹Am requires a higher operating voltage than ¹³⁷Cs for the fixed 2 mV threshold chosen. If an operating voltage is selected which supports counting ²⁴¹Am, the instrument can also count higher energy photons from isotopes such as ¹³⁷Cs and ⁶⁰Co. That is a low energy emitting isotope is chosen to run the voltage plateau before selecting the operating voltage. Follow the procedures in the E-600 interface program manual to run a voltage plateau from 400 to around 840 volts in 10 volt increments.

Once the high voltage plateau is complete, a high voltage can be chosen to operate with while operating the instrument in the gross counting mode. Looking at the ²⁴¹Am voltage plateau above, an operating voltage in the 750 to 800 Vdc range supports monitoring photons equal to or higher than ²⁴¹Am photons (60 keV).

Remember that with our current channel configuration, we are counting the signal from both the lower and upper windows. In reality, we can also choose to count just the information in the upper window when operating in gross counting mode since we have set the lower and upper thresholds at the same 2 mV operating point. A small fraction of the photons will exceed the lower threshold and not exceed the upper threshold when the instrument is configured in this manner.

Defining a PHA Channel

This section will discuss the steps necessary (only two) to set up a PHA channel to monitor a specific isotope. For our example the instrument will be configured to monitor the 662 keV photon emitted by the ¹³⁷Ba daughter of ¹³⁷Cs.

In this case, we will define a 2mV window with the lower and upper thresholds set to 4.00mV and 6.12mV respectively.  Use one click of the mouse to select the lower counting window in the "Selected Window" box and run a voltage plateau to determine the optimum operating voltage which "centers" the ¹³⁷Cs photon in the preset window. That's really all there is to it.

Start by selecting one of the three available channels and setting the parameters to agree with the above screen.

Follow the procedures in the manual to run a voltage plateau from 400 to around 540 volts in 10 volt increments.

 From the voltage plateau curve, an operating voltage can be chosen which will place the ¹³⁷Cs peak in the defined window. To do this simply select the highest point on the line representing the lower counting window. (The thinner of the two lines.) In the voltage plateau above, one can see that a voltage of 480 Vdc is the optimum choice.

Clicking on the left (<<) and right (>>) chevrons on the graph menu bar moves the vertical marker bar to the desired high voltage selection point so that the Set HV button can be clicked to set the high voltage without going to the channel parameters edit dialog box.

Determining PHA Window Width 

In the previous section, the PHA channel was set up with a 2 mV wide window by setting the lower threshold to 4.00 mV and the upper threshold to 6.12 mV. The choice of window width is largely up to the users needs; however the following table provides some insight into why the 2 mV window width is a good compromise for many applications.

Table 1 - Effect of window width on efficiency and background

Table 2 - Change in count rate due to temperature with a 2mV window (4.00 mV - 6.12mV), normalized to 20C value

Table 3 - Change in count rate due to temperature with a 1mV window, normalized to 20C value

The PHA channel user is torn between the desire to decrease the PHA window width to both increase the ability to reject unwanted photons above and below the target energy and reduce the operating background and the need to increase the window width to improve the counting efficiency.

The 1 mV window offers about the narrowest window which can be used with this hardware. The resolution of the upper threshold is around 0.25 mV so it doesn't make any sense to operate with less than about a 0.5 mV window.  However, as can be seen in Tables 2 and 3, the effects of temperature are greater with the narrowed window, particularly at lower temperatures and lower energies.  In a temperature stable environment, a 1 mV window offers low background and reasonable sensitivity.

For outdoor use or environments that do not have a stable temperature, the 2 mV window offers a good compromise for low background, high sensitivity and reduced temperature variation effects.

Operating with a 3 mV or 4 mV window instead of a 2 mV window increases the relative counting efficiency but gives a significant increase in background countrate.

It is very quick and easy to connect the E-600 to your PC and alter the window width and see what works best for your application.

Note: The exact setting of the thresholds and high voltage is critical in PHA mode.  The small variations in threshold calibration and high voltage resolution mean that swapping smart probes between E-600 instruments is not recommended if PHA mode is to be used.

PHA Effectiveness

While the E-600 is not a laboratory instrument designed for high precision multi-channel analysis, it can be quite effective as a field instrument for rough qualitative isotopic identification.

One of the limits of this system is the relatively poor resolution of NaI detectors and the Compton scatter into lower windows from higher energy photons which are absorbed by other than photoelectric effect interactions.

The Compton and pair production interactions which predominate the high energy photon interactions result in detector pulses which are not limited to a narrow band around the central energy of the primary photon.

Having said this, it is useful to understand what can and cannot be expected in terms of the ability of a PHA channel to reduce the interference from other than the target energy.  Table 4 shows the crossover effect for a number of isotopes with a 2 mV window.


Table 4: Crossover, count rate in window (4.00 mV - 6.12 mV) compared with peak count rate

Conclusion

The intent of this application note is to take the mystery out of a task which is normally considered difficult with a portable instrument.

Setting up PHA channels on the E-600 requires no tools except your PC since no hardware adjustments are required. Simply set the appropriate fixed window width in the channel parameters and run a single high voltage plateau to determine the best operating voltage.

Computer control of the high voltage, lower threshold and upper threshold in combination with the ease of use the Windows^® E-600 Interface Program provides allows the casual user to configure the E-600 and a SPA-3 for any desired isotope in a matter of minutes.

Note: The exact setting of the thresholds and high voltage is critical in PHA mode.  The small variations in threshold calibration and high voltage resolution mean that swapping smart probes between E-600 instruments is not recommended if PHA mode is to be used.