Measurements with shielded gamma counters

We started taking counts on each floor of NSC building with 3 side shielded gamma couters(SPA-3 and LEG-1). Khristian Pettigrew who is really interested on cosmic radition, joint with our group to carry on this project.




Khristian is wrapping up the lead bricks with paper sheets











Some views of 3 side shielded gamma counter. Lead bricks used to shield the 3 sides and the bottom of the counters. Hence, the detector only responses to gamma rays which are comming from up





























Khristian is preparing the setup to get measurements on 2 floor of NSC building.

How well do you know about the natural background radiation?

It looks like that the more we make measurements of the natural background radiation inside the Science Center Building the more questions are arising. How well does one know about the nature of the background radiation?

We should start posting new measurements with the shielded gamma counters.

Counts on different floors

Counts on different floors were measured using the two gamma detectors, SPA-3 and LEG-1. On each floor counts were measured with placing the detectors on the LEAD bricks and without LEAD bricks as shown in the following pictures.


Placing the detector on the LEAD bricks














Placing the detector on the cart without LEAD bricks














The following plots show gamma counts on each floor of NSC building of GSU. 1st and 6th floors are basement and roof respectively.

Taking measurements on different floors of NSC

Taking measurements on 4th floor of NSC

Taking measurements on 5th floor

Taking measurements on roof of NSC

Prabha (grad. student) is observing gamma counts

Operating high voltage for detectors

1.0 micro Ci Cobalt 60 source was used and placed near the detectors as showing in the following pics.

Threshold(R27) and Gain(R22) potentiometers (at side of the ESP-2) are adjusted to the maximum position in clockwise direction. However, I am not aware of the output values of both Threshold(R27) and Gain(R22). The nominal threshold value for both SPA-3 and LEG-1 detectors is 10mV which is given in the manual.

HV is changed from 700V to 1200V with a step size of 50V. The nominal HV for both detectors is 1000V which is given in the manual. However, the maximum HV which can be given for the detectors is not mentioned in the manual.

The following plots are made by reading the # of counts per minutes for different HV values. In fact, # of count per minute is calculated by taking counts for 10 minutes.

In the above plot it can be clearly found a plateau region. Hence, 1100V is selected as a suitable HV for LEG-1 detector.

In the above plot there is no plateau region. Since we don’t know the maximum HV for the detector, it is not suitable to increase the HV furthermore. However, 1100V is selected as a suitable HV for SPA-3 detector.

ESP High Voltage Measurements

The high voltage output of the ESP-2 is measured by connecting the Keithley HV probe. Measurements were taken using the HP multimeter. When the probe is connected all the readings are reduced by a factor of 1000.









This graph is plotted with set-HV in ESP on the “x” axis, and measured-HV on the “y” axis, when the probe is connected.


Since this shows a linear relation, we can use the Keithley HV probe for larger high voltage measurements.