RSDetection
User’s Manual
March 2016
S131-200-UM Rev A
Page 18
g
4
O
PERATION
4.1
HPIC
P
RINCIPLES OF
O
PERATION
The HPIC was designed with the objective of achieving the best balance between sensitivity,
energy response, stability, measurement range, and accuracy. No one technology has the optimal
characteristics for each of these specifications, but the HPIC was selected because it was the best
overall solution. The detector consists of a 10-inch stainless steel outer sphere that contains
approximately 25 atmospheres of argon. Figure 6 illustrates the basic detector, with outer 10-inch
cathode and inner 2-inch anode.
In operation, a high-voltage bias of -400 volts is applied to the outer shell, while keeping the anode
at ground potential. When gamma photons pass through the detector they interact with the steel
wall and the argon gas to generate ionized argon gas molecules. The amount of ionization
produced is primarily a function of the number of photons, the gamma ray energy, and the
incident direction of the photons.
In practice it is assumed that the directional effects average out and that the energy and flux are
the most important. The ionization produced will be swept to the anode and cathode by the
electric field within the detector; this generates an electric current that is a function of the incident
flux and photon energy.
10" SS Sphere 1/8"
2" Spherical anode
25 atmospheres
of Argon
Ceramic to Metal Seal
Anode support rod
Figure 6 HPIC Diagram
The effect of energy on the output is important since the desired relationship is for the energy
response to be “air like.” This means the response of the detector will be proportional to the
ionization produced in air. The actual energy response is described in the next section.
The range of the detector is also an important factor. To guarantee a linear output with gamma
intensity it is necessary to collect all the ions generated before they recombine and therefore are
not measured. To ensure this occurs the bias voltage must be high enough to collect all of the ions.
This condition is called saturation. The higher the gamma field, the higher the voltage required to
achieve saturation.
