The pulse height obtained is on the order of several volts. The field strength is so great that the discharge, once ignited, continues to spread until amplification cannot occur, due to a dense positive ion sheath surrounding the central wire (anode). . This is where the number of ion pairs level off and remain relatively independent of the applied voltage. This leveling off is called the Geiger plateau that extends over a region of 200 to 300 volts. In the G-M region, the gas amplification factor depends on the specific ionization of the radiation to be detected. This region is used extensively in radiation monitoring, but not used by the NIS
Geiger-Mueller Region (V)
The applied voltage is so high that, once ionization takes place in the gas, there is a continuous discharge of electricity, so that the detector cannot be used for radiation detection.
Continuous Discharge Region (VI)
LIST the type of detector used in each of the following nuclear instruments:
The source range normally uses a proportional counter, while the intermediate and power ranges use ionization chambers. A compensated ion chamber is used for the intermediate range. The power range uses an uncompensated ion chamber.
STATE the reason gamma compensation is NOT required in the power range
Uncompensated ion chambers are utilized in the power range because gamma compensation is unnecessary; the neutron-to-gamma flux ratio is high. Having a high neutron-to-gamma flux ratio means that the number of gammas is insignificant compared to the number of neutrons.
DESCRIBE how a wide range fission chamber is used to detect neutrons
Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U-235. The neutrons interact with the U-235, causing fission. One advantage of using U-235 coating rather than boron is that the fission fragment has a much higher energy level than the alpha particle from a boron reaction.
- Neutron-induced fission fragments produce many more ionizations in the chamber per interaction than do the neutron-induced alpha particles from boron. This allows the fission chambers to operate in higher gamma fields than an uncompensated ion chamber with boron lining. Fission chambers are often used as current indicating devices and pulse devices simultaneously. Because of the fission chamber’s dual use, it is often used in “wide range” channels in nuclear instrumentation systems. Fission chambers are also capable of operating over the source and intermediate ranges of neutron levels.
An Alpha particle
consists of two neutrons and two protons
In respect to a gas filled detector
sufficient voltage creates a cascade effect, releasing more electrons
When radiation enters a proportional counter
the gas becomes ionized
A gamma spectrometer uses a
What does the neutron interact with in a wide range fission chamber?
The U-235 coated detector wall