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7 Non-conventional Detection Techniques
7.8 Solid-State Track Detector
In recent years, solid-state track detectors are gaining ever-increasing importance as
a powerful experimental tool in the study of a number of nuclear phenomena. These
detectors are made of insulating materials such as mineral crystals (e.g., quartz glass,
and mica sheet), glass (e.g., pyrex and sodalime ), and certain synthetic plastics
(like lexan, makrofol, etc.). The system takes advantage of the fact that energetic
heavy ions such as fission fragments moving through the insulating material leave
behind narrow continuous trails of damage which can be made visible under an
optical microscope after suitable chemical etching. Chemical etching is based on
the principle that large free energy associated with the disordered structure makes
damage trails more chemically reactive than the normal material. If a substance
containing damage trails is immersed in a suitable chemical, those damaged sites
which intersect a surface are preferentially leached.
The technique has several advantages over conventional semiconductor counter
or the time-consuming radiochemical methods of fission studies, the most attractive being its simplicity. The detector is very selective and effectively discriminates
against processes other than fission and thus affords convenient means for studying
fission events, in an essentially background free situation.
7.9 Low-Level Counting
In low-level counting (for isotopes other than β-emitters), especially for measuring
food contamination by radioactive isotopes, on fall out of nuclear explosion to the
atmosphere, etc., one is not concerned with the efficiency of counting, but in reducing
the background activity to a value as low as 0.2 cpm. Because in low- level counting
the activity of a sample may be of the order of 10–30 cpm. With a background
of ordinary G.M. counter (whose background counts are normally 10–20 cpm), it
becomes impossible statistically to distinguish activity due to either the sample or
the background. One can think of reducing the background by increasing shielding
of the counter with a thick lead wall. But one cannot reduce the background to
less than 10–20 cpm even by making lead of 30 cm thickness. Lead, being the end
product of all radioactive series, is bound to be contaminated with a few natural
radioactive substances, which can contribute toward the background radiation. Hence
some modification to the assembly of the G.M. counter and some modification to
the electronic circuit are made to lower the background to almost 0.1 cpm. This is
achieved by two processes namely Anti-Coincidence counting and Co-incidence
counting systems. These are discussed here.
7 Non-conventional Detection Techniques
7.8 Solid-State Track Detector
In recent years, solid-state track detectors are gaining ever-increasing importance as
a powerful experimental tool in the study of a number of nuclear phenomena. These
detectors are made of insulating materials such as mineral crystals (e.g., quartz glass,
and mica sheet), glass (e.g., pyrex and sodalime ), and certain synthetic plastics
(like lexan, makrofol, etc.). The system takes advantage of the fact that energetic
heavy ions such as fission fragments moving through the insulating material leave
behind narrow continuous trails of damage which can be made visible under an
optical microscope after suitable chemical etching. Chemical etching is based on
the principle that large free energy associated with the disordered structure makes
damage trails more chemically reactive than the normal material. If a substance
containing damage trails is immersed in a suitable chemical, those damaged sites
which intersect a surface are preferentially leached.
The technique has several advantages over conventional semiconductor counter
or the time-consuming radiochemical methods of fission studies, the most attractive being its simplicity. The detector is very selective and effectively discriminates
against processes other than fission and thus affords convenient means for studying
fission events, in an essentially background free situation.
7.9 Low-Level Counting
In low-level counting (for isotopes other than β-emitters), especially for measuring
food contamination by radioactive isotopes, on fall out of nuclear explosion to the
atmosphere, etc., one is not concerned with the efficiency of counting, but in reducing
the background activity to a value as low as 0.2 cpm. Because in low- level counting
the activity of a sample may be of the order of 10–30 cpm. With a background
of ordinary G.M. counter (whose background counts are normally 10–20 cpm), it
becomes impossible statistically to distinguish activity due to either the sample or
the background. One can think of reducing the background by increasing shielding
of the counter with a thick lead wall. But one cannot reduce the background to
less than 10–20 cpm even by making lead of 30 cm thickness. Lead, being the end
product of all radioactive series, is bound to be contaminated with a few natural
radioactive substances, which can contribute toward the background radiation. Hence
some modification to the assembly of the G.M. counter and some modification to
the electronic circuit are made to lower the background to almost 0.1 cpm. This is
achieved by two processes namely Anti-Coincidence counting and Co-incidence
counting systems. These are discussed here.