78
5 Ionization Counters
coaxial anode wire. The inner glass tube, containing the cathode and anode is filled
with a mixture of gases, argon and alcohol (Fig. 5.14). Usually, the space in between
the two coaxial cylinders can accommodate about 12–14 ml liquid. The inner glass
wall of the counter acts like a window in this counter.
5.10.1 The Assembly of the Liquid G.M. Counter
A liquid G.M. counter generally rests on a metal base plate (Fig. 5.13), such that
the cathode (M) touches the metal base (B) and the central anode dips in mercury
(N ). Positive potential is applied through the mercury pool (N ). The entire tube is
shielded with lead bricks (shown by broken line, C), to reduce the activity due to
background cosmic radiation. Care should be taken to see that no visible light enters
into the counter when a positive potential is applied to the anode wire, as the liquid
G.M. counter can respond to photons of visible light as well. It is observed that if light
enters the counter, the count rate shoots up so high that the scalar gets jammed due
to fast counting rate. This effect is known as Joshi’s effect. Therefore, it is essential
that liquid G.M. counter be protected from external light and background radiation
by a well-designed lead chamber.
5.10.2 Thickness of the Window and Density Correction
It is worth noting that in a liquid G.M. counter, thickness of the inner glass and thin
film of silver on the window makes a total thickness of the wall to about 50 mgcm
−2
(unit of thickness considers the product of density and the actual thickness of the
material). Therefore, low energetic β-particles (approximately lower than 0.5 MeV)
cannot be counted by this liquid G.M. counter, because it cannot penetrate the glass
window to reach the active zone of the counter.
5.10.3 Necessary Precautions While Using Liquid G.M.
Counter
1. Density correction: Since the radioactive sample is in liquid form, the density of
liquid also plays a major role in affecting counting efficiency. Higher the liquid
density more the absorption of β-particles by the liquid (i.e., loss of radiation
due to self-absorption by the liquid itself) and hence lower is the efficiency of
counting. If liquids with two different densities are to be counted, to compare
their activity, a density correction has to be made. Their activities recorded are
converted to a condition as if both measurements were recorded in same type of
5 Ionization Counters
coaxial anode wire. The inner glass tube, containing the cathode and anode is filled
with a mixture of gases, argon and alcohol (Fig. 5.14). Usually, the space in between
the two coaxial cylinders can accommodate about 12–14 ml liquid. The inner glass
wall of the counter acts like a window in this counter.
5.10.1 The Assembly of the Liquid G.M. Counter
A liquid G.M. counter generally rests on a metal base plate (Fig. 5.13), such that
the cathode (M) touches the metal base (B) and the central anode dips in mercury
(N ). Positive potential is applied through the mercury pool (N ). The entire tube is
shielded with lead bricks (shown by broken line, C), to reduce the activity due to
background cosmic radiation. Care should be taken to see that no visible light enters
into the counter when a positive potential is applied to the anode wire, as the liquid
G.M. counter can respond to photons of visible light as well. It is observed that if light
enters the counter, the count rate shoots up so high that the scalar gets jammed due
to fast counting rate. This effect is known as Joshi’s effect. Therefore, it is essential
that liquid G.M. counter be protected from external light and background radiation
by a well-designed lead chamber.
5.10.2 Thickness of the Window and Density Correction
It is worth noting that in a liquid G.M. counter, thickness of the inner glass and thin
film of silver on the window makes a total thickness of the wall to about 50 mgcm
−2
(unit of thickness considers the product of density and the actual thickness of the
material). Therefore, low energetic β-particles (approximately lower than 0.5 MeV)
cannot be counted by this liquid G.M. counter, because it cannot penetrate the glass
window to reach the active zone of the counter.
5.10.3 Necessary Precautions While Using Liquid G.M.
Counter
1. Density correction: Since the radioactive sample is in liquid form, the density of
liquid also plays a major role in affecting counting efficiency. Higher the liquid
density more the absorption of β-particles by the liquid (i.e., loss of radiation
due to self-absorption by the liquid itself) and hence lower is the efficiency of
counting. If liquids with two different densities are to be counted, to compare
their activity, a density correction has to be made. Their activities recorded are
converted to a condition as if both measurements were recorded in same type of