494 9 Particle Physics – I
9.39 Estimate the gas multiplication required to count a 2 MeV proton which gives
up all its energy to the chamber gas in a proportional counter. Assume that the
amplifier input capacitance in parallel with the counter is 1. 5 × 10 −^9 F and that
its input sensitivity is 1 mV. Energy required to produce one ion pair is 35 eV.
9.40 Calculate the pulse height obtained from a proportional counter when a 14 keV
electron gives up all its energy to the gas. The gas multiplication factor of the
proportional counter is 600, capacitance of the circuit is 1. 0 × 10 −^12 F and
energy required to produce an ion pair is 35 eV
9.41 The plateau of a G.M. Counter working at 800 V has a slope of 2.0% count
rate per 100 V. By how much can the working voltage be allowed to vary if the
count rate is to be limited to 0.1%?
9.42 An organic-quenched G.M. tube has the following characteristics.
Working voltage 1,000 V, Diameter of anode 0.2 mm, Diameter of cathode
2.0 cm. Maximum life time 10^9 counts.
What is the maximum radial field and how long will it last if used for 30 h
per week of 3,000 counts per minute?
[University of New Castle]
9.43 A G.M. tube with a cathode and anode of 2 cm and 0.10 mm radii respectively
is filled with Argon gas to 10 cm Hg pressure. If the tube has 1.0 kV applied
across it, estimate the distance from the anode, at which electron gains just
enough energy in one mean free path to ionize Argon. Ionization potential of
Argon is 15.7 eV and mean free path in Argon is 2× 10 −^4 cm at 76 cm Hg
pressure.
9.44 A S^35 containing solution had a specific activity of 1m Curie per ml. A 25 ml
sample of this solution mass was assayed.
(a) in a Geiger-Muller counter, when it registered 2,000 cpm with a back-
ground count of 750 in 5 min; and
(b) in a liquid scintillation counter, where it registered 9,300 cpm with a back-
ground count of 300 in 5 min.
Calculate the efficiency as applied to the measurement of radioactivity and
discuss the factors responsible for the difference in efficiency of these two
types of counters.
[University of Dublin]
9.45 The dead time of a G.M. counter is 100μs. Find the true counting rate if the
measured rate is 10,000 counts per min.
[Osmania University]
9.46 A pocket dosimeter has a capacitance of 5.0 PF and is fully charged by a
potential of 100 V. What value of leakage resistance can be tolerated if the
meter is not to lose more than 1% of full charge in 1 day?
9.47 A G.M tube with a cathode 4.0 cm in diameter and a wire diameter of 0.01 cm
is filled with argon in which the mean-free-path is 8× 10 −^4 cm. Given that the