Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

508 CHAPTER 9
9.23. An air-wall air-filled ionization chamber whose volume is 100 cm 3 gives a saturation
current of 10 −12 A when placed in an X-ray field. If the temperature was 27 ◦ C and
the atmospheric pressure was 740 mm Hg, what was the radiation exposure rate?
9.24. (a) A resistor of what value, to be placed in series with the ion chamber (Problem
9.23), is required to generate a voltage drop of 10 mV?
(b) If the capacitance of the chamber is 250 pF, what is the time constant of
detector circuit?
(c) How much time is required before the meter will read 99% of the saturation
current?
9.25. A pocket dosimeter has a capacitance of 5 pF and a sensitive volume of 1.5 cm3 .
What is the charging voltage if it is to be used in the range 0–200 mR (0–51.5 μC/kg,
0–1.75 mGy air kerma) and if the voltage across the dosimeter should be one-half
the charging voltage when the dosimeter reads 200 mR (1.75 mGy air kerma)?
9.26. A Geiger-Müller tube has a capacitance of 25 pF. The time required to collect all
the positive ions is 221 × 10−6 seconds. In order to produce sharp output pulses,
it is desired to limit the time constant of the detector circuit to 50 μs.
(a) What is the value of the series resistor?
(b) If 108 ion pairs are formed per Geiger pulse, what is the upper limit of the
output voltage pulse?
9.27. A GM counter has a resolving time of 250 μs. What fraction of the counts is lost
due to the counter’s dead time if the observed counting rate is 30,000 cpm?
9.28. The fact that the gas multiplication in a proportional counter is very much less than
that in a Geiger counter means that a pulse amplifier for use with a proportional
counter must have a lower input sensitivity than one used with a Geiger counter.
Calculate the input sensitivity for an amplifier to be used with a 2-in. diameter,
hemispherical, windowless gas-flow proportional counter whose capacitance is 20
pF and which is operated to give a gas amplification of 5 × 103 . Assume that the
output pulse is “clipped” to one-half its maximum height.
9.29. What is the sensitivity of a thermal neutron detector whose volume is 50 cm3 and which is filled with 96% enriched 10BF3 to a total pressure of 70 cm Hg at a
temperature of 20◦C? 9.30. If the BF3 tube of Problem 9.29 is used as a current ionization chamber, what
saturation current would result from a thermal flux of 109 neutrons/cm2 /s?
9.31. How long would it take for the sensitivity of the BF3 detector of Problem 9.30 to
decrease by 10%?
9.32. What is the sensitivity for 1-MeV and for 10-MeV neutrons (amps per neutron per
cm2 /s) of an ion chamber that is filled with CH4 gas to a pressure of 760 mm Hg,
if its volume is 500 cm3 ?
9.33. A 1000-MBq (27-mCi) 60Co source is lost. At what distance can the lost source
be detected with a survey meter whose sensitivity is 0.05 mR/h (0.013 μC/kg/h)
above background?
9.34. The thermal-neutron flux from a moderated 252Cf neutron-calibration source is
determined by irradiating a gold foil of dimensions 1 cm (diameter) × 0.013 cm
(thickness) for a period of 7 days at a distance of 100 cm from the source. The