Page 180 - Radiochemistry and nuclear chemistry

164 Radiochemistry and Nuclear Chemistry

6.2. What is the minimum energy that an a-particle must have to be detected by a GM tube having a mica (the
density is approximately equal to aluminum) window of 1.5 mg cm-2?
6.3. For an irradiation experiment it is necessary to extract a beam of deuterons from an accelerator. The
projectile energy is 22 MeV D +. For this purpose the beam is deflected and permitted to pass through a thin
titanium foil (density 4.5 g cm-3). Assuming that ktPlMt -'~ - kz02M2 -'~ (Bragg-Kleman rule), what is the
maximum thickness of the foil? Give the answer in millimeters.
6.4. Make a rough estimate of the range in air for a 1 MeV a, 1 MeV H + and 1 MeV e- using the plot in Fig.
6.7. The energy to form an ion pair in air is 14.6 eV but assume that twice as much energy is lost through
excitation.
6.5. What is the range of a 6.3 MeV a-particle in (a) aluminum, (b) nickel, (c) platinum?
6.6. What is the 7-ray flux from a 3.7 GBq e~ source at a distance of 3 m? Assume r = 1.
6.7. What is the maximum range in millimeters of/~-particles from T, 14C, 32p and 9~ in a photographic
emulsion if its absorption efficiency is assumed to be the same as aluminum? The density of the emulsion is
assumed to be 1.5 gcm -3.
6.8. The Era, x of 32p/~-particles is 1.71 MeV. To what electron velocity does this correspond?
6.9. In a laboratory an irradiation area must be designed for 7-radiography using a 3.7 x 10 II Bq e~ source.
For this purpose a cubic building is erected with an interior side length of 2 m. The desired flux reduction is 106.
How thick must the wall be and how much will the shielding material cost (i.e. not including labor costs) if it is
made of (a) concrete? (b) lead? Assume lead blocks cost $1.50 per kg and concrete $40 per m 3.
6.10. An experiment is done with e~ which emits 0.05860 MeV 7. The detector used is a Nal crystal. What
photo peaks will be observed if the electron binding energies in sodium are K 1072 and L 63 eV, and in iodine
K 33 170 and L 4800 eV?
6.11. A human body may be considered as consisting of water. Radiation from 137Cs is absorbed by a 15 cm
thick body. How much is the 7-ray flux reduced by the body, and how much of the beam energy (~ plus ~/) is
absorbed?
6.12. For iron the mass attenuation coefficients are" at 0.5 MeV 7, 0.083; at 1.0 MeV, 0.059; at 1.5 MeV, 0.047
cm 2 g-l. Calculate the corresponding one-tenth values.
6.13. An absorption curve of a sample emitting/~- and 7-rays was taken with aluminum absorber using a
gas-flow proportional counter. The data obtained were:

Absorber Absorber
thickness Activity thickness Activity
(gcm -2) (counts min -I) (gcm -2) (counts rain -l)

0 5800 0.700 101
0.070 3500 0.800 100
0.130 2200 1.00 98
0.200 1300 2.00 92
0.300 600 4.00 80
0.400 280 7.00 65
0.500 120 10.00 53
0.600 103 14.00 40

(a) Estimate the maximum energy of the D-spectrum. (b) Find the energy of the 7-ray.
6.14. A 40-story high modern business building is supported by 0.9 m thick pillars of reinforced concrete. The
insurance company must check that the number of iron bars are as many as required, and therefore they want to
investigate the pillars by 7-radiography. What exposure times are required for (a) a small 200 OBq ~0 Co source,
(b) for a large 150 TBq source? Use the same film data as in 6.9.2.
6.15. A swimming-pool reactor produces a flux of 3 x 1016 thermal neutrons m -2 s -l at 1 m from the reactor
center. Assuming a parallel beam of neutrons diffusing up to the surface of the pool where the neutron flux is
measured to be 10 s n m -2 s-l, calculate the thickness (x m) of the water layer required. For thermal neutrons the
flux is reduced exponentially with the exponent x L -l, where L is the diffusion length (2.75 em in H20).
6.16. In a sample of 10.4 TBq of old fission products, the average 7-raY energy is 0.5 MeV and on the average
0.4 7's are emitted per/~-decay. (a) What is the lead shielding required to reduce the 7 flux to 102 7 cm-2 s-l