1997 COMPREHENSIVE EXAM (Part II)

QUESTION 1

You are the health physicist responsible for setting up an air-sampling system in the exhaust vent of a nuclear facility which emits both particulates and radioiodine.

POINTS

10	A.	Define isokinetic sampling and discuss its significance in setting up the air-sampling system. What is the effect on the representative nature of the air sample if the system is anisokinetic?
10	B.	Assuming anisokinetic sampling conditions, list two principle factors which contribute to the error relative to collection of a representative sample. Number your responses. Only the first 2 will be graded.
10	C.	Sampling line losses can range widely (from zero to 100 percent). List five factors that lead to sample line losses. Number your responses. Only the first 5 will be graded.
10	D.	If copper tubing is used for the sampling line in the air-sampling system you are setting up for particulates and radioiodines, what would be the effect on the air sample? Why?
10	E.	Briefly describe the function of using a cascade impactor air sampler in the initial design of a sampling system that minimizes sampling line losses.

QUESTION 2

An inhalation incident involving airborne ⁶⁰Co and ¹³¹I occurred at a radiochemistry laboratory. The worker immediately took a shower and changed clothes, then received a whole body count. Assume that the ⁶⁰Co was a class Y compound and the ¹³¹I was class D. Apply ICRP 30 methodology to answer the following questions.

GIVEN:

⁶⁰Co data:
	T_1/2 = 5.2 years For inhalation class Y: ALI_stoch = 30 mCi Committed dose equivalent (CDE) in lungs: 3.4x10^-7 Sv/Bq (f_N-P, f_T-B, f_P) = (0, 0, 100)
		where: f_N-P, f_T-B, f_P are the fractional contributions of the CDE to the reference tissue from initial depositions in the nasal passages, tracheo-bronchial, and pulmonary regions, respectively.

Fraction of initial intake remaining in whole body as a function of inhaled
particle size (in microns) and elapsed time:

Elapsed Time	___ Inhaled Particle Size___
(days)	1 µm	5 µm	10 µm
0 1 5 10 15 20	0.63 0.57 0.18 0.14 0.13 0.12	0.91 0.80 0.10 0.06 0.05 0.05	1.00 0.87 0.09 0.04 0.04 0.03

ICRP 26 recommended weighting factors

Organ or tissue
Gonads
Breast
Red bone marrow
Lung
Thyroid
Bone surfaces
Remainder

W_T
0.25
0.15
0.12
0.12
0.03
0.03
0.30

Fraction of intake deposited in the lung compartments

AMAD	_________DEPOSITION_________
(microns)	D_N-P	D_T-B	D_P	SUM
1 5 10	0.30 0.74 0.87	0.08 0.08 0.08	0.25 0.09 0.05	0.63 0.91 1.00

Correction for particle size:

POINTS:

10	A.	The ⁶⁰Co component of the whole body count result was 21 mCi. Assuming that the activity median aerodynamic diameter (AMAD) of the aerosol was 1 mm, estimate the intake, expressed in %ALI, based on the whole body count result. Show all work.
10	B.	Calculate the committed effective dose equivalent (CEDE) for an inhalation intake of 25 mCi of 1 mm AMAD class Y⁶⁰Co. Show all work.
10	C.	For this part only, assume that the CEDE due to ⁶⁰Co was 50 mrem. The worker had an ¹³¹I intake that resulted in 600 mrem committed dose equivalent (CDE) to the thyroid. Assume that the thyroid is the only significantly irradiated organ or tissue. During the same monitoring period, the worker also received 250 mrem due to external radiation exposure from ⁶⁰Co. What is the total effective dose equivalent (TEDE) to the worker during the monitoring period? Show all work.
20	D.	Another worker inhaled 30 mCi of class Y ⁶⁰Co. The AMAD was determined to be 10 mm. Calculate the committed dose equivalent (CDE) to the lungs. Show all work.

QUESTION 3

GIVEN:

The number of induced genetic defects (I) per generation for a given population is expressed as:

I = 0.05(Sd/D_g)
where:	S = spontaneous occurrence of defects in the population, d = dose equivalent to an average exposed individual in the population, D_g = genetic doubling dose equivalent (assume 250 mSv).
This assumes that both mother and father are exposed. I must be corrected by a factor of 1/2 if only one of the parents is exposed.

Relative risk (R) of genetic defects in a population is defined as: R= (S+I)/S

Dose and Dose Rate Effectiveness Factor for cancer induction (DDREF) = 2.0

POINTS

	A.	The linear-no-threshold model is most commonly used to describe dose response for induction of cancer by radiation.
10		1.	Name two other models and describe (or draw) the shapes of their dose response curves. Number your responses. Only the first 2 will be graded.
5		2.	What characteristic of the linear-no-threshold model makes it useful as a basis for radiation protection purposes? Justify your answer.
6	B.	What will be the most likely effect from a 30 rem dose equivalent delivered to the fetus 3 days, 3 weeks, and 3 months after conception?
	C.	The following questions relate to genetic effects of radiation:
4		1.	What does the term "genetic doubling dose" mean? What two types of studies were used to determine the genetic doubling dose?
10		2.	What is the relative risk of genetic defects (per generation) caused by an average dose equivalent of 4 mSv to each individual in a population of 40,000 men?
	D.	The concept of collective dose is sometimes used in assessing the potential harm which low level radiation may cause in a large population.
5		1.	In applying collective dose, the dose response is assumed to have what shape?
5		2.	Several additional assumptions regarding the attributable risks, exposed population or exposure conditions must be made in order to properly estimate risk using the collective dose concept. Name one of the assumptions. Only the first response will be graded.
5	E.	A population of 40,000 individuals was exposed, on average, to 4 mSv. Estimate the excess number of cancer deaths that might be expected in the remaining lifetime of this population. List any assumptions

QUESTION 4

You are a health physicist using a commercial ionization chamber survey instrument to perform general area surveys in a highly contaminated area containing mixed fission products.

GIVEN:

The instrument has the following characteristics:
	Volume = 200 cm³ Gas fill = ambient air Sliding shield = 0.34 cm phenolic	Window = 7 mg/cm² aluminized mylar Walls = 0.16 cm phenolic (plastic) Collection bias = 50 Volts

In addition, you have the following data:

	Density	Mass attention coefficient (g) (cm²/g)
Phenolic Air	(g/cm³) 1.25 1.29E-3	@ 0.5 MeV 0.091 0.087	@ 1.0 MeV 0.067 0.063

Thicknesses of Ionization Chamber Walls Required for Establishment of Electronic Equilibrium (from ICRU #20)
	Photon Energy (MeV)	Thickness (g/cm²)
	0.02 0.05 0.1 0.2 0.5 1 2 5 10	0.0008 0.0042 0.014 0.044 0.17 0.43 0.96 2.5 4.9



	1 erg = 6.2 x 10¹¹ eV w = 35 eV/ion pair 10⁷ ergs = 1 Joule 1 amp = 6 x 10¹⁸ ions/sec

Beta Range-Energy Curve (from Radiological Health Handbook)

POINTS

10	A.	Describe how this instrument can be used to determine the separate dose rates for betas and gammas in a mixed field.
15	B.	List three of the parameters that affect the correction factor needed to convert the meter reading to actual beta dose rate. Number your responses. Only the first 3 will be graded.
15	C.	1.	What is the maximum gamma ray energy for which full electronic equilibrium is established in the sliding shield?
		2.	What is the maximum energy beta particle that would be stopped by the sliding shield? Do not use rules of thumb.
	D.	Regarding electronic equilibrium:
5		1.	Briefly explain the meaning of "electronic equilibrium."
5		2.	What is the consequence if full electronic equilibrium is not established?

QUESTION 5

You are a health physicist at a manufacturing facility which produces depleted uranium plates. An employee of one of your customers has recently learned that the plates are radioactive and has expressed concern about her exposure. She has been working for three years in a warehouse where plates of depleted uranium are stacked on pallets forming an extended source. She has never worn any dosimetry. Her job duties involve visually inspecting the pallets and occasionally handling the plates. You have been asked to serve as a health physics consultant to your customer.

GIVEN:

Density of Air at STP:
Density of Water:

Specific Activity of depleted uranium:

Glove mass:
Glove total surface area:

0.001293 g/cm³
1.000 g/cm³

3.6 x 10^-7 Ci/g

200 g
400 cm²

Attached graph of "Dose rate from an extended source of depleted uranium," applicable to materials of low atomic number

Emissions from depleted uranium (partial list) (MeV)

U-238	4.1, 4.2	--	--
Th-234	--	0.1, 0.2	0.06, 0.09
Pa-234	--	2.3	0.8, 0.1
U-234	4.7, 4.8	--	--

Figure 1:
Doser rate from an extended source of depleted uranium

Attenuation, mg/cm²

Source: Handbook of Safety Procedures for Processing Depleted Uranium, Army Material Command Handbook, No. AMCHDBK-385-1.1-89, Department of the Army, Washington, D.C.

Expansion of Figure 1 for Range of 0 to 100 mg/cm²

POINTS:

10	A.	Calculate the shallow dose rate to the skin of the hands when handling the plates with and without gloves. If the wearing of gloves does not effect work performance, would you advise adopting the wearing of gloves as a standard practice? Show all work; justify your recommendation.
10	B.	Individuals walking through the warehouse maintain a distance of about one meter from the loaded pallets. Assuming that the loaded pallets essentially form a semi- infinite source relative to the individuals, what is the dose rate in air at a distance of 1.0 m from the pallets? Show all work.
10	C.	Assuming that eye protection is not used and that visual inspection of the pallets is performed for 2.5 hours a day at an eye-to-source distance of 30 cm, calculate the annual eye dose equivalent. Was the annual eye dose equivalent limit exceeded? State this limit as part of your evidence. State all assumptions; show all work.
10	D.	If you were the RSO for this warehouse facility, what radiological safety practices would you recommend be implemented? List five recommendations and provide a brief description for each. Number your responses. Only the first 5 will be graded.
10	E.	The plot of dose rate versus mass density thickness (Figure 1) displays three distinct slopes within regions A, B, and C. Describe the physical and/or radiological processes that account for this.

QUESTION 6

You are tasked with the design and fabrication of a spherical shield for a point source. The source currently has a 2.5 cm iron shield; your task is to add an outer lead shield. The configuration is shown below.

GIVEN

Isotope: ⁶⁰Co Source Strength: Up to 15 Ci _Co60 = 1.32 R-m²/Ci-hr Pb linear attenuation coefficient for ⁶⁰Co = 0.679 cm^-1 Fe linear attenuation coefficient for ⁶⁰Co = 0.35 cm^-1
Buildup factors for a point source:	Pb: 1 + (µx/3) Fe: 1 + µx where x is the thickness of the shield

POINTS

35	A.	Neglecting the dose buildup effect, what is the minimum thickness of lead that must be added to the existing iron shield to reach a desired exposure rate of 2.5 mR/hr at the surface of the shield? Show all work.
15	B.	For this part only, assume a total shield (lead + iron) thickness of 22 cm. Calculate the expected exposure rate on the outside surface of the shield if the exposure rate without buildup is 2.5 mR/hr. Assume the energy spectrum is not significantly degraded as it penetrates the iron shield. Show all work.

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