RPT 223 Instructional Resources

RPT 223 Instructional Resources

Module 3: Using Track Etch Dosimeters to Monitor Neutron Radiation

Table of Contents:

Resources Key 2

Module Readings and Homework 2

Primary Scenario “Dose monitoring during external beam radiation therapy” 2

Transfer Scenario “Dose monitoring during soil moisture gauging using PuBe source” 2

Module Assessment Items 3

Primary Scenario “Dose monitoring during external beam radiation therapy” 3

ACAD References 4

Resources Key

Module Readings and Homework

Primary Scenario “Dose monitoring during external beam radiation therapy”

Transfer Scenario “Dose monitoring during soil moisture gauging using PuBe source”

Refer to readings and homework for primary scenario above.

Module Assessment Items

Note: If instructors wish to increase the difficulty of any item, then we suggest you use it as the basis for an in-class discussion, and / or require students to write an explanation for why a particular choice is correct.

Primary Scenario “Dose monitoring during external beam radiation therapy”

1.  A consulting company is designing the shielding for a therapy room and has suggested the use of a polyethylene lining on the walls of the room. Fast neutrons born in the walls from photonuclear reactions must pass back through the polyethylene to get into the therapy room.
Assuming that the neutrons are reduced in energy because of interactions in the polyethylene, but that essentially none are absorbed, select all statements which are true.

a.  The resulting thermal neutrons deliver more dose in the room than do the fast neutrons (F).

b.  Unless the number of neutrons is reduced by absorption, the neutron dose is the same as when no polyethylene is used (F).

c.  The energy of the neutrons has no effect on the neutron dose in the therapy room (F).

d.  The neutron dose is decreased because the energy of the neutrons has been decreased. (T)

2.  Two neutron dosimeters are placed in a fast neutron field: one is a neutron sensitive TLD and one is a track etch dosimeter. Assume that the two dosimeters are placed in air in a neutron field containing both fast and thermal neutrons. Check ALL that are correct.

a.  Both dosimeters will respond the same to both the thermal and fast neutrons in these radiation fields (F).

b.  The TLD will detect the thermal neutrons, but it will not be able to detect the fast. (T)

c.  The track etch will detect the fast neutrons, but it will not be able to detect the thermal (T)

d.  Even though the dosimeters respond to different energy neutrons, both will accurately measure dose (F).

3.  Two neutron dosimeters are placed in a fast neutron field: one is a neutron sensitive TLD and one is a track etch dosimeter. Assume that the two dosimeters are placed on a plastic phantom simulating the torso of the human body. Check ALL that are correct.

a.  The track etch detector will detect some of the fast neutrons which are scattered in the phantom (T)

b.  Both dosimeters will respond to the fast neutrons in the neutron field. (T)

c.  The TLD will detect some of the fast neutrons which are scattered in the phantom. (T)

d.  Both dosimeters will be able to accurately measure dose from both thermal and fast neutrons (F)

4.  Assume the same conditions as item 3 above but both detectors are covered with a filter of Cd on the side opposite from the phantom and closest to the source. Check ALL that are correct.

a.  The Cd cover is used as a beta filter and will not affect the response of either dosimeter (F)

b.  The Cd cover will change the response of the track etch dosimeter to the neutron field (F).

c.  The Cd cover will eliminate the response of both detectors to thermal neutrons (F).

d.  The Cd cover on the TLD will eliminate its response to the thermal neutrons in the field. (T)

5.  The dose delivered by fast neutrons is primarily due to the scattered nuclei from scattering interactions of the fast neutrons with matter. Consider the case of fast neutrons scattering off of human tissue as compared to steel. In both cases, the material is thick enough that all neutrons are slowed to thermal energies. How would this phenomenon differ for these two materials? Check ALL that are correct.

a.  More nuclei will be scattered in steel than in tissue. (T)

b.  The dose in the two materials will be similar. (T)

c.  Most scattering in tissue will be with hydrogen. (T)

d.  The neutron scattered nuclei don’t deliver much dose. (F)

6.  There are several mechanisms for producing neutrons. Select all statements that are true about these mechanisms.

a.  High energy photons can produce neutrons in materials because the energy of the photons can overcome the binding energy of a neutron in the nucleus. (T)

b.  Neutrons can be easily produced in the target of a medical therapy accelerator by bombarding it with high energy electrons (F).

c.  An (alpha, n) PuBe source creates neutrons by fusing together an alpha particle and a Be nucleus, which split apart yielding a neutron and Li. (T)

d.  Additional neutrons can be created by bombarding almost any material with thermal neutrons which release fast neutrons (F).

7.  There have been a few accidents in which acceleratory based therapy machines have malfunctioned and delivered much more dose than prescribed. In such a situation, assume a patient receives a whole body dose of 4 Sv in a single treatment. Check ALL that are correct.

a.  The patient will likely die within minutes from acute radiation sickness syndrome (F).

b.  The patient will have no immediate reaction but will have a slight increase of cancer in his / her lifetime (F).

c.  If the patient survives, there will still be a slight increase in the risk of cancer over his/her lifetime. (T)

d.  With medical attention the patient has a chance for recovery without long term effects. (T)

8.  A measurement has been made in the containment building of an operating nuclear power plant and a dose of 30 mRads/hr (0.3 mGy/hr) is measured. A radiation protection technician has been asked to convert this to a dose equivalent in mRem/hr (or mSv/hr). Check ALL that are correct.

a.  Since the quality factor for uncharged particles such as gamma-rays is 1, the quality factor for neutrons is also the same since they are uncharged.

b.  The number of neutrons needed to create a given dose equivalent is much smaller for fast energy neutrons compared to thermal neutrons. (Correct)

c.  The quality factor (thus the conversion from dose to dose equivalent) is highly dependent upon the energy of the neutrons causing the dose. (Correct)

d.  Without knowing the energies of the neutrons detected, it is not possible to convert the neutron dose to neutron dose equivalent. (Correct)

9.  In a CR-39 track etch dosimeter, the tracks are caused by recoiling nuclei. What factors affect the likelihood that a track is caused by an interaction with a specific nucleus (C, O, H, etc.) in the CR-39 dosimeter?

a.  The neutron cross section of nucleus.

b.  The energy of the incoming neutron.

c.  The angle of scatter of the recoiling nucleus.

d.  All of the above. (CORRECT)

10.  Why does the dose equivalent in Rem or Sv range from 5 – 20 times the dose (in Rads or Gy) for neutrons?

a.  Neutrons more readily interact with tissue than do gamma-rays or betas, therefore creating more dose.

b.  Neutrons are much larger than gamma-rays or electrons, therefore creating more dose.

c.  Neutrons have much higher energy than gamma-rays or betas, therefore they create more dose.

d.  Neutrons create large, charged recoil nuclei that cause more biological damage per unit of energy deposited. (CORRECT)

11.  How do track etch dosimeters measure dose?

a.  They estimate the energy deposited from the number and depth of neutron created damage tracks. (CORRECT)

b.  They become activated and the resulting emitted radiation from activated isotopes can be measured.

c.  They interact with thermal neutrons, creating proton recoil nuclei that create damage tracks.

d.  They heat up through interactions with neutrons, resulting in a temperature increase which can be measured.

12.  Based upon the evaluation of a track etch dosimeter, it is estimated that 3 mrad (0.03 mGy) of neutron dose has been delivered at thermal (<1 eV) and 5 mrad (0.05 mGy) of neutron dose has been delivered at approximately 1 MeV. What is the dose equivalent in mRem and mSv?

Answer:

3 mrad * 5 + 5 mrad *10 = 65 mRad

or

0.03 mGy * 5 + 0.05 mGy * 10 = 0.65 mSv

13.  Compared to the problem above, what would happen if all 8 mrad (0.08 Gy) of neutron radiation was between 100 keV and 2 MeV in energy?

a.  Both the dose and the dose equivalent stay the same.

b.  The dose decreases and the dose equivalent stays the same

c.  The dose remains the same but the dose equivalent decreases.

d.  The dose remains the same but the dose equivalent increases. (CORRECT)

14.  Why does CR-39 have a useful dose response range starting at a minimum of about 20 mRem?:

a.  Below 20 mRem (0.2 mSv) no tracks are formed because the neutrons have insufficient energy at this dose level.

b.  Below 20 mRem (0.2 mSv) the tracks created do not contain sufficient damage to be etched.

c.  Below 20 mRem (0.2 mSv) there are too few tracks formed to provide a statistically valid result. (CORRECT)

d.  Doses below about 20 mRem (0.2 mSv) are small enough to not be recorded, so there is no need to detect them.

15.  Why does CR-39 have a useful dose response range up to about 5 Rem (50 mSv)?

a.  Above this dose, so many tracks are formed that they tend to overlap and not be accurately counted. (CORRECT)

b.  Above this dose the CR-39 material disintegrates due to too much radiation damage.

c.  The neutron energy is so high that tracks created are longer than the size of the CR-39 chip.

d.  5 Rem is the federal limit for radiation workers and there is no need for a dosimeter that reads beyond this value.

16.  What happens as the result of the neutron dose that has been measured for the cancer therapy room?

a.  The neutron dose didn’t need to be determined because it was a small fraction of the X-ray dose.

b.  The neutron dose is found to be so small that it can be neglected. (CORRECT)

c.  The neutron dose is a significant part of the dose a patient gets from therapy and needs to be included.

d.  The neutron dose equivalent is large compared to the X-ray dose because the quality factors are much larger.

17.  The following dosimeter systems have all been used to measure the neutron dose in a therapy room. Which systems are likely to under-predict dose due to pulse-pileup or resolving time issues? Select all that apply.

a.  Bonner spheres (CORRECT)

b.  BF3 probe (CORRECT)

c.  Neutron sensitive OSLD

d.  Track etch

e.  6LiI scintillator (CORRECT)

f.  Neutron sensitive TLD

ACAD References

Module Using Track Etch Dosimeters to Monitor Neutron Radiation Dose

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