9/13/2012
Radiation Biology Lecture #7
Neutron Interactions
Jeffrey Bryan, DVM, MS, PhD, DACVIM(Oncology)
[email protected]
1
9/13/2012
Neutron Interactions
• Neutrons have approximately the mass of a proton
• Not charged
• Relativistic, Fast, Epithermal, Thermal
Relativistic Neutrons
• Energy > 10 MeV
•
32S + n => 32
P + p+ (11H = p+)
• Nuclear binding energy is 7‐8 MeV
• Takes much force to knock out a proton
2
9/13/2012
Relativistic Neutrons
• Space travel
•
14N + n => 14C + p+
•
14C is radioactive and p+ is reactive Fast Neutrons
• Have a kinetic energy of 0.5 MeV up to 8‐10 MeV
• These are functionally high LET radiation
• Have minimal electron interaction because they have no charge
3
9/13/2012
Fast Neutrons
• Produced by nuclear fission at MURR (2 MeV)
• Can also be produced by fusion
• Can be slowed by moderation
Fast Neutrons
• Collide with nuclei releasing energy
• Most efficient with Hydrogen
• n + 11H => n + recoil proton
4
9/13/2012
Fast Neutrons
p+
n
Recoil Proton
p
n
Fast Neutrons
• Best stopped by water and paraffin
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
5
9/13/2012
Fast Neutrons
• Neutron/proton interaction results in a high ratio of energy transfer
• Average ΔE/E = ½ or 50% between n, p
• Larger nuclei result in a much smaller loss of E per interaction
Fast Neutrons
• Neutrons penetrate the body readily
• Body is mostly water
• High frequency of event with high LET
• Generate p+, also high LET
6
9/13/2012
Epi/Thermal (Slow) Neutrons
• Thermal/epithermal neutrons
• Much more likely to interact with nucleus
• Lower velocity = increased LET
Epi/Thermal (Slow) Neutrons
• Thermal neutrons have a larger neutron absorption cross‐section
• Cross‐section describes the likelihood of a neutron being captured in a nucleus
• Expressed in barns
7
9/13/2012
Epi/Thermal (Slow) Neutrons
Boron cross‐section
http://en.wikipedia.org/wiki/Neutron_absorption
Reactor Production of Isotopes
•
152Sm + n => 153Sm
•
130Te + n => 131I
Iodine must be eluted in alkaline solution
•
64Zn + n => 64Cu + p+
•
64Ni + p+
=>64Cu + n (cyclotron)
8
9/13/2012
Spallation Reaction
• Induced nuclear disintegration by the addition of a particle
• n enters nucleus, elevating energy state
• Nucleus disintegrates
https://honchemistry.wikispaces.com/Nuclear+reaction
Neutron Capture Therapy
• Gordon Locher, 1936 • Binary Treatment
– Selective accumulation of 10B
– Irradiation with nth
• The BNCT reaction – 10B(n,α)7Li
• High LET radiation
18
9
9/13/2012
Neutron Capture Therapy
• Important program Mizzou
• BNCT
Radiation and Living Tissue
• Henri Becquerel carrying an isotope vial noted that a red spot developed under his shirt pocket
• Clarence Dally was the first American radiation fatality in 1904
• Marie Curie died of leukemia
• Led to “skin erythema dose” experiments
10
9/13/2012
Radiation and Living Tissue
• Exposed skin through holes in a lead plate for differing times
• 400‐600 RAD results in skin color change
Short‐term (Early) Effects
•
•
•
•
•
•
•
•
Nausea
Fatigue
Skin erythema
Alopecia
Intestinal disorders
Fever
Blood disorders
Desquamation (loss of skin layers)
11
9/13/2012
Long‐term (Late) Effects
• Cancer
• Birth defects
• Cataract formation
• Unknown future generational risk
Measuring Radiation Exposure
• Roentgen (R, r) is measured in an air‐filled ionization chamber between charged plates
12
9/13/2012
Measuring Radiation Exposure
• Roentgen is a measure of radiation exposure
• Useful only for penetrating rays (x and γ)
• Does not equate directly with dose
Measuring Radiation Exposure
• 1 R of exposure is approximately equivalent to 1 RAD of dose
• Why is this approximate?
• Depends on LET, relative biological effectiveness (RBE) of radiation
13
9/13/2012
Measuring Absorbed Dose
• Unit: Rad (Radiation absorbed dose)
• Measured in tissue or water, not air
• Rads of radiation are iso‐ionizing (create the same number of ionization)
• Doesn’t matter if it is rays or particles
Relative Biological Equivalent
• RBE
• So… if it takes more of the comparator radiation, RBE is smaller and if takes less RBE is bigger
14
9/13/2012
Measuring Absorbed Dose
• Roentgen Equivalent Man (REM)
• REM = RBE X Dose (Rads)
• For 200 Rads of fast neutrons with an RBE of 10, REM = 2000
Measuring Absorbed Dose
• For electrons, RBE is 1
• x‐rays and γ‐rays produce high energy electrons
15
9/13/2012
Examples
• A radiation worker at the Chernobyl power plant receives a dose of 100 Rad of fast neutrons and 100 Rad of high‐energy gamma rays during the core breach. If the RBE of neutrons (quality factor) is 10, what is the total dose to this man in REM?
Examples
• 100 Rad of neutrons
• 100 Rad X 10 (QF) = 1000 REM
• 100 Rad of γ‐rays
• 100 Rad X 1 (QF) = 100 REM
• Total dose = 1100 REM
16
9/13/2012
Examples
• A radiation worker at a power plant receives an inadvertent exposure to 100 REM of radiation due to a safety failure on a containment vessel door. If 70 REM were from fast neutrons and 30 REM were from high‐energy gamma rays, how many Rads
were from each.
Examples
• 70 REM of neutrons
• 70 REM / 10 (QF) =7 Rad • 30 REM of γ‐rays
• 30 REM / 1 (QF) = 30 Rad
• Total dose = 37 Rad
17