Transcript P202 Lecture 2

Typical Decay scheme II
•Nuclei can decrease their
proton number by one in
three ways, positron
emission (the most common)
Electron capture (much
more rarely; see next slide),
or proton emission (very
rare).
•Decay rates expressed in
terms of Becquerrel (1/sec)
or Curies (37 GBq)
http://www.nucleide.org/DDEP_WG/Nuclides/Na-22_tables.pdf
Absorption length for gammas
(in lead and aluminum)
From E. Segre,
“Nuclei and Particles” 2nd
ed. (1977)
Examples
1. The LENS neutron source at IU creates roughly 4e4 neutrons per
pC of proton charge (incident at 13MeV) through the reaction:
9Be(p,n)9B
in a 1mm thick target. Estimate the cross section for this reaction
(Note: Fro Be r=1.85 g/cm3 and W=9.01g/mole).
2. A foil of natural In that is 1.0 mm thick is placed in a thermal neutron
beam (v=2200 m/s) of flux 107n/cm2.s. In has a molecular weight of
114.8 g/mole, and a density of 7.31 g/cm3. 116In is a beta emitter and we
will assume that it is only produced in a state that decays with a 54 min
half life.
a). What is the flux of neutrons on the back side of the foil?
b). If the foil is in the beam for 1.0 min, what is the activity due to 116In?
c). What is the 116In activity if the foil is in the beam for 10 hours?
The information at the website on the following slide may be useful.
Cross Sections
http://www.ncnr.nist.gov/resources/n-lengths/
Alpha Decay
T&R Fig. 12.11
http://en.wikipedia.org/wiki/File:Alpha1spec.png
Lecture 23
Potential Barrier: Alpha decay
The deeper the “bound” state is below the top of the barrier, the lower will be
the kinetic energy of the alpha particle once it gets out, and the slower will be
the rate of tunneling (and hence the longer the half-life). Figures from Rohlf
“Modern Physics from a to Zo”.
Radiation Shielding
•Different types of radiation penetrate through matter with
different ranges. Alpha particles are very easily stopped
(doubly charged and relatively slow), beta particles are
relatively easy to stop, gamma rays need very heavy
shielding, and neutrons are the hardest to shield against.
•https://reich-chemistry.wikispaces.com/b.sulser+and+k.nagle+powerpoint+presentation
neutron
Radiation Dose
•Dose:
•1 Gray = 1 J/kg of whatever radiation (energy deposited
per unit mass), supplants the RAD (Radiation absorbed
dose)
•Roentgen (older unit, radiation needed to produce a
certain charge per unit mass (1 esu /cm3 of dry air). This
corresponds to 0.258 mC/kg.
•Equivalent Dose
•1 Sievert: absorbed dose multiplied by factors to account
for relative biological effectiveness for particular radiation
type (b, g, n etc.; energy etc. ) and body part involved.
NOTE units are the same as the Gray, but the meaning is
quite different.
•REM (Roentgen Equivalent Man): 1 REM = 10 mSv
Radiation Dose
NOTES:
•Prior to 1990 the
weighting factor was
referred to as the “Quality
Factor and you will still
see this term used.
•There is some
controversy over the
appropriateness of the
weighting factors
(especially for alphas)
http://www.ccohs.ca/oshanswers/phys_agents/ionizing.html
Effects of Radiation
•“LD50/60” Dose that would result in death for 50% of the
population so exposed within 60 days (Lethal Dose to 50% of
the population)
•LD50/60 limit for gamma radiation is roughly 450 RAD (or
4.5Gray) for whole-body exposure
•Threshold lethal Dose (2 Gy, whole body exposure)
•Beyond these acute dose issues, future development of
cancer is also a concern
•Doses at ~10 cSv appear to produce no increased risk of
cancer
•Occupational limits for radiation workers are set at levels
below this to be conservative (typically 50 mSv/yr for
radiation workers).
•Typical background exposure in the US is of the order of
3.5 mSv/yr (as high as 8 mSv/yr in Colorado mountains).
Sources of Background
Radiation
•Cosmic Rays
•Naturally occurring radioactive nuclei
•40K this is the most abundant radio-nuclide in your body
•14C (e.g. about 50 times/sec one C atom in the DNA of
one of your cells is converted to N by beta decay).
•222R, a decay product from 238U, and a common concern
in buildings
•Medical tests
•Man-made nuclides (fallout, waste, release etc.).
Sources of Background
Radiation
http://web.princeton.edu/sites/ehs/osradtraining/backgroundradiation/background.htm
Effects of Radiation
•Recall: 1 rem is roughly 10mGy for gammas
•Typical background radiation is 350 mrem/yr, airline travel
gives roughly 0.4-1 mrem/hr (4-10 mSv/hr)
http://www.physics.isu.edu/radinf/risk.htm
See also:
http://trshare.triumf.ca/~safety/EHS/rpt/rpt_4/node20.html
Radiation Dose
•Different types of radiation at a given energy have different
“Relative Biological Effectiveness”, and different parts of the
body have different susceptibilities to radiation, so you have to
be a bit careful about how you quote numbers.
•Today medical physicists discuss dose in Gray to specific
organs, rather than Sieverts etc..
Relative Biological Effectiveness
NOTES:
•Different parts of the body have
different susceptibilities to
radiation (in terms of their
likelihood of developing cancer
after a given exposure)
•This is taken into account in
planning radiation treatments for
cancer.
http://www.ccohs.ca/oshanswers/phys_agents/ionizing.html
Relative Biological Effectiveness
Note that a 1.5 Gy dose
of Carbon ions has the
same biological effect
as a 4.5Gy dose of
photons (for this
particular cell type).
http://en.wikipedia.org/wiki/Relative_biological_effectiveness
Applications of Radiation
•Radiation and radioactive materials are used in numerous
applications today, we’ll touch on only a few of these:
•Nuclear Medicine
•Diagnostics (x-rays, CAT scans, PET scans, etc.)
•Cancer treatment (x-rays/gamma-rays, radio-nuclides,
protons, heavy ions)
•The key here is that rapidly reproducing cells (such
as CANCER, in children/fetuses) are more
susceptible to radiation damage AND CANCER
cells are less able to repair the damage radiation
causes (at least that is the dogma, there is some
conflicting information on this).
•Dating of artifacts (archeologic, organic, geologic etc.)
•trace element analysis
•Nuclear power
Proton Radiotherapy
http://en.wikipedia.org/wiki/Proton_therapy
http://mpri.org/science/vstreatments.php
Nuclear Fission
KEY element (from CALM):
•Large, unstable, nuclei (2)
•Neutron activated dissociation (3)
•These are both necessary but not
sufficient:
•“Chain reaction (8)” but what does that
mean?
•One neutron induces a reaction that
produces MULTIPLE neutrons out (to
sustain the reaction) (6) THIS IS THE
KEY ingredient.
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html
Fission products
http://en.wikipedia.org/wiki/Fission_products
http://www.euronuclear.org/info/encyclopedia/f/fissionproducts.htm
Nuclear Reactors (LWR’s)
Boiling water reactor (BWR)
Pressurized water reactor (PWR)
66% of US reactors are this type
http://reactor.engr.wisc.edu/power.html
Yucca Mountain
http://www.ocrwm.doe.gov/ymp/about/why.shtml
American Nuclear Plants
http://www.nrc.gov/info-finder/reactor/
Nuclear Waste depots
http://en.wikipedia.org/wiki/Radioactive_waste
Nuclear Decay Chains
4n chain: Thorium
series
4n+1 chain
Neptunium series
4n+2 chain
Radium series
Does not include the 4n+3 chain or Actinium series which terminates in 207Pb
(Wikipedia does not have so nice a graphic for that chain; from:
http://en.wikipedia.org/wiki/Decay_chain
About 1 part per trillion of atmospheric
carbon is 14C, thanks to this mechanism.
•Dates have to be calibrated to account for historic variations
in the production and distribution of 14C in the atmosphere
(thank goodness for the Bristlecone pine tree). Figs from:
•http://www.ndted.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating
.htm
and http://en.wikipedia.org/wiki/Radiocarbon_dating
Various Dating schemes
See article at: http://physics.info/half-life/
Radiocarbon Dating of a Hypothetical Organic Sample
age (half-lives)
age (years)
14C
0
0
128
128 × 1012
1
1
5,730
64
128 × 1012
0.5
2
11,460
32
128 × 1012
0.25
3
17,190
16
128 × 1012
0.125
4
22,920
8
128 × 1012
0.0625
5
28,650
4
128 × 1012
0.03125
6
34,380
2
128 × 1012
0.015625
7
40,110
1
128 × 1012
0.0078125
(atoms)
12C
(atoms)
14C
: 12C (ppt)*
Note; this is really a toy example (1014 carbon atoms is only 2 ng of sample,
typically you would need much more; estimate how much more)
Various Dating schemes
See article at: http://physics.info/half-life/
Potassium-Argon Dating of a Hypothetical Mineral Sample
age (half-lives) age (109 years)
40K
(atoms)
40Ar
(atoms)
40K
: 40Ar
0
0
64
0
–
1
1.26
32
32
1:1
2
2.52
16
48
1:3
3
3.78
8
56
1:7
4
5.04
4
60
1 : 15
5
6.30
2
62
1 : 31
6
7.56
1
63
1 : 63
Potassium-Argon Dating of a Hypothetical Mineral Sample
Various Dating schemes
See article at: http://physics.info/half-life/
Radioisotopic Dating Techniques
range (years past)
technique
dateable items
lead 210
1
–
150
lake and ocean sediments, glacial ice
carbon 14
1
–
40,000
previously living things
uranium series
1
–
400,000
bone, teeth, coral, shells, eggs
potassium-argon
10,000
–
3 billion
minerals, igneous rocks
uranium-lead
1 million
–
4.5 billion
minerals, igneous rocks
rubidium-strontium
60 million
–
4.5 billion
minerals, igneous rocks
Early Particle discovery: W-
From E. Segre “Nuclei and Particles”, 2nd edition
Particles appear as tracks (in bubble chambers in the early days, in electronic trackers of
various sorts today) that are bent by a magnetic field. By measuring curvature, track length etc.
things like half-life, momentum, charge etc. can be determined.
Early categorization of Particles
Early collider experiments started to reveal more and more particles, and
people started to question whether they were truly “Fundamental”, but did
allow for the prediction of “missing particles that were later found.
Attempts to rationalize this “zoo” of particles led Gell-Mann (and
independently Zweig) to suggest more fundamental building blocks (based
largely on the observation of patterns [symmetries] in the properties of the
particles; they appeared in families of 1, 8, 10, 27 etc. members]:
The Standard Model
http://newsimg.bbc.co.uk/media/images/41136000/gif/_41136526_standard_model2_416.gif