Post on 16-May-2015
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Smart Women in Physics Together:
Undergraduate MentoringAsk female graduate students all of your questions about
majoring in physicsapplying for graduate schoolbeing a woman in physics!
Friday, April 10th at 2:30pm
Winnings CoffeeWest side of Harvard, South of CentralFunded by the GPSA Project Committee
Today: Radiation Dose Definitions, Medical Applications,
PET scan 3-D renderingSource: wikipedia
Small group exercise!
I place on the table a small piece of material and say, “this is radioactive.”
How many different properties of the material can you think of that you would want to characterize, in order to know everything about its radioactivity?
Form small groups, and brainstorm. Have a leader / spokesperson write down results.
?
What kind of radiation is emitted? Alpha, beta, gamma.
Mass/weight“half-life” (time for ½ to decay)Color…?Temperature…What is the element?
Measuring the amount of radioactivity or dose
We can think about amount of radiation in two ways.
Describe the source of radiationPhysics-based, rigorous
Describe the absorbed radiation doseComplicated biology and other factors
There are rigorous ways to characterize the source of radiation
What type of radiation does the material emit?X-rays, gamma rays, beta radiation, alpha particles
How much radiation per second?curies, becquerels, counts per second
What is the half-life of the material?Minutes, days, years, …
What is the average energy per particle or ray emitted?
There are rigorous ways to characterize the source of radiation
What type of radiation does the material emit?X-rays, gamma rays, beta radiation, alpha particles
How much radiation per second?curies, becquerels, counts per second
What is the half-life of the material?Minutes, days, years, …
What is the average energy per particle or ray emitted?
There are two main units of radioactivity
The becquerel (Bq) is defined as one radioactive decay event per second
1 Bq = 1 reaction / second
The curie (Ci) is based on the radioactivity of about 1 gram of radium-226
1 Ci = 37 billion Bq = 37 billion reactions / second
Marie CurieNobel prizes in
Physics and Chemistry
Henri BecquerelNobel Prize in Physics
Example: 1 kilogram of “typical” soil has400 Bq of Potassium-40 -- or 11 nanocuries
There are rigorous ways to characterize the source of radiation
What type of radiation does the material emit?X-rays, gamma rays, beta radiation, alpha particles
How much radiation per second?curies, becquerels, counts per second
What is the half-life of the material?Minutes, days, years, …
What is the average energy per particle or ray emitted?
Radioactive decay is a random process
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Example: Phosphorus-32 decay into Sulfur-32
Half-life is the average amount of time for ½ of the atoms to decay
P-32 half life ~14 days
Decay rate is proportional to the number of radioactive atoms remaining
So, radioactivity of a sample decreases with the same half-lifeApplets: http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html
http://lectureonline.cl.msu.edu/~mmp/applist/decay/decay.htm
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Radioactivity (becquerels) and Half-life are inversely related
Longer half-life = lower radioactivityStable isotope has an infinite half-life
Shorter half-life = higher radioactivityHighly radioactive isotopes will disappear
quickly
Clicker Question—Half life / radioactivity
Let’s say you have two samples, each with the same number of atoms in them.
Sample “A” has half-life of two weeks.Sample “B” has half-life of 4 billion years.
Which will have a higher radioactivity?
A or B
Clicker Question—Half life / radioactivity
Let’s say you have two samples, each with the same number of atoms in them.
Sample “A” has half-life of two weeks.Sample “B” has half-life of 4 billion years.
Which will have a higher radioactivity?
A or B
Shorter half-life = higher radioactivityIf the half-life is infinite, it is not radioactive
There are rigorous ways to characterize the source of radiation
What type of radiation does the material emit?X-rays, gamma rays, beta radiation, alpha particles
How much radiation per second?curies, becquerels, counts per second
What is the half-life of the material?Minutes, days, years, …
What is the average energy per particle or ray emitted?
The energy per emitted particle indicates how deeply it can penetrate materials and how much damage it can do
Examples:P-32 beta 1.7 megaelectronvolts (MeV)Ra-226 alpha 4.9 MeVRn-222 alpha 5.6 MeV K-40 beta 1.3 MeV
However, the type of radiation (alpha, beta, gamma, neutron) tends to be more important
Mega = 10^6 or 1 million
Clicker question—classifying radiation
If held 1 inch away from your body, which radiation source is more likely to cause cancer?
A) Source with radioactivity of 1 million becquerels (Bq)
B) Source with radioactivity of 1 billion becquerels (Bq)
C) Impossible to determine
Clicker question—classifying radiation
If held 1 inch away from your body, which radiation source is more likely to cause cancer?
A) Source with radioactivity of 1 million becquerels (Bq)
B) Source with radioactivity of 1 billion becquerels (Bq)
C) Impossible to determineThere are many other things you would need to know:What type of radiation do each emit? Is there any shielding in between?
Results of homework assignment
A few homework responses
Adrian: If I became a luddite and lived in a hut in the woods at sea level I might save myself 80 mrem or so.
Laura: Only 2.010 mrem or 0.6 % of my annual radiation exposure can be attributed to man made sources
David: Unavoidable background radiation is from food and water, there is absolutely nothing I can do to change that.
Measuring the amount of radioactivity or dose
We can think about amount of radiation in two ways.
Describe the source of radiationPhysics-based, rigorous
Describe the absorbed radiation doseComplicated biology and other factors
Classifying the radiation dose is very complicated
Biological effects depend on acute versus chronic dose
Interaction of radiation with cells has a number of possible outcomes…depends on numerous factors
Characteristics of the radiation
Characteristics of the biological system (e.g. general level of health)
Nevertheless, attempts are made to define dose
Radiation doses
In terms of energy, absorbed dose is measured in grays (Gy)1 Gy = 1 joule per kilogram of material
Biological dose is measured in sieverts (Sv) or rem1 Sv = effect of 1 joule per kilogram1 Sv = 100 rem
1 sievert = Q * gray “Q” is some fudge factor
For photons (x-ray, gamma) Q = 1Beta rays, Q = 1Low energy neutrons, Q = 5; medium energy Q = 20; high energy
Q=5Alpha, Q = 20
Some relevant radiation dose values
Acute radiation doseAbout 3 Sv (whole body) will cause radiation poisoning and
50% chance of death.
Medical imaging procedures typically in the milli-sievert range
Chronic radiation doseNatural background about 2.4 milli-Sv per year
What dose will cause cancer? Difficult to determine and also random.
Medical applications of radiation
Destruction:Radiation Therapy
Imaging:X-rays
Imaging:Positron emission tomography
Imaging:Gastrointestinal imaging
Positron emission tomography (PET scanning)
Radiation Therapy
Extremely high but localized dose, meant to destroy tumor cells.20 or more localized grays
Also a “whole body” dose for killing immune system prior to bone-marrow transplant.~ 10 grays (3 usually lethal)
Some slides about cancer radiation therapy
http://youtube.com/watch?v=ZtENJI1K1hA&feature=related
Clicker Question—Acute Radiation Dose
I said earlier that a dose of about 3 sieverts (Sv) will kill someone about 50% of the time. Let’s say someone is exposed to 100 sieverts of radiation. Can you say almost certainly they will die?
A) YES!B) No!
Clicker Question—Acute Radiation Dose
I said earlier that a dose of about 3 sieverts (Sv) will kill someone about 50% of the time. Let’s say someone is exposed to 100 sieverts of radiation. Can you say almost certainly they will die?
A) YES!B) No!The information is not specific enough.
If it happens over the whole body in a short time, then probably YES…
However, if it happens over 100 years, then probably not.
Also if localized, also will not die
There is no easy way to put a concrete number on radiation dose
Ionizing radiation almost NEVER makes something turn radioactive!
In order for an object to become radioactive, the nuclei in the atoms must change…”nuclear reactions” required.
Typically:
Neutrons are the most effective at causing nuclear transmutation…
Radiactive decay very rarely releases neutrons
The person does NOT becomeradioactive … his nuclei are unchanged
Nuclear reactionReleases
ionizing radiationCauses
Chemical reactions
Clicker question--transmutation
Food irradiation or “cold pasteurization” often uses gamma rays to kill any microorganisms in the food. There are no neutrons involved.
Does the food then become radioactive (only mildly, though)?
A) YesB) No
Clicker question--transmutation
Food irradiation or “cold pasteurization” often uses gamma rays to kill any microorganisms in the food. There are no neutrons involved.
Does the food then become radioactive (only mildly, though)?
A) YesB) No
There are safety concerns, but NOT because of radioactive food! We can understand the physics
The risks need to be balanced against the benefits…In US:325,000 hospitalized per year from food poisoning!5,000 deaths / year!
Protons and neutrons are made of quarks
Quark model of a protonwww2.cnrs.fr/presse/communique/709.htm
“Up” quarks charge = + 2/3 each
“Down” quark-1/3 charge
Arpad Horvath
Quark model of neutron
Quarks and Leptons are the constituent parts of matter.
Murray Gell-MannNobel Prize 1969
There are six quarks
Up Charmed Top
Down Strange Bottom
Charge = +2/3
Charge = -1/3
Similarly, there are six leptons
Increasing mass
Electron Muon Tau
Electron neutrino
Muon neutrino
Tauneutrino
Antimatter consists of the 6 anti-particle quarks and
6 anti-particle leptons
Under normal conditions, quarks do not exist in free space.
Fermilab particle accelerator
Billion-dollar physics experiments were required to detect quarks for fleeting instants in time
Beta-decay can transform a neutron into a proton
Quarks
(Electron)
It is a decay of a “down quark” into an “up quark”
Clicker question--Quarks
An up quark has an electric charge of +2/3.A down quark has an electric charge of -1/3.Which of the following combinations would a proton
consist of?
A) Up, up, upB) Down, down, downC) Up, down, downD) Up, up, downE) Up, up, down, down
Clicker question--Quarks
An up quark has an electric charge of +2/3.A down quark has an electric charge of -1/3.Which of the following combinations would a proton
consist of?
A) Up, up, upB) Down, down, downC) Up, down, downD) Up, up, downE) Up, up, down, down
+2/3 +2/3
-1/3Take home message about quarks:Nucleons are composed of three quarks.Particle Physicists can predict a lot about matter from the quark / lepton models,Including some radioactive decay properties