Radiation Interactions

Document Sample
Radiation Interactions Powered By Docstoc
					Chapter 8 - Interaction of Radiation with Matter
 the chemical effects of radiation depends on the
      composition of matter and the amount of energy
      deposited by the radiation

 it is practical to divide high energy radiation into
   (1) charged particles (e-, e+, , etc.) and
   (2) uncharged particles (n)
   (3) electromagnetic radiation ().

 neutrons and -radiation produce recoil atomic ions,
     products of nuclear reactions and electrons as
     charged secondary ionizing particles.

 the terms direct and indirect ionizing radiation are often
      used for (1) and (2+3) respectively.
                     Ionizing Radiation
                                                       ed
                                   e-
                                               scatter oton
                                                       ph
                                          par ticle or
particle or photon
                                                              e-
                                                         electron

                                                                    ion-pair

         atom or molecule
                                                           +
                                               positive ion or molecule




                                                              n
                                                           tio
                                                         za
                                                      ni
                                                    io
                                                 ry

                                                           ys
                                               na

                                                         ra
                                           co


                                                      -
                                          se


                        primary ionization track
CLOUD CHAMBER ALPHA RADIATION

              Alpha decay (two energies)

              212Bi   — 208Tl (6.09 MeV)
              212Po   — 208Pb (8.78 MeV)

              Note particle reaction

              14N   + 4He  1H + 17O
BUBBLE CHAMBER (CERN)




                        6.04
Excitation
Ionization
            Rutherford Scattering




Coulomb effect
                  Bremsstrahlung

             Bremsstrahlung
“Braking radiation”




    electron in
        e-


                                   X-rays
  The Photoelectric Effect

Photoelectric Effect
                          photoelectron
                                  e-



   in



                       Note that the -ray
                       is totally absorbed
                       in the photoelectric
                       effect
         Compton Scattering

Compton Scattering
                          Compton electron
                                   e-



   in




                        sc
                         at
                           te
                             re
                               d
       Pair Production
Pair Production
                                  electron
                                     e-



 in
                                                  0.51 MeV   
                            +



                            annihilation     e-




                 0.51 MeV    
              Neutron Capture

                  Capture

particle in




                                excited nucleus
                   Particle Reaction

               Nuclear Transformation             particle out



high energy
 particle in




                                       excited nucleus
                      Energy Transfer

Edep = Ein + Q - Eout

   Ein is the energy of the radiation entering the volume
   Eout is the energy of the radiation leaving the volume
   Q is the sum of all Q-values for nuclear transformation that have
      occurred in the volume

 for a beam of charged particles, Ein = Ekin

 for -rays, Ein = E
 if no nuclear transformations occur, Q = 0
 for neutrons which are captured and for radionuclides
       which decay in the absorber, Q > 0; in the latter
       case, Ein = 0.
           Linear Energy Transfer (LET)
stopping power, S, is defined as
                           S = -(dE/dx)
which is the rate of energy loss per unit length of the
   matter (The mass stopping power is S/, where  is the
   density.)
S is also referred to as the linear energy transfer (LET) of
   the substance for the given radiation.
Bethe’s equation for LET of charged particles

         dE       4z 2e 4     2m e v 2                  2
LET                     NZ ln         ln 1      
                                                      2

         dx coll m e v
                         2
                                     I                      
                            dE      Mz 2
                  LET         
                            dx coll   E
                      Energy Loss
 specific energy loss of a particle in matter is called the
   stopping power
                         ˆ  dE loss
                         S
                                       dx J / m
 the stopping power of a material is determined by its
   atomic composition and is almost independent of the
   chemical binding of the atoms
 stopping power is a function of the particle velocity and
   changes as the particle is slowed down.
“specific ionization'' is the number of ion pairs produced
   per unit path length
                         J
                              dN j
                                     dx bion  pairs / mg
J depends on the particle and its energy
                                 Energy Loss

 the relationship between S and J is                S  wJ  J / m 
                                                     ˆ
 the mass stopping power, S/, is commonly expressed
   in units of MeV/g•cm2
 another important concept is the linear energy transfer
   (LET) of charged particles. It is defined as the energy
   absorbed in matter per unit path length traveled by a
   charged particle
                                                              dE abs
                    high energy electrons, -rays   LET 
   increasing LET




                    -particles, soft X-rays
                                                                       dx
                    protons
                    deuterons                    S  LET  E x ( brems)
                                                  ˆ
                    -particles
                    heavy ions (C,N,O)        dE loss      dE abs
                    fission fragments                                Ex
                                                      dx          dx
            LET Values for Radiation
Radiation   Energy    Range in air    Range in   LET in water
             MeV     (15 oC, 1 atm)    water      keV/micron
                          cm            mm
Electron      1          405            4.1         0.24
             10         4200            52          0.19
Proton        1          2.3          0.023          43
             10          115           1.2           8.3
Deuteron     10           68           0.72          14
Alpha        10         10.5           0.11          92
           1.25                                    0.25
  Uncharged Radiation - Gamma and X-Rays

 when neutrons or photons having the incident particle
   energy Ein are absorbed, a certain fraction of energy Etr is
   transferred into kinetic energy of charged particles when
   traversing the distance dx.
 the energy transfer coefficient is defined as
                              dE tr
                      tr            dx  m 1 
                                E in
 If we neglect the bremsstrahlung associated with the
   absorption of the secondary charged particles formed in the
   initial absorption processes, Etr is the energy absorbed
   (Eabs)                  dE    abs
                     a               dx  m 1 
                                E in
                                 Radiation Tracks
distance between ion pairs
1000 nm -rays
 500 nm       fast e-                                        bremmstrahlung

   1 nm       , slow e-                                                 -
                    branching track
                       < 5000 eV         -
                                     -   +       +
                                                     -                             -track in mica
                                                             +
                                             +                                      spur < 100 eV
                                         -                    track > 5000 eV
                                                                                                  +
                                                                                                        ionizing
          column ionization                          +
                                                             -                        -
         along primary track                                                                           projectile
~ 2 nm
                -
                                                             -                            -
                                                         +
                    +            +           +                   +
                                     +                               -   ~ 35 eV              +

                         -   -    +
                            +        -                                                                "hot atom"
          spur from secondary of ~10 - 100 eV
         ~1 - 3 ion pairs and 3 - 10 excited atoms
                  per 500 nm track length
                      Ion-Pair Formation
          absorber           w (eV) j (eV) w-j (eV)
          He(g)                 43        24.5       18.5
          H2(g)                 36        15.6       20.4
          O2(g)                31.5       12.5        19
          air                   34         15         19
          H2O (l or g)          38         13         25
w = energy for formation of ion pair        j = ionization potential
                      w - j = excitation energy
 since atomic excitation energies are < 5 eV, several excited
    atoms are formed for each ion pair formed
 it is more difficult to obtain reliable values for liquids and
    solids. They differ widely; w ranging from 1300 eV per ion
    pair in hexane to about 5 eV per ion pair in inorganic solids.
                    Radiation Dose

exposure is the energy flux of unperturbed photon
   radiation hitting matter

1 Roentgen (R)  exposure in air that produces 2.5810-4
   C/kg (1.611015 ion pairs) or 8.810-3 J/kg of absorbed
   energy assuming 34 eV per ion pair

absorbed dose per unit mass, D = dEabs/dm

SI dose unit — 1 Gray (Gy)  1 J/kg ( 100 rad )
                 rad = radiation absorbed dose

dose rate is absorbed dose per unit time (SI), Gy/s
                 Positron Annihilation

Positron Annihilation
                               0.51 MeV   
           +



           annihilation   e-




0.51 MeV    
Gamma Dose vs. Energy
Specific Ionization
Particle Ranges in Aluminum




                               Mz 2
                         LET 
                                E
                              0
                               1
                       R         dE
                             dE 
                         E0
                             
                             dx 
 Radiation Attenuation (Absorption) Measurements
               lead collimators


      0                                
                    dx




source
                 absorber                   detector


                      r

R  k det      k det   det A det
                                        A = N
                      sampl nA n = emissions/decay
   abs0     0 
                      4 r 2
Calibrated Absorber Sets
                                                 Absorption Curves

                                    Absorption curve for 32P -radiation
log counting rate (cpm)




                                                average range is most useful
                                           extrapolated range due to “straggling”


                                                                        units of absorber
                                                                      thickness in mg/cm2
                              background (cpm)



                  0.0
                          0                      C3        C4
                                            average     extraploted           x
                                        Absorber thickness
                                                      mg/cm2
                                    Radiation Relative Ranges
                                     heavy ions ( — p — d — Z+)
                          1.0
 I    
   or
I0 0
 Relative trransmission




                                                                   neutron —  — X-ray
                          0.5



                                               
                                                 -                 monoenergetic
                                                                     electrons




                          0.0                                                            x
                                0      C1 C2                           C3 C4
                                       avg max                         avg max
                                           Absorber thickness
     Alpha (Charged Particle) Absorption

 -particles emitted naturally with energies from
  4 to 9 MeV
 -particles are much heavier than electrons and
  are not deflected — tend to travel in straight
  lines
 average energy of secondary electrons is ~100
  eV
 -particle shows a maximum number of ion-
  pairs formed near the end of their range (Bragg
  peak) — after Bragg peak, charged particle
  gains electrons to become a neutral atom
       Bragg Peaks




Bragg peak



             particle direction
                      Proton Radiation Therapy




Figure 1: Presents the path of a single proton    Figure 2: A comparison of the amount of radiation
as it enters the body and deposits the vast       delivered with conventional (high energy X-rays)
majority of its energy at a single point. This    radiation therapy versus proton therapy.
phenomenon is referred to as the Bragg peak.      Conventional therapy is distinguished by a
We are able to manipulate the depth at which      relatively high entrance dose and an exit dose. By
this takes place by controlling the speed of      contrast, proton therapy has a much lower
the proton. In addition, we are able to "stack"   entrance dose and no exit dose. The goal in
protons in a row to cover an area in which a      radiation therapy is to minimize damage to healthy
tumor is growing thereby concentrating the        tissue by minimizing the tissue exposed in the
radiation directly at the tumor site with         entrance and exit doses. Proton therapy causes
minimal damage to surrounding healthy tissue      much less damage to healthy tissue surrounding
(See Figure 2).                                   the tumor.
                Alpha Particle Range

for -particles, the range in air ( 4 - 9 MeV )
             R  E3/2 (MeV)  0.40 E3/2 (mg/cm2)
in other absorbers
                   R  0.173E3/2A1/3 (mg/cm2)
the value of R is expressed as mg/cm2 which is a function
    of thickness times density.

What is the range of a 5 MeV -particle in Al ( = 2.7 g/cm3)?

 R = 0.173E3/2A1/3 = 0.173  5.03/2  271/3 = 5.80 mg/cm2

       R = 5.80 mg/cm2  2700 mg/cm3 = 0.0021 cm
             Beta-minus Absorption

 -particles quite different from heavier charged
  particles (p, , etc.)

 -particles are highly scattered in each collision

 ~95% of -particles are stopped in the first half
  of the total range

 half-thickness value is amount of absorber
  required to stop 50% of the -particles
         Beta-minus Absorption

Mechanisms of  energy loss
   Ionization
   Excitation
   Bremsstrahlung
   Positron annihilation (for +)
   Cerenkov radiation
                   Bremsstrahlung

     Bremsstrahlung
                             dE dx  brems
                                                
                                                  E elec (MeV)Z
                              dE dx    coll
                                                         800

electron in
    e-


                           X-rays

acceleration of electron
  by charged nucleus        Al      1% brems 99% ioniz
                            Pb      10% brems 90% ioniz
       Bremsstrahlung

                 147Pm   -spectrum



100
                   Cerenkov Radiation
      REFRACTIVE INDEX                   Cerenkov
            nr = 1.0     nr > 1.0        emission
                                               c vac
                                            c
        fast  -                               nr
                                                   c
                                           sin  
                                                   v

                    cone of emission     Cerenkov
                                         emission
nr(H2O) = 1.33 nr(Plexiglas)=1.5
 with energy > 0.6 MeV travel faster than light in water
energy loss is <0.1% of other mechanisms
               Cerenkov Radiation

                         University of Virginia




University of Illinois
    Beta Absorption Curves

 FG IJ
 g Al  0.543E ( MeV)  0160
R
  H K
  cm 2         max        .
    Beta Backscattering
Backscattering
                  Gamma Ray Absorption

unlike charged particles, gamma-rays are stopped in just
    one or a few interactions

for thin absorbers:

                               0e   m x
                        A N A  e ZN A
                  m         
                         M         M
m = linear attenuation coefficient    = total attenuation coefficient
 = density                           M = average atomic weight
Z = average atomic number             NA = number of atoms/volume
a, e = probability of interactions with electrons
        Gamma Ray Absorption

                                  x
                         0e
                                     x
        x          abs ( x )  e
                    abs  scatt
              
    




               - x
              e
               Gamma Ray Shielding

since gamma-ray attenuation follows an exponential
   decay, it is not possible to calculate a thickness that
   “stops” all of the gammas

half-thickness value (reduces gamma intensity by one
   half):
                            ln 2
                     x 12 
                             
tenth-thickness value (reduces gamma intensity by an
   order of magnitude)

                              ln 10
                    x 110   
                                
Gamma Ray Shielding


                  chart shows
                  thickness of
                  absorber
                  required to
                  reduce
                  gamma ray
                  intensity
                  by factors
                  of 10
                 Photoelectric Effect
 Photoelectric Effect
                  photoelectron



                                  E e  E   E be ( K ,L ,M ...)
                         e-



     in




                        photoelectron emission not possible
                          if E < Ebe(K,L,M…)
photo decreases with increasing E
photo decreases from K > L > M > N …
edge effects (Kedge, Ledge, etc.) similar to X-rays are seen
   with heavy elements
X-ray fluorescence and Auger emission accompanies the
   photoelectric effect
                                       Compton Effect
 Compton Scattering
                   Compton electron
                                                  h  h'  K. E.electron
                                  e-



     in

                 sc
                                         '  
                                                   h
                                                  m0 c
                                                      b          g       b
                                                       1  cos    C 1  cos    g
                  at
                   te
                       re
                          d


                                               Cb electron g  2.426  1012 m

 Compton scattering increases with absorber Z and decreases with
   increasing E
 Compton scattering occurs with most weakly bound electrons so
   Ebe is negligible
 extensive secondary ionization is caused by the scattered electrons
 X-ray fluorescence and Auger emission accompanies Compton
    scattering
 since scattered electron has a range of energies, Compton gammas
    have a broad energy spectrum
                                              Pair Production
  Pair Production
                              electron
                                 e-
                                                              E   102 MeV  E e   E  
                                                                     .
    in
                                              0.51 MeV   
                        +



                        annihilation     e-




             0.51 MeV    




 E must be greater than 1.02 MeV for pair
   production to occur
 pair production increases with increasing E
 + annihilation produces two 0.51 MeV gamma-
   rays emitted at 180°
       Gamma Ray Absorption Processes
partial gamma absorption processes:

               = coh + phot + Comp + pair

   coh = coherent scattering (Bragg or Rayleigh scattering)

   phot = photoelectric effect

   Comp = Compton scattering

   pair = pair production

scattering:   s  coh and Comp

absorption: a  phot , pair , and Comp
          Gamma Ray Absorption Data




http://physics.nist.gov/PhysRefData/Xcom/html/xcom1.html
Gamma Ray Absorption Domains
              Neutron Absorption

 collimated beams of neutrons are attenuated by
   a thin absorber in much the same manner as
   gamma rays
 for thick absorbers, neutrons are generally
    reduced in energy until they approach
    thermal energies and are captured
 capture process probabilities are exceedingly
    complex and vary greatly with neutron energy
 shielding is generally accomplished by using
   high proton densities (H2O, paraffin) to
   induce scattering
                Radiation Shielding

for an ideal point source, with no backscattering

n = number of photons/decay           nA  x
A = source activity                       e
                                     4 r 2


                                     x



                                                    detector



                                      r
                   Dose “Build-up”

                   x
     B 0e
 must take into account
   multiple Compton and
   Rayleigh scattering plus
   gammas generated by
   pair production
 build-up factor, B, may
   exceed a factor of 10
 B cannot be calculated
   but is estimated from
   empirical data
Relative Radiation Ranges
        Radiation Interactions with Matter
If we leave out direct interaction of very high energy
   radiation with atomic nuclei, the rest of radiation
   chemistry involves interactions with orbital electrons
   and these consist mainly in excitation and ionization
   as the primary act. The final products are the result
   of secondary reactions due to these excited and
   ionized species.
Excitation             + H2O ~~ H2O*
                      e- + H2O ~~ H2O* + e-

Ionization             + H2O ~~ H2O+ + e-
                      e- + H2O ~~ H2O+ + 2 e-

Radical formation      + H2O ~~ H• + •OH + e-
                Radiation Chemical Yield

G-values
old units
G(x) number of molecules of x transformed per 100 eV
      absorbed energy, G < 10

SI units
G(x) units are mole/J

           1 mole/J = 9.649106 molecules per 100 eV
                Effect of Radiation on Metals

 builds up a static charge
 displaces atoms from lattice positions




                                                   decrease
               electrical resistance   density
    increase




               volume                  ductility
               hardness
               tensile strength


 become brittle due to trapped gases at crystal
  boundaries (H2, He)
 thermal annealing and radiation annealing may causes
  lattice defects to migrate
 Effect of Radiation on Inorganic Crystals
can cause electron trapping or create electron “hole”,
particularly at impurity sites
local excess (or deficiency) of electrons may alter
 electronic absorption bands to produce color centers
 (used to color gems)
   LiCl   white  yellow
   LiF    white  black
   KCl    white  blue
heating release electrons and removes color centers,
usually accompanied by the emission of light.
This gives rise to thermoluminescent dosimetry: used
to measure radiation dose and for archaeological
dating of pottery
           Thermoluminescent Dosimeters (TLD)
TLD Dosimeters are one of the most advanced application of Thermoluminescent Detectors. The
reason for this is that, according to the new 10 CFR Part 20 Guidelines of the US NRC, modern TLD
Dosimeters measure the Skin Dose, Eye Dose and Deep Dose specified in this regulation, in addition
to the dose due to other nuclear particles. Thus, modern TLD Dosimeters consist of at least four
separate TL detectors, which are shielded by materials of various compositions and thicknesses, to
allow for discrimination of photon energies and the nature of the incident radiation.

As a result of irradiation, some solid substances undergo changes in some of their physical
properties. These changes amount to storage of the energy from the radiation. Since the energy is
stored, these materials can be used for dosimeters.




Advantages (as compared to film dosimeter badges)
    includes:
· Able to measure a greater range of doses
· Doses may be easily obtained
· They can be read on site instead of being sent away
    for developing
· Quicker turnaround time for readout
· Reusable

Disadvantages
· Each dose cannot be read out more than once
· The readout process effectively "zeroes" the TLD
          Thermoluminescent Dosimeters (TLD)




TLD manufacturing differs from company to company, so specific chip arrangement and composition
may vary. Most badges are lithium fluoride (LiF) and calcium fluoride (CaF). Lithium has two stable
isotopes; 6Li is sensitive to neutrons, but 7Li is not. Neutrons interact in 6Li via the reaction:
6Li(n,)3H. In fact the reason that 6Li is a special nuclear material (SNM) is that this same reaction is

used for the production of tritium for nuclear weapons. Badges that measure betas and gammas have
at least one chip behind a mylar window, to allow some energy discrimination of betas and soft x-
rays. This chip would be used to assign the shallow dose. Another chip would be behind a layer of
plastic about 600 mg/cm2 thick. This chip is designed to measure deep dose or whole body dose. One
of these is usually 7LiF, the other is CaF. Both of these measure gamma dose. CaF is more sensitive
to low energy gammas than 7LiF.
Thermoluminescent Dosimeters (TLD)
                 A glow curve can be obtained from the heating
                 process. The light output from TL material is not
                 easily interpreted. Multiple peaks result as the
                 material is heated and electrons trapped in
                 "shallow" traps are released. This results in a
                 peak as these traps are emptied. The light output
                 drops off as these traps are depleted. As heating
                 continues, the electrons in deeper traps are
                 released. This results in additional peaks. Usually
                 the highest peak is used to calculate the dose
                 equivalent. The area under the curve represents
                 the radiation energy deposited on the TLD.
                     Thermoluminescent Dating
Thermoluminescence dating is in its developmental stages. Except for doing
simple authenticity tests of art objects, thermoluminescence dating is not
generally accurate enough for archaeological standards. There are many
factors which have to be taken into account and each of these factors has its
own random error. This, combined with poorly understood measurement
errors, make the accuracy of thermoluminescence dating only about 15%
accurate for a single sample and 7 to 10% accurate for a suite of samples in a
single context.

Thermoluminescence dating is used for rocks, minerals and pottery. It dates
items between the years 300-10,000 B.P. It is based on the fact that almost all
natural minerals are thermoluminescent. Energy absorbed from ionizing
radiation frees electrons to move through the crystal lattice and some are
trapped at imperfections. Later heating releases the trapped electrons,
producing light.


Exposure of the sample to light must be avoided at all costs to avoid an underestimation of the age.
               Thermoluminescent Dating
Measurement of the intensity of the luminescence can be used to determine
how much time has passed since the last time the object was heated. The light
is proportional to the amount of radiation absorbed since the material was last
heated. Natural radioactivity causes latent thermoluminescence to build up so
the older an object is the more light is produced. Therefore,
thermoluminescence dating is actually determining the last time a crystal was
heated and electrons were released. The minerals that are used for
thermoluminescence dating are quartz, feldspar, diamond and calcite.

The last time a crystal was reheated and its electrons were released is known
as a clock resetting event. This usually occurs when the items are heated to
350 ºC. Therefore, in archaeology, thermoluminescence dating works best for
ceramics, cooking hearths, incidentally fire-cracked rocks and deliberately fire
treated rocks such as flint or chert.

When collecting samples for thermoluminescence dating, several samples from different
vessels not smaller than 1 gram should be taken. Samples should not be exposed to heat
and powdery examples should not be exposed to bright light. A sample of the earth also
needs to be collected so environmental radiation can be tested. The wetness of the soil
and sample should also be recorded. Samples should be placed in a polyethylene bag
and sealed with electrical tape.
              Thermoluminescent Dating
To test the date, three steps are taken:

1. Measure sample’s intensity of luminescence

2. Relate luminescence intensity to radiation dose. Irradiate sample with a
   calibrated radioactive source

3. Determine the dose per year that the sample has been exposed to

The formula used in this equation is:

               accumulated
                   dose
     Age
             ______________
     =
                dose rate


Dose Rate = dose accumulated each year
            Radiation Damage and Annealing




Radiation damages crystal structure and   Heating the sample allows the crystal
creates “color centers” due               structure to be repaired and as the
displacement of electrons and atoms in    electrons and atoms return to nearly
the crystal                               their original positions, energy in the
                                          form of light is emitted.
             Thermoluminescent Dating




fine grain                              inclusions
Thermoluminescent Dating




             Daybreak Nuclear 1100
             Automated TL / OSL reader

             OSL - optically stimulated luminescence



                       http://flint.mater.unimib.it/nozioni_uk.htm
Thermoluminescent Dating
                        Thermoluminescent Dating
                                                                                   WHO: Valdivia culture

                                                                                   WHERE: Southern coastal valleys,
                                                                                   Ecuador

                                                                                   WHEN: 3200-1500 B.C.

                                                                                   WHAT: This female figure is one of the
                                                                                   oldest creations in clay in the Americas.
                                                                                   The Valdivia culture of Ecuador dates
                                                                                   from approximately 3500 B.C. and
                                                                                   represents one of the earliest ceramic
                                                                                   traditions in this hemisphere. Valdivia
                                                                                   potters were very accomplished artists
                                                                                   and a multitude of similar figures, as
                                                                                   well as beautifully made ceramics, have
                                                                                   been found in archaeological sites from
                                                                                   the earliest time periods. These figures
   Valdivia (Ecuador) culture: comparison                                          may have been used for fertility rituals,
                                                                                   but it is equally likely that they were
   between TL and 14C chronologies.                                                used for curing ceremonies.
                                                                                   Height: 10.5 cm

                                                    Valdivia                       HOW: Hand-modeled brown
                                                                                   earthenware, incised decoration.
(ca. 3500 – 1500 B.C.)
Valdivia is the most ancient culture of sedentary agriculturalists and potters who inhabited the present territory
of Ecuador, and one of the earliest culture in South America. The ruins of Valdivian towns are located along
the river basins of the Ecuadorian coastal strip. This fact leads us to draw the conclusion that this people could
take advantage of the fertile river plains for agricultural purposes, as gardening was their most important
subsistence activity. Valdivian inhabitants cultivated Indian corn (maize), kidney beans (fréjol), cassava,
cotton-plants and archira (plant of the Cannacae family whose roots are edible). The diet was complemented
with game (specially deer) and offshore fishing. They also practiced the gathering of mollusks and crustaceans
along the beach, in mangroves and estuaries. The houses were built orderly around a ceremonial square.
Thus, a large village was formed, which become the nucleus of numberless small hamlets.
                   Thermoluminescent Dating




We can observe the glow curves of unirradiated samples (NTL) as well as those
irradiated with 90Sr beta radiation at four different artificial doses (NTL+ATL). The dose
response curves obtained by the additive dose method. Interpolation of these curves to
zero TL intensity (TL=0) gives the dose equivalent (Q).

The dose equivalent, Q, takes into account that all types of radiation do not have the
same energy loss when passing through materials.
                  Radiolysis of Water

                        H2O
           excitation                  ionization                     Time
                                                                      scale

       H2O*                              H 2O + + e -                 10-16 s


                                             H2O



H• + •OH           H2 + •O        H3O+ + •OH                   e-aq   10-14 s



                             formation of molecular products
                                in the spurs and diffusion
                                 of radicals out of spurs




       e-aq, H•, H2, H2O2, H3O+, •OH                                  10-7 s
                         G-Values for Water

                                              -
Radiation      G(-H2O)   G(H2)   G(H2O2)   G(e aq)   G(H•)   G(•OH)   G(•HO2)

H2O(l)          0.43     0.047    0.073     0.28     0.062    0.28    0.0027

gamma and
fast e-

H2O(l)          0.294    0.115    0.112    0.0044    0.028   0.056     0.007

12 MeV 

H2O(g)          0.85     0.05      0       (0.31)    0.75     0.85

gamma and e-

units are -moles/J in irradiated neutral water

Possibility of production of explosive mixtures of H2
  and O2 in highly radioactive fields
         Radiolysis of Aqueous Solutions

 for solutions less than 0.1 M in solute, water is the only
      important reactant
 use scavengers to react selectively with radicals at
     higher solute concentrations

 chemical changes in dilute solutions depend on solute’s
reactivity with e-aq , •H, and •OH

 the hydrated electron, e-aq, is a strong reducing agent,
            E0 = +2.9 V (comparable to F2)
 H• is a weaker reducing agent, E0 = +2.3 V

 •OH is a strong oxidizing agent, E0 = -2.7 V (acidic) and
E0 = -1.8 V (basic): [comparable to Na(s), E0 = -2.71 V]
         Radiolysis of Aqueous Solutions
 H• can be considered to be a weak acid
   e-aq + H+  H•                      pKa = 9.6

 in strongly alkaline solutions, •OH decomposes to O-, a
       strong nucleophile
      •OH  O- + H+ pKa = 11.9

 the perhydroxyl radical, •HO2 is formed in oxygenated
      solutions
      e-aq + O2  •O2-
      H• + O2  •HO2
      •HO2  •O2- + H+     pKa = 4.7
 in saline waters, the dominant reactions are
      •OH + Cl-  HClO-
      HClO- + H+  Cl• + H2O
      Cl- + Cl•  Cl2-
                     Fricke Dosimeter

G(Fe3+) = 2 G(H2O2) + 3 [G(e-aq) + G(•H) + G(•OH2)] + G(•OH)



e-aq + H+  H•
H• + O2  •HO2
•OH + Fe2+  Fe3+ + OH-
•OH2 + Fe2+  Fe3+ + HO2-
•OH2- + H+  H2O2
H2O2 + Fe2+  Fe3+ + OH- + •OH
                      Dose Measurements
dosimeters
        primary measurement
              calorimetric measurement of heat evolved
              collection of ions formed in a gas
        secondary measurements
              chemical changes in a liquid (Fe2+/Fe3+)
              exposure of photographic film
              excitation of atoms in a glass or crystal

Fricke dosimeter (spectrophotometric measurement)

A = change in absorbance at 304 nm                        A
                                                D(Gy) 
G(Fe3+) = yield of Fe3+ in mol/J
x = cell path length (m)
 = density of solution (1024 kg/m3 at 20 °C)
                                                             c h
                                                        xG Fe 3
 = molar absorbance 217.4 m2/mol at 304 nm
Fricke Dosimeter
              Pen Dosimeter




measure the rate at which a static voltage is
  discharged from a gas-filled chamber
    (similar to discharging a capacitor)
               Sources for Irradiation

reactors

accelerators
     EBA (electron beam accelerators)
sealed sources
      60Co
     137Cs
     192Ir
INEL Irradiation Facility
   Industrial Irradiation Processes




   clear topaz           blue topaz
little gem value         irradiated
      Industrial Irradiation Processes



production of
192Ir for

industrial
radiography
sources




                           JAERI
               Industrial Irradiation Processes




Pipe 93 x 2,6mm, broken weld at pipe
junction, Ir 192, 25 Ci, 6 sec


Requirement of maximum source activity
limits as follows:
Co-60: 400 GBq
Ir-192: 1500 GBq
Se-75: 3000 GBq
                  Industrial Irradiation Processes




                                                                      Deep infected ulcer on
Early erythema in the
                                                                      the upper medial part of
frontal and antelateral right
                                                                      right thigh six months
side of the chest 11 days
                                                                      after being unawarely
after the exposure to an
                                                                      exposed to a 164 GBq
iridium-192 source (185
                                                                      (4.4 Ci) cesium-137
GBq, 5 Ci) mounted in a
                                                                      source
pen-size source holder for
industrial radiography
which was placed to the
pocket of the worker's
overall and kept there for      Tense painful bulla of the left
about two hour                  palm on day 20 evolving from
                                erythema with early blistering
                                which had appeared on day 10
                                after the initial contact for a few
                                minutes with the iridium-192
                                source.
         Industrial Irradiation Processes
Sterilization( 25 - 50 kGy )
       medical supplies
       contact lens solutions
       laboratory animal feed

Wire and cable ( 50 kGy)
      cross linking insulator sheathing

Shrinkable film and tubing ( 40 - 100 kGy )
      increase cross-linking of polymers, thermally
      expand followed by rapid quench. Polymer retains
      “memory” of pre-expansion dimensions

Curing of surface coating and inks ( 10 - 100 kGy)
      increase cross-linking to bond polymers to surfaces
           Industrial Irradiation Processes




1 Two-stage traveling-wave rf
   linac
2 Collimator head for medical
   treatment beam
3 380-V motor generator to
   convert to 50 Hz
4 8-MW Klystron and waveguide
   for 3000 MHz rf
5 Water cooling system
6 Operator's console and data   Yale University
   acquisition system
       Irradiation Research Topics at Yale
Medical Applications
      •Absorbed dose measurements
      •Real-time sensor development
      •Benchmark measurements for shielding
Industrial Applications
      •Electron-beam curing
      •Radiation hardness testing
      •Electron-beam sterilization
      •Beam diagnostics
      •Industrial CT scanning
Environmental Applications
      •Municipal waste sterilization
      •Hazardous compound degradation
E-Beam Physics Applications
      •Channeling radiation
      •Materials modification
Industrial Irradiation Processes
Food Irradiation
Food Irradiation - Linear Accelerator at Iowa State University
                               The Accelerator
                               Electrons are concentrated and accelerated to 99
                               percent of the speed of light. This produces rapid
                               reactions on the molecules within the product.
                               The Electron Beam Linear Accelerator machine
                               generates and accelerates electrons to energies
                               of 5, 7.5, or 10 MeV with beam power of up to
                               10kW.
                               A stainless steel plate may be placed under the
                               scanning horn to convert the electrons to X-rays
                               at an energy level of 5 MeV to allow very thick
                               penetration at low doses; however, this increases
                               irradiation time considerably.

                               The Irradiation Process
                               A cart system moves the products to be irradiated
                               under the electron beam at a predetermined
                               speed to obtain the desired dosage. Multiple carts
                               move products in and out of the irradiation area
                               continuously with throughput up to 500 pounds
                               per hour. Maximum product dimensions are 24
                               inches wide and 36 inches long. Product
                               thickness depends on density and electron
                               energy. For example, 3.5 inches is the maximum
                               thickness for meat. Using X-rays increases
                               thickness to several feet for various products.
               Countries Approving Food Irradiation


                                                Europe
Asia
                                                Belgium
Bangladesh
                                                Croatia
China
                                                Czech Republic
India
                                                Denmark
Indonesia
                                                Finland
Iran
                                                France
Japan
                                                Germany
Korea
                                                Hungary
Pakistan
                                                Israel
Philippines
                                                Italy
Russian Federation
                                                Netherlands
Syria
                                                Norway
Thailand
                                                Poland
                                                Spain
Africa
                                                Ukraine
Algeria
                                                United Kingdom
South Africa
                                                Yugoslavia
South America
                                                North America
Argentina
                                                Canada
Brazil
                                                Costa Rica
Chile
                                                Cuba
Peru
                                                Mexico
Uruguay
                                                United States
Food Irradiation (University of Florida)
Food Irradiation (University of Florida)
Food Irradiation (University of Florida)
             Food Irradiation Examples
 Food irradiation rules US Food and Drug Administration

Product                            Dose permitted (kGy)
Wheat and wheat powder
    Disinfest insects                          0.2-0.5
White potatoes
      Extend shelf life                        0.05-0.15
Spices and dry vegetable seasoning
      Decontamination/disinfest insects        30 (max)
Dry or dehydrated enzyme preparations
      Control insects and microorganisms       10 (max)
Pork carcasses or fresh non-cut processed cuts
      Control Trichinella spiralis    0.3 (min)-1.0 (max)
Food Irradiation Examples
                     Calculations for Chapter 8

The mass attenuation coefficient for 1.0 MeV -rays in tungsten is about 0.0662 cm2/g.
   Calculate the thickness of tungsten (in cm) necessary to reduce a 1.0 Ci source to
   1.0 mCi.

define the equation                         lin  x
for -ray absorption           I       I0e

define the initial                               define the final
intensity of the       I0  1.0 Ci             intensity of the   I  0.001 Ci
-rays                                           -rays

define the mass                                           2
attenuation coefficient                           cm
for 1.0 MeV -rays              mass  0.0662 
                                                  gm
in W

define the density                            gm
of W                        W  19.3 
                                                 3
                                              cm
                  Calculations for Chapter 8

calculate the linear
attenuation coefficient                                           1
from the mass coefficient    lin   mass  W    lin  1.278 cm
and the density

solve the attenuation         I
                            ln         linx
equation for the length,x
                               I0 

                                     I
                                  ln 
                             x 
                                     I 0
                                                     x  5.407 cm
                                     lin
                             Chapter 8 - Glossary
absorption coefficient       fission fragments                radical scavengers
absorption curve             Fricke dosimeter                 radiolysis of water
absorption edge              Gray (Gy)                        range
attenuation                  G-value                          Rayleigh scattering
average range                half-thickness value             reactors
backscattering               hydrated electron                relative dose rate
Bragg peak                   hydroxyl radical                 Roentgen (R)
Bremsstrahlung               impurity sites                   sealed sources
bubble chamber               indirect ionizing radiation      secondary electrons
build-up factor              index of refraction              secondary ionization
calibrated absorber          industrial irradiation           shrinkable film and tubing
Cerenkov radiation           ion pair                         source activity
charged particles            ionization                       specific energy loss
charged particles            ionizing radiation               static charge
cloud chamber                lattice defects                  stopping power
coherent scattering          linear attenuation coefficient   straggling
collimated beam              linear energy transfer (LET)     thermoluminescent dosimetry
color centers                mass attenuation coefficient     uncharged particles
Compton gamma                mass stopping power
Compton scattering           neutron absorption
cone of emission             pair production
continuum spectrum           phonons
Coulomb effect               photoelectric effect
counting efficiency          point source
cross-linking of polymers    radiation absorbed dose
direct ionizing radiation    radiation chemistry
dose rate                    radiation dose
dosimeters                   radiation shielding
electromagnetic radiation    radiation tracks
electron beam accelerators   radical formation
electron “hole”

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:4/29/2013
language:English
pages:101
tao peng tao peng fuzhou http://
About 1234567