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Shielding of Ionizing Radiation

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    Radiation Protection Basics

    Shielding of IonizingRadiation

    Dr.Verius Michael

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Overview

    General Remarks Shielding of electromagnetic radiation

    (photon radiation) Shielding of b-Radiation

    Shielding of -Radiation (Shielding of Neutron-Radiation)

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    Radiation Protection Basics

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    muc

    ujucu

    General Remarks

    In shielding of ionizing radiation we have to differbetween charged and neutral radiation:

    Charged radiation (,b,p):It exists a finite thickness of an absorber, so that the primary

    particle radiation is absorbed totally

    Rmax

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    Radiation Protection Basics

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    muc

    ujucu

    General Remarks

    Charged radiation (,b,p):

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    Radiation Protection Basics

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    muc

    ujucu

    General Remarks

    Neutral radiation (n,g):An entire shielding of the whole neutral radiation is never

    possible

    if the thickness of an absorber is big enough, the remaining

    radiation can be reduced to a minimal amount!!

    DmaxDtolerance if dAbsdmin

    For shielding dimensioning two questions have to be answered:

    1. Is a pre-existing shielding thick enough to guaranteea dose below Dtolerance?

    2. Which strength and what material is needed to reach thedemanded minimum dose limit?

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    In RP the shielding of x-Rays and gRays are very important inmedicine due to their large active range.

    Both are electromagnetic radiation or photon radiation depending on

    their energies (wave-particle dualism).

    For easier calculations of shieldings the concept of a point-shaped

    source is used.

    The generation of secondary photons has to be considered when

    dimensioning radiation shieldings (working with build-up factors).

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    Schematic graphics of shielding andradiation:

    1...direct (remaining) radiation

    2...absorbed radiation3a...scattered radiation (by the

    absorber) which hits notthe person

    3b...secondary (build-up) radiation (bythe absorber), hitting theperson

    3c... secondary (build-up) radiation

    (by surrounding materialslike floor), hitting the person

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    This figure shows, that the absorber/absorber material

    cannot be chosen randomly.

    In 3b we can see the radiation is build-up (Aufbau-

    Effekt) by the absorber and led to the person, whichwouldnt have reached without an absorber!!!

    demand on small absorbers which are very close to

    radiation source!!!

    This build-up effect depends strongly on absorber thickness and used photonenergies Eph>500keV

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    The total dose rate of a person (in distance r) is

    estimated by summing up all partial dose rates of

    different photon energies.

    Considering the source Q is point shaped, the precisecalculated dose rate in a point P is:

    )exp(),(

    )cos(*4

    )(01,0)(

    )( 2 bbEBr

    EkEpA

    ED

    =

    &

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    )exp(),(

    )cos(*4

    )(01,0)(

    )( 2 bbEBr

    EkEpA

    ED

    =

    &

    )(ED&

    ... dose rate at energie E [Sv/h]

    ... activity of point source [Bq]

    ... probability of photon emission at energy level E [%]

    ... conversion factor for different energies E

    ... build-up factor for different absorber materials coefficients b and energies E

    ... shielding path length; b is dependent on absorption coeff. t (Photoeffectcoeff.), scatter coeff. s and the pair production coefficient k

    ... penetration angle

    A

    )(Ep

    )(Ek

    ),( bEB

    )(Eb

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    If a source emits only at one energy, the formula reduces to:

    )exp(),(4

    )(2

    bbEBr

    EkAD

    =

    &

    1 2 3

    Part 1:Dose rate of a source in a distance of rwithout shielding

    Part 2: Additional rate of dose rate due to Build-Up radiation generated by

    absorber and/or surrounding materials like floor, walls, devices,etc...(led to the person). For E

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    If a source emits more than on one energy level, the calculations should be

    performed for every occuring energy, but normally performed just for the

    most penetrating energy (in general the highest energy).

    Example: Radioactive substance,

    Emax=1,4MeV and Activity=3,7*1010Bq

    request: dose rate in 1m distance has to be lower than 10Sv/h

    wall thickness of lead: >=110mm (from a table not listed)

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    For estimation of proper shielding thicknesses several tables are available.

    There you find values for Half-value thickness or 1/100th-value

    thickness,etc..

    82120,051,33Co 60

    145027,7100,30.10.3Ir 192

    0.84670.10,61051,50,30.06W(K)0.010,060,20.0010.010,0342098940,006Fe(K)PbCuAlPbCuAlPbCuAl[MeV]

    d1/100 [mm]d1/2 [mm]/ [cm/g]EnergyRadiation

    Attention: In case of very large shieldings, the attenuation of theradiation can be significant lower due to the Build-Up Factor!!

    /...mass attenuation coefficient

    d1/2... absorber thickness, where the dose rate decreases to one half

    d1/100... absorber thickness, where the dose rate decreases to one hundredth

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of electromagnetic

    radiation (photon radiation)

    Rule of Thumb

    for Radiation Protection (in air and energies 0.01Mev...2MeV):

    4GBq, 1m

    1mSv/h4MBq, 1m 1Sv/h4kBq, 1m 1nSv/h

    This rule of thumb is a very rough estimation and does not replace a

    precise calculation in terms of dimensioning!!!

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    Radiation Protection Basics

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    muc

    ujucu

    Shielding of b-radiation

    As previously mentioned, a interaction with matter and electrons or positrons,

    leads to the so called Bremsstrahlung.

    Therefor a proper shielding has to be considered for bradiation:

    Its always constructed of a thick (d1) and leight material and a thin (d2) and

    heavy material.

    b-source

    absorber

    g

    d1 d2

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    Radiation Protection Basics

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    Shielding of b-radiation

    The first is used for absorbing the bradiation (d1>Rmax) and should

    contain matter with a low atomic number ( few secondary radiation is

    generated).

    The second material with high density (i.e. Pb) is used for absorbing the

    seondary bremsstrahlung.

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    Radiation Protection Basics

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    ujucu

    Shielding of b-radiation

    Maximum range of bradiation is estimated:

    16,0546,0 maxmax,

    =

    ER

    Rmax...max. range of beta-radiation [cm]

    Emax... max. energy [MeV]

    ... density of absorber [g/cm]

    0.67mm0.9mm2mm3mm7.6mm6.0mRs.max

    11.358.923.72.710.0013P [g/cm3]

    PbCuNaJAlH20Air

    Maximum range of bradiation of P32 , Emax=1,7MeV

    15143

    4.34.11

    1.71.70.5

    0.430.40.2

    0.140.130.1

    Soft Tissue [mm]Air [m]

    Rmean

    Mean b -Energy [MeV]

    Mean range ofb- -radiation in air and

    soft tissue [ICRP 38]

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of b-radiation

    Additional Comments:

    Since PET (Positronen-Emissions-Tomography) becomes more and more

    popular, peculiar attention has to be paid on RP in this field of

    radiodiagnostics. Here short life b+-radiating substances as F18, O15 and

    others are injected. By pair recombination with tissue electrons thesepositrons are annihilated and emit hard gradiation (E g=511keV) which has

    to be shielded with heavy absorbers.

    Special care and attention should be put on the doctors fingers (injections

    of patients)!! Finger dosimeter!!

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of -radiation

    Shielding against alfa-radiation is not necessary due to the short active range

    within matter it gets absorbed allready in the most upper skin layer or by

    clothes.

    But its crucial to avoid contamination of body surfaces, because a possible

    incorporation could lead to a severe irradiation within the body as a result

    of its high relativ biologic effectiveness (20 times higher than x-rays!!)

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    Radiation Protection Basics

    Dr.Michael Verius

    muc

    ujucu

    Shielding of neutron radiation

    For the attenuation of fast neutrons (0,1MeV < En < 50MeV) only absorbers with low

    atomic numbers are used. The transfer of the incoming energy is provided by elastic

    collisions with those materials; special materials with Hydrogen is used (H2O,

    Hydrocarbons, Paraffin ,...).

    Heavy materials such as lead are more or less without any effect, cause there is no

    electrical interaction with the radiation (neutral).

    During attenuation of neutrons very within the leight material hard y-raxs are

    generated, which have to be shielded by heavy materials (lead,...).

    neutronsource

    leight (H1-compunds ) heavy(lead)

    absorber

    gn

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    Radiation Protection Basics

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    muc

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    Example calculation

    See slide page 12:

    Emax=1,4MeV and Activity=3,7*1010Bq

    request: dose rate in 1m distance has to be lower than 10Sv/h

    3,7*1010Bq @10mSv/h (see rule of thumb)

    10mSv/h / 10Sv/h = 1000 (attenuation factor)

    with d1/100 + 3*d1/2 we can reach the requested dose rate

    ( for Co60: 82mmPb +3*12mm Pb=118mmPb).


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