Post on 06-Apr-2018
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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
Dr.Michael Verius
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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
Dr.Michael Verius
muc
ujucu
General Remarks
Charged radiation (,b,p):
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Radiation Protection Basics
Dr.Michael Verius
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
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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
Dr.Michael Verius
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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
Dr.Michael Verius
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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
Dr.Michael Verius
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
Dr.Michael Verius
muc
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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
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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
Dr.Michael Verius
muc
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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
Dr.Michael Verius
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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
Dr.Michael Verius
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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
Dr.Michael Verius
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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
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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
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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
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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
Dr.Michael Verius
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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).