-ray (Röntgen - Pécsi...
Transcript of -ray (Röntgen - Pécsi...
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-ray (Röntgen)
radiation
József Orbán, Department of Biophysics, 2012 nov.
• Formation of X-ray
• Properties
• Interaction with matter
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Hand mit Ringen: print of Wilhelm
Röntgen's first "medical" x-ray, of his
wife's hand, taken on 22 December
1895 and presented to Professor
Ludwig Zehnder of the Physik Institut,
University of Freiburg, on 1 Jan 1896
http://en.wikipedia.org/wiki/X-ray
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Wilhelm Conrad Röntgen; 1895 (1845-1923, physics Nobel prize: 1901.)
Chatodray tube emission:
• fluorescating salt crystal,
• can not be deflected neither by electric nor by
magnetic field.
• Differently absorbed by different materials.
• X-ray: Named due to its unknown source. 1901. Nobel-prize
http://en.wikipedia.org/wiki/Wilhelm_Conrad_R%C3%B6ntgen
Wavelength: 0,01-10 nm (10-11-10-8 m)
Energy: 0,1-100 keV (~ 10-17 – 10-14 J)
High ionising effect!
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Röntgen- (X-ray), as
electromagnetic
radiation
wave, particle
energy
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IR
UV
1 eV = 1,6*10-19 J
E= hn = hf
v= ln = lf
g and X
g and X
Röntgen
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Formation of X-ray
(Röntgen-) radiation
X-ray tube
characteristic / breaking radiation
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Heated cathode
anode
(Wolfram-Rhenium alloy)
Oil cooled rotor
X ray
cathode tube
- Low pressure gas filled glass tube
(10-6 bar = 0.13 Pa)
- due to the high voltage between the
anode and cathode elecrons leave
the material of the cathode
- that accelerate due to the electric
votage
- they impact to the anticathode
(anode)
- and induce X ray radiation.
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electron
1. Ejection of inner
electron: ΔEkin →
Eionisation
2. Replacement by outer
electron
3. Emission of energy
excess:
ΔEn→1 → Ephoton=hf
Attention! All transition has its
own energy:
3 → 1 E3-1
2 → 1 E2-1
3 → 2 E3-2
where E3-1 > E2-1 > E3-2
Characteristic
radiation
n = 1
K shell
n = 2
L shell
n = 3
M shell
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electron
The initial (i) and final (j)
energy levels determine
the energy of emitted
photon:
ΔEi→j → Ephoton= hfi-j
Characteristic
radiation
Attention! All transition has its
own energy:
3 → 1 E3-1
2 → 1 E2-1
3 → 2 E3-2
where E3-1 > E2-1 > E3-2
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Atomic energy levels 0
Energ
y
K series
emission
hf
N
M
L
K
L series
emission
M series
emission
2 → 1: Kα
3 → 1: Kβ
4 → 1: Kγ
5 → 1: Kδ
2 → 2: -
3 → 2: Lα
4 → 2: Lβ
5 → 2: Lγ
3 → 3: -
4 → 3: Mα
5 → 3: Mβ
Only well defined transitions are
allowed.
Only well defined energy
differences.
Line type emisszion spectrum
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electron
The electron interacting
with nucleus is deflected,
decelerates!
Difference of original (Einit)
and final (Efinal) kinetic
energy determines the
energy of emitted photon:
ΔEinitial→final = Einitial – Efinal →
Ephoton= hf
Breaking
radiation
Continuous emission
spectrum.
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c: 1.1x10-9 V-1 (constant)
U: accelerating voltage (several kV)
I: current (~ mA)
Z: atomic number (W:74)
IZcUPRtg
2
Less then 1% is converted to
RTG radiation of the input
energy!
99% heat loss! →
Requires cooling! (rotation)
X ray cathode tube
exit slit
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kVU nm0 l
2345,1U0 l
Vm10x345,1210x6,1
10x3x10x6,6
e
hcU 7
19
834
0
l
With increasing accelerating voltage:
• l0 decreases
• I, intensity increases (for all l)
JeVE 19106.1
Duane-Hunt rule
Rela
tive inte
nsity
wavelength (nm)
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Rela
tive inte
nsity
wavelength (nm)
Characteristic: line type emission
radiation
Breaking: continuos emission
radiation
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X-ray (Röntgen) as
electromagnetic
radiation
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X-ray (Röntgen) tube
Generator
Imaging unit
Control unit
X-ray instrument:
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Radiation (general)
20
1
rII Intensity dependence on distance
from a point source
Spread along straight line (without reaction with matter)
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The intensity of EM-radiation decreases while it
passes through a substance.
The number of photons decrease, but their energy
remains constant (case of absorption) or could
decrease (case of Compton scattering).
The decreasement (attenuation) is exponencial:
EM-radiation in interaction with matter (macroscopic description)
I(0) = I0: incident intensity
μ: linear attenuation coefficient
x: depth of intrusion (pathlength)
xeIxI )0()(
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sample/body
Reflection Transmission Absorption
I ~ A2
I: intensity
A: amplitude
I ~ n
n: photon
number
I0 I
Scattering
DETECTION
Low significance!
Significant! Important for imaging!
Imaging error source!
Light – matter interaction
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Phenomena decreasing intensity
Absorption depends on:
l: wavelength, depends on substance
composition
Z: atomic number of element
D: constant
d: distance traveled in substance (pathlength)
Number of atoms along the path:
n=N/V: volumetric density
A ~ λ3Z4dD
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The total energy of the photon is absorbed by an electron of
an atom. Ionisation occurs:
positively charged ion and freely moving electron is created
The photon disappears (ceased)!
Photoeffect (photoelectric phenomenon)
hf: energy of absorbed
photon
Ebinding: binding energy of
electron
1/2m0v2 : kinetic energy of
electron
Photoeffect -> secunder RTG radiation (characteristic)
2
021 vmEhf binding
photoelectron
photon
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EM radiation interacting with free or weakly bound
electrons.
During the effect an photon with
energy and an electron with
impulse undergo an ellastic collision.
Compton-scattering
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Compton-scattering
impulse- and
energy conservation laws incoming photon scattered photon
scattered electron
Detection effects:
- energy decreased → detector may not be sensitive for lower energy photons
→ decreased intensity = increased attenuation
- scattering
→ some photons ”deflected out” → ↓ intensity = ↑ attenuation
→ some photons deflected in wrong direction → ↓ image resolution