-ray (Röntgen)

radiation

József Orbán, Department of Biophysics, 2012 nov.

• Formation of X-ray

• Properties

• Interaction with matter

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

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!

Röntgen- (X-ray), as

electromagnetic

radiation

wave, particle

energy

IR

UV

1 eV = 1,6*10-19 J

E= hn = hf

v= ln = lf

g and X

g and X

Röntgen

Formation of X-ray

(Röntgen-) radiation

X-ray tube

characteristic / breaking radiation

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.

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

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

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

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.

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

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)

Rela

tive inte

nsity

wavelength (nm)

Characteristic: line type emission

radiation

Breaking: continuos emission

radiation

X-ray (Röntgen) as

electromagnetic

radiation

X-ray (Röntgen) tube

Generator

Imaging unit

Control unit

X-ray instrument:

Radiation (general)

20

1

rII Intensity dependence on distance

from a point source

Spread along straight line (without reaction with matter)

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()(

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

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

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

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

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