PRODUCTION OF X-RAY & X-RAY SPECTRUM

BY

Dr Nahla Nagy

X-RAY EMETION SPECTRUM

History of Radiology

November 11, 1895, Wilhelm Conrad Roentgen

Roentgen the first Nobel prize in physics in 1901

How They Were Discovered

Röntgen discovered the new ray while

working with a cathode tube in his

laboratory.

The tube was a glass bulb that had

positive and negative electrodes inside.

When the air was removed from the

tube, and a high voltage was applied it

produced a florescent glow.

Röntgen-Cathode tube

To further observe the rays he

positioned a screen in front of the tube.

He began placing various objects

between the screen and the tube that

was emitting the X-rays.

He discovered that the rays or “invisible

light” passed right through pieces of

black paper and thin sheets of

aluminum and copper but that the light

did not pass through blocks of lead and

his bones, and instead these objects

casted shadows on the screen.

1st X-ray? Roentgen’s Wife’s Hand

X-ray of a colleagues hand after presenting the“new ray” to the Physics – Medical Association

X-Ray-Tube

The main parts of the x-ray tube areCathode/filament

Typical electron current is 0.1-1.0 A for short exposures (< 100 ms)

Anode/targetGlass/metal envelopeAccelerating voltage

Typical voltage is 20-150 kVp

X-Ray-Tube

X-RAY PRODUCTION

X-rays are produced when electrons are

accelerated and collide with a target

Bremsstrahlung x-rays

Characteristic x-rays

X-rays are sometimes characterized by the generating voltage

0.1-20 kV soft x-rays 20-120 kV diagnostic x-rays 120-300 kV orthovoltage x-rays 300 kV – 1 MV intermediate energy x-rays > 1MV megavoltage x-rays

BREMSSTRAHLUNG

Bremsstrahlung:(braking) x-rays are produced

when incident electrons interact with nuclear

electric fields, which slow them down (brake)

and change their direction.

BREMSSTRAHLUNG

Bremsstrahlung x-rays produce a continuous spectrum of radiation, up to a maximum energy determined by the maximum kinetic energy of the incident electron.

The closer the electron passes to the nucleus, the greater the interaction of the incident electron with the nucleus, and the higher the energy of the resulting x-ray.

The energy of the x-rays varies from

zero to the maximum kinetic energy of

the electron (x-ray tube kVp).

The probability of bremsstrahlung goes

as Z2, hence high Z targets are more

effective than low Z

Maximum photon energies correspond to minimum x-ray wavelengths.

-The majority of x-rays produced in x-ray tubes are via the bremsstrahlung process.

-Bremsstrahlung x-ray production increases with the accelerating voltage (kV) and the atomic number (Z) of the anode.

BREMS. EMISSION-CONTINUOUS

Characteristic radiation

Characteristic radiation is the result of ionization and is produced when inner-shell electrons of the anode target are ejected by the incident electrons.

-To eject a bound atomic electron, the incident electron must have energy greater than the binding energy.

CHARACTERISTIC X-RAY

The resultant vacancy is filled by an

outer-shell electron, and the energy

difference is emitted as characteristic

radiation (e.g., K-shell x-rays, L-shell x-

rays), as shown in Fig.

                                                

  

Characteristic x-rays occur only at discrete energy levels, unlike the continuous energy spectrum of bremsstrahlung.

-Each anode material emits characteristic x-rays of a given energy, as listed in Table 2.1.

K-shell characteristic x-ray energies are always slightly lower than the K-shell binding energy. (Table 1.4 lists K-shell binding energies).

-K-shell electrons are ejected only if incident electrons have energies greater than the K-shell binding energy.

-For tungsten, K-shell characteristic x-rays are only produced when the applied voltage exceeds 69.5 kV (K-shell binding energy is 69.5 keV).

For molybdenum, K-shell characteristic x-rays are only produced when the applied voltage exceeds 20 kV.

-L-shell radiation also accompanies K-shell radiation, but because L-shell characteristic x-rays have very low energies, they are absorbed by the glass of the x-ray tube.

-Only K-shell characteristic x-rays are important in diagnostic radiology.

CHARACTERISTIC EMISSION-LESS POLYENERGETIC

X-RAY EMISSION SPECTRUM

BREMSINTENSITY

X-RAY ENERGY

X-RAY EMISSION SPECTRUM

BREMSINTENSITY

X-RAY ENERGY

K-CHARACTERISTIC

L-CHARACTERISTIC

X-RAY EMISSION SPECTRUM

CHARACTERISTIC

BREMS

MAXIMUM ENERGY OF AN X-RAY IN THE BEAM

= kVp OF OPERATION= kVp OF OPERATION

kVkVpeakpeak

GREATEST # OF PHOTONS IS EMITTED WITH ENERGY:

APPROX. 1/3 OF MAX PHOTON ENERGY

THANK YOU