X-ray production and applications

by: Dr. Ahmed M. Maghraby

I - Discovery

During the early 1890’s many physicists had been

studying electrical conduction in gases at low pressures.

Wilhelm Conrad Roentgen was also working with gas

discharge tubes and in December 1895 he announced

the discovery of a new type of radiation, which he called

X-rays because their nature was then almost completely

unknown.

First case of human injury from X-rays appears to have

been recorded early in 1896.

X rays were discovered in 1895 by W. C. Roentgen, who called them X rays because their nature was at first unknown; they are sometimes also called Roentgen, or ROntgen, rays. X-ray line spectra were used by H. G. J. Moseley in his important work on atomic numbers (1913) and also provided further confirmation of the quantum theory of atomic structure. Also important historically is the discovery of X-ray diffraction by Max von Laue (1912) and its subsequent application by W. H. and W. L. Bragg to the study of crystal structure.

II - Some Historical Events

III - Nature of X rays

X ray, invisible, highly penetrating electromagnetic

radiation of much shorter wavelength (higher frequency)

than visible light. The wavelength range for X rays is

from about 10-8 m to about 10-11 m, the corresponding

frequency range is from about 3 × 1016 Hz to about 3 ×

1019 Hz (1 Hz = 1 cps).

IV - Electromagnetic radiation

The electromagnetic spectrum plotted on a logarithmic energy scale.

V - Production

Gas-filled X-ray tubes were used for X-ray production, the electrical discharge between the electrodes depended upon the presence of residual gas left in the tube.

At lower pressure more voltage was required to start discharge, so the tube was said to be hard, if pressure of gas increased, the tube is said to be softened.

Although gas-filled tubes are not the current source of X-rays, we still refer to hard X-rays to those produced by high voltage and more penetrating than soft.

Tube operation was tested by examining the bones of the left hand close to the tube while the right hand catching a fluorescent screen, this action was responsible for large number of malignancies at left hand of early radiologists.

Early type of gas-discharge X-ray tube. Positively charged gas ions bombard cathode and liberate electrons. When these electrons strike anti-cathode, X-rays are produced.

In 1913, W.D. Coolidge announced the development of a hot cathode X-ray tube in which current and voltage were independently variable, and without need to electric discharge gas.

When a filament is heated in a high vacuum, the most energetic electrons leave the wire to form an electron cloud around the filament, if the second electrode has a sufficiently high positive potential, all of electrons emitted by the filament will flow to the target, or anode.

Emission is highly dependent on the temperature, hence radiation quantity could be varied.

anode made of platinum, tungsten, or another heavy metal of high melting point

V - Production

Modern types of X-ray tubes

1 – Bremsstrahlung: x rays that are emitted when high

speed charged particles suffer rapid acceleration.

When a beta particle passes close to the nucleus, the strong attractive coulomb forces causes the beta particle to deviate sharply from its original path.

In accordance, the beta particle loses energy by electromagnetic radiation at a rate proportional to the square of the acceleration.

Mechanisms VI - Production

For the purpose of estimating the bremsstrahlung hazard,

the following relation may be used:

F = 3.5 x 10-4 ZE

Where f is the fraction of the incident bta energy converted

into photons. Z is the atomic number, and E is the

maximum energy of the beta particle, MeV.

2- Radiation is emitted by the electrons of the anode atoms when incoming electrons from the cathode knock electrons near the nuclei out of orbit and they are replaced by other electrons from outer orbits.

Mechanisms VI - Production

VII – Efficiency of X-ray production

Many of the decelerations experienced by the incident electrons are too small to small to produce X-ray photons and consequently produce heat at anode.

Efficiency of X-ray production is defined as:

Total radiated X-ray energy/Total energy delivered to anode

Kramer’s analysis leads to the following expression:

Eff. = VZ x 10-9

Where V is the applied voltage, Z is the atomic number of target

From previous equation, radiated power = V2IZx10-9 Watt

VIII- X-ray spectrum

The spectrum of frequencies given off with any particular

anode material thus consists of a continuous range of

frequencies emitted in the first process, and

superimposed on it a number of sharp peaks of intensity

corresponding to discrete frequencies at which X rays

are emitted in the second process. The sharp peaks

constitute the X-ray line spectrum for the anode material

and will differ for different materials.

IX - Applications

There are many and different applications for X-rays. Most applications are based on their ability to pass through matter:

A- Radiography:

1- Normal X-rays for skeletal bones.

2-Mammography.

3-Dental X-rays.

4-CT (Computerized Tomography) scan.

5-Paintings analysis.

IX - Applications

6-Security gates and tools.

7- X-ray microscopy or micro-radiography (provide enlarged images of the structure of opaque objects).

B- X rays therapy: X-rays can destroy living tissue and can cause severe skin burns on human flesh exposed for too long time. This destructive power is used in X-ray therapy to destroy diseased cells.

C- X-ray fluorescence.

D- X-ray diffraction.

X – X-ray diffraction

In 1912 Von Laue succeeded in demonstrating X-ray diffraction using a crystal as a diffracting material.

In simple cubic crystal such as NaCl, atoms are arranged in a regular alternating manner with an inter-atomic spacing or diffraction grating constant ‘d’.

These atoms serve as scattering or diffracting centers for an X-ray beam.

The ray diffracted at B and enter the detector must travel a distance CBD = 2d sin θ

farther than a similar ray

diffracted at A, when path

difference is equal to an integral

number of wavelength nλ, the

diffracted waves will add

constructively and reach detector

in phase:

n λ = 2d sin θ

θ is the angle of incidence.

X – X-ray diffraction

Source Detector

A

B C D

XI - X-ray shielding

X-ray shielding is divided into two categories: source shielding, and structural shielding.

1- source shielding: shielding for the X-ray tube (lead housing), which is divided by NCRP into two types:

i- Diagnostic type: built so that the leakage at 1m cannot exceed 100mR/h = (30 μC/kg)/h, at maximum tube potential and maximum continuous current.

ii- Therapeutic type:

a- for X-ray generators that are incapable of kVp = 500: built so that the leakage at 1m cannot exceed 1R/h = (300 μC/kg)/h, at maximum tube potential and maximum continuous current.

b- for X-ray generators that are capable of kVp = 500 or more: built so that the leakage at 1m cannot exceed 1R/h = (300 μC/kg)/h, or 0.1% of the useful beam exposure rate whichever is greater at maximum tube potential and maximum continuous current.

2- Structural shielding: To protect against (useful X-rays, leakage radiation, and scattered radiation).

It encloses X-ray tube (which is protected), and the space in which is located the object being irradiated.

Requirements for structural shielding: I- maximum kilovoltage at which the X-ray tube is operated. II- maximum milliamperes of beam current. III- workload factor (W), which is a measure for the amount of use of

an X-ray machine in units of (miliampere-minutes per week). IV- use factor (U), which is the fraction of the workload during which

useful beam is pointed in the direction under consideration. V- Occupancy factor (T), which reflects the degree or type of occupancy

of the area in question.

XI - X-ray shielding

XI - X-ray shielding Occupancy factor

Control space, workrooms, darkrooms, waiting rooms, restrooms used by occupationally exposed personnel, kids play areas.

Full Occupancy

T = 1

Elevators, uncontrolled parks, corridors, rest rooms used by non occupationally personnel.

Partial occupancy

T = 1/4

Stairways, far elevators, traffic closets,..etc. Occasional occupancy

T = 1/16

XI - X-ray shielding The beam pass through the object and is attenuated

to an acceptable level by the primary protective barrier, while the secondary protective barrier protects against the leakage and the scattered radiations

XII – Synchrotron Radiation