Production of x-rays

X-rays are produced when fast moving electrons are suddenly stopped by a metal target. At the time of their discovery by a German Physicist Ron6tgen, their nature was unknown and hence their name x-rays. Generally, x-rays are uncharged electromagnetic radiations of short wavelength and high penetrating ability (power).

X-rays are produced in an x-ray tube:

E.H.T Glass tube

Target

Cathode

Electron beam Oil out

Low voltage Cooling fins

Oil in

Filament X-rays Copper anode

Current in the filament emits electrons at the cathode by thermionic emission. These electrons are then attracted towards the anode by the high potential difference that exists between the cathode and anode. On striking the target, the electrons transfer their kinetic energy to the metal target. About 99.5% of this energy is converted to heat at the target and only 0.5% of the energy is responsible for the production of x-ray radiations.

As such, the material of the target must be one that has a high melting point like molybdenum or tungsten. The anode should also be a good thermal conductor like copper so as to ensure efficient dissipation of heat.

Further cooling at the anode is enhanced by a circulation of oil around the anode and the presence of cooling fins. In some tubes, the target is made in such away to rotate so as to change the point of impact and thus reduce wear and tear.

The target is inclined at an angle to direct the x-rays out of the tube. The glass tube is also evacuated to prevent interference with the electron beam before reaching the target. The cathode is concave in shape to focus the emitted electrons to the target. The high potential difference is used to accelerate the emitted electrons towards the anode.

The x-ray tube is well shielded using lead which absorbs any stray rays thereby protecting the user.

32.2: Properties of X-rays

- Travel in straight lines with the speed of light in air; 3.0x10⁸m/s. When an opaque object like a bone is placed on the path of x-rays a sharp shadow of the object is formed on the screen.

- They carry no charge. Hence x-rays are not deflected by either magnetic fields or electric fields.

- Ionize air molecules on their paths by knocking off electrons in them.

- They cause certain substances and salts to fluoresce.

- They cause photographing emulsion, a property used in x-ray photography.

- They cause photoelectric effect when incident on the surface of some metals.

- They can readily penetrate matter. The degree of penetration depends on the density of the material and the quality of the x-rays.

- They obey the wave equation v=fλ.

- They undergo interference, reflection, refraction and diffraction effects.

32.3: Energy and Quality of x-rays

When an electron of charge e is accelerated by a voltage V applied across the tube, the electron gains an amount of energy equivalent to eV (electron volts). This energy is converted into kinetic energy of the electron;

i.eeV=K.E

eV= ½mѵ², where m- mass of the electron (m=9.11x10^-31kg) and ѵ- the velocity of the electron.

Also, according to Plank’s theory, the energy of any electromagnetic radiation x-rays included is given by;

Energy, E= hf, where h- is Plank’s constant and f- is the frequency of the radiation.

Hence for x-rays; eV=½mѵ²=hf=hѵ/λ.

Generally, most energetic x-rays are those with higher frequency or shorter wavelength while the least energetic x-rays are those with lower frequency or longer wavelength.

The energy of x-rays depends on the accelerating potential between the cathode and the anode. The higher the accelerating potential, the higher the energy of the electrons.Since it is the energy of the electrons that is converted into x-rays, the higher the energy of the electrons the higher the energy of the x-rays.

X-rays produced by high energetic electrons or high accelerating voltage are referred to as hard x-rays. They are high quality x-rays, have very high frequency and high penetrating power.

X-rays produced from low energy electrons or low accelerating voltage are called soft x-rays. They are low quality x-rays, have low energy content, low frequency and low penetrating power.

32.4: Intensity of X-rays

Intensity of x-rays refers to the number of x-rays produced per second. It depends on the number of electrons striking the target per second. This is controlled by the filament current. The higher the filament current the higher number of electrons emitted and hence the greater the intensity of the x-rays.

32.5: Detection of X-rays

X-rays can be detected by:

- Using a fluorescent screen. The screen glows when struck by the x-rays.

- Using a photographic plate. The plate is blackened when exposed to x-rays.

32.6: Uses of X-rays

In medicine

- Detection of fractures, displaced bones or other strange objects within the body.

- Destruction of cancerous growths and other malignant growths.

- Testing densities of bones.

- Detection of lungs with tuberculosis.

In industries

- Detection of flaws in metals and welded joints.

- Checking percentages of certain elements in an ore.

- For security checks in airports.

- To check the purity or genuineness of certain precious stones like gold, silver etc.

- To sterilize surgical equipment before packaging.

- Detection of leakages in water pipes.

In crystallography

- To study the crystal structure of substances.

32.7: Dangers of X-rays

Excessive exposure of living body tissues to x-rays may lead to damage or killing of the cells. X-rays can cause deep rooted burns, mutation and serious diseases.

These can be minimized by:

1. Limiting the exposure time of living tissues to x-rays.
2. X-ray sources should be well screened or shielded.

Example 32.1 (take h=6.63x10^-34Js, e=1.6x10^-19C, m_e=9.11x10^-31kg and ѵ=3.0x10⁸m/s)

1. Calculate the energy of x-rays whose frequency is 3x10¹⁶Hz.

E=hf =6.63x10^-34Jsx3x10¹⁶Hz

=1.989x10^-17J

1. In an x-ray tube, an electron is accelerated by a potential difference of 1kV.
2. a) Determine the velocity of the electron as it is reaching the target.

eV=½mѵ²

1.6x10^-19Cx1000V=½ x9.11x10^-31kg xѵ²

ѵ²=3.5126x10¹⁴

ѵ=

1. b) How much kinetic energy will the electron have acquired when it hits the target?

eV=K.E= 1.6x10^-19Cx1000V

=1.6x10^-16J

1. Explain how you can increase:
2. a) Quality of x-rays.

By increasing the accelerating potential between the cathode and the anode.

1. b) Intensity of the x-rays.

By increasing the filament current so that more electrons are emitted per unit time.

1. An x-ray tube operates at 10kV and a current of 15mA. Calculate the number of electrons hitting the target per second.

I=ne

15x10^-3A=nx1.6x10^-19C

n=9.375x10¹⁶electrons.

1. An x-ray tube operates at 20kV. What is the shortest wavelength in its x-ray beam?

eV=hѵ/λ

1.6x10^-19x20000V= (6.63x10^-34Jsx3.0x10⁸)/λ

λ=6.2156x10^-11m

1. State any differences between x-rays and cathode rays.

- X-rays are uncharged while cathode rays are charged.

- X-rays are produced in an x-ray tube while cathode rays are produced in a cathode ray tube.