X-rays and Laser
Daniel Vlk
Institute of Biophysics
Origin of X-rays
X-rays are beams of electromagnetic radiation which originate in
atomic electron shells. Wavelength <10nm.
They are formed by photons of very high energy (substantially
higher than the energy of ultraviolet light photons). In practice,
they are produced in X-ray tubes – evacuated glass tubes
involving two electrodes – the anode and the cathode.
Electrons escape from the hot cathode surface (thermoemission
of electrons), and then they are rapidly accelerated in the space
between the hot cathode and the (cold) anode by very high
electric voltage (e.g. 150,000 volts).
The energy of electrons striking the anode is given by the term
½m.v2 which is equal to their original potential energy Ue.
Origin of X-rays
Subsequently, these electrons are suddenly decelerated in a
tungsten (W) target which is a part of the anode. It means
that their kinetic energy is transformed into X-ray and heat
energy.
However, on average, only a small part (less than 1 %) of
this energy is transformed into high energy X-ray photons.
More than 99 % of their energy is transformed into heat. In
the rare case of total electron energy transformation into a
single X-ray photon, the energy of the photon is:
where hfmax is the maximum energy of the liberated photon.
Such photons possess maximum possible frequency fmax or
the shortest possible wavelength λmin.
Origin of X-rays
The energy spectrum of these X-rays is continuous.
The X-rays produced in this way are called
“Bremsstrahlung” (i.e. deceleration radiation) which is a
German word used in honour of the German physicist W.C.
Roentgen, the discoverer of X-rays.
X-ray photons can be also produced by jumps of electrons
from outer to the innermost shells of some heavy atoms.
There are vacancies (unoccupied sites) formed by impact of
accelerated electrons, which must be filled immediately by
some other electron.
The spectrum of X-rays produced in this way consists of
lines representing discrete energies of photons. This is the
characteristic radiation.
Origin of X-rays
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Laser
Light Amplification by Stimulated Emission of Radiation.
The first ruby laser was constructed by T.H. Maimann in
1960. Main parts of a laser:
active medium
optical resonator
source of excitation energy
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Laser
Principle of the laser: alternating excitation and deexcitation.
Electrons of the atoms of the active medium are excited by the
source energy („optical pumping“).
Thereafter they are deexcited by a stimulating photon, new photons
of the same energy arise and the effect is repeated – amplification
occurs.
In the so-called three-level laser, the third energy level is broad, thus
it is not necessary to use monochromatic (i.e. monoenergetic) light
for optical pumping. Because of small energy difference between the
second and third energy level, the electron transition to the second
energy level is spontaneous („thermal“) – electrons are waiting for the
stimulating photon there.
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Lasers
Solid l. (compact, semiconductor): ruby laser (694,3 nm),
neodymium (1,06 µm),
Semiconductor l. – based on the principle of
electroluminescence.
Liquid l. An organic dye solution is used as active medium.
Advantage: can be tuned to different wavelengths (from near
IR, VIS to UV range).
Gaseous l.. Important for medicine. Helium-neon laser (1,06
µm) and ion lasers (argon and krypton). CO2-N2-He-laser
etc.
Plasma l. Active medium is plasma, fully ionised carbon –
irradiates soft X-rays.
Lasers can operate in two modes: continuous and pulsed
Laser power ranges from 10-3 to 104 W. Low-power lasers
(soft-lasers) are used mainly in physical therapy. High-power
lasers are used as surgical tools (laser scalpel).
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Effects of laser radiation
Laser light is monochromatic and coherent. This allows us to
concentrate the laser beam on a small area and to reach a high
output density. The laser beam can be guided by mirrors, lenses,
or optical fibres.
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Effects of laser radiation
Thermal effects depend on the power density of light and its
wavelength. They are exploited mainly in surgery and
microsurgery.
Non-thermal effects are typical for soft-lasers, they depend little
on the wavelength – based on a molecular action mechanism
(action on enzymes of the respiratory chain, enhancement of
mitochondrial DNA replication, enhancement of enzyme activity).
Laser light also has a photodynamic effect – chemical changes of
inactive substances irradiated by laser light of certain wavelength
can lead to formation of (e.g. biologically) active derivatives.
Laser applications
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Soft-laser application
Surface applications – short wavelength, deep
applications – long wavelength (near IR).
laser pens are the simplest devices, based on
laser diodes, fed by batteries, constant power
setting.
Small lasers (pocket) with exchangeable probe,
different frequency modes are possible.
Tabletop lasers – user comfort, many functions
and applications.
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High-power laser application
General surgery:
A laser can serve as an optical lancet cutting without contact. The blood
vessels are coagulated and the cut practically does not bleed.
Ophthalmology:
Besides being the light source of many optical instruments used for
examination, the main use is photocoagulation of retina and photoablation
of cornea to correct re
In dentistry, in oral surgery and endodontics, and for precise preparation of
the tooth enamel and dentine.
Dermatology - the main applications are photocoagulation of varicose
veins, wart removal, skin lifting, depilation and tattoo removal.
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Laser applications - medical
caries removal
Face lifting
removal of warts
Surgical laser unit
Lasers
devices: CT, MRI, radiotherapy systems, laser surgery,
eye-lens corrections, DVDs etc
bioeffects: thermal and photochemical damage to skin,
retina as eye-lens can focus laser to a very intense point
on the retina, cornea burn
Laser Protection Adviser (LPA) and Laser Protection
Supervisors
laser controlled areas
local rules
appropriate training
protective eye-wear
Maximum Permissible Exposure levels
Laser Classes
classes 1 - 4 in increasing power
Class 1: Inherently safe (max permitted limit cannot be
exceeded) because laser is very low power or housed
in an enclosure that does not allow harmful levels of
exposure (eg laser printer, CD drive)
Class 2: low power where safety is afforded by blink
mechanism of eye (eg laser lecturing pointer)
Class 3A and 3B: direct beam viewing could be
hazardous
Class 4: high power devices. Direct beam and
reflections hazardous.