Application of a photo effect in medicine.
Electrovacuum or semi-conductor devices, which principle of work it is based on a photo effect, name photoelectronic. We shall consider an arrangement of some of them.
The most widespread photoelectronic device is the photo cell.
The photo cell based on an external photo effect, will consist of a source of electrons — the photocathode K on which light gets, and anode A. All system is concluded in a glass cylinder from which air is pumped out.
The photocathode is a
photosensitive layer, it may be directly put on a part of inner
surface of a cylinder. In picture the diagram of inclusion of the
photocathode in a circuit is given.
Basic parameter of a photo cell — its sensitivity expressed by the attitude of force of a photocurrent to appropriating light stream.
To an increase of force of a photocurrent apply
Fig.1. also photo cells in which there is a dependent dark category in inert gas, and secondary electronic issue — the emission of electrons which is resulting from bombardment of a surface of metal by a bunch of primary electrons. The last finds application in photoelectronic multipliers (FEM).
Scheme of FEM is shown in fig. 3. Falling on the photocathode K photons lets out electrons which are focused on the first electrode Э1.
As a result of secondary electronic issue from it electrode takes off more electrons, than falls on it. There is as though a multiplication of electrons.
Fig. 2.
Being multiplied by the following electrodes, electrons as a result form time strengthened in hundred thousand a current in comparison with a primary photocurrent.
FEM apply mainly to measurement of small radiant streams, in particular them register a superweak bioluminescence that is important at some biophysical researches.
Work of the electron-optical converter is based on an external photoeffect (EOC) , the image intended for transformation from one area of a spectrum in another, as well as for strengthening brightness of images.
T
he
diagram of the elementary electronic-optical converters (EOC) is
resulted on fig. 4. The light image of object 1 projected on the
translucent photocathode K, it will be transformed in the electronic
image 2. Accelerated and focused electric Fig.
3.
field of electrodes Э
electrons get on luminescent screen E.
In medicine EOC are applied for strengthening of brightness of the
x-rayed image, it allows considerably to decrease the dose of
irradiation of man.
If signal from EOC to give as an involute on the televisional system, then on the TV screen it is possible to get " the thermal" image of objects. Parts bodies having different temperatures differentiate on the screen either a color at the coloured image or light, if image black-white. Such technical system is called thermal image, is used in a thermography.
В
ентильные
photo cells offer advantage before vacuum as work without a source of
a current. One of such photo cells — copper-oxide — is presented
on the diagram fig. 5.
The copper plate serving one electrodes,
Fig.
4.
becomes covered by a thin layer of
protoxide copper Сu2SО
(semiconductor). On protoxide the transparent layer of metal (for
example, gold Аu)
which serves the second electrode is put to copper.
I
f
a photo cell to shine through the second electrode between electrodes
there will be a photo-electromotive force, and at short circuit of
electrodes, in an electric circuit the current depending on a light
stream will go.
Sensitivity gated photo cells reaches several thousand microampere on a lumen. On the basis of highly effective gated photo cells with efficiency, equal 15 % for a sunlight, create special solar batteries for a feed of the onboard equipment of satellites and spacecrafts. Dependence of force of a photocurrent on light exposure (light stream) allows the use of photo cells as luxmeter, that finds application in a sanitary-and-hygienic practice and at photographing for definition of an exposition (in exposure meter).
The some people gated photo cells (sulphureous-thallic, germanium, etc.) are infrared-sensitive, them apply to tracking down heated invisible bodies, as though expand possibilities of vision.
Other photo cells (selenic) have the spectral sensitivity close to a human eye, it opens possibilities of their use in automatic systems and devices instead of an eye as objective receivers of a visible range of light.
The phenomenon of the photoresistor is based on the phenomenon of photoconductivity also. The elementary photoresistance (fig. 6) represents a thin layer of the semiconductor 1 with metal electrodes 2; 3 — an insulator. Photoresistance, as well as photo cells, allow to define some light characteristics and are used in automatic systems and the measuring equipment.
Thus, a photoeffect is the phenomenon connected with deliverance of electrons of a firm body (or liquids) under action of electromagnetic radiation.
Distinguish:
1) an external photoeffect – emission of electrons under action of light (photoelectronic issue),
radiations, etc.;
2) an internal photoeffect – an increase электропроводности semiconductors or dielectric under action of light (photoconductivity);
3) gated a photoeffect – excitation by light of electromotive power on border between metal and the semiconductor or between diverse semiconductors.
Photoionization of gases sometimes also is named a photoeffect.