Transcript 投影片 1 - 伯特利中學:BETHEL HIGH SCHOOL

X-ray
1. Use of X-ray
a. Medical diagnosis ……………………………………….1
b. Industry …………………………………….……………….2
c. Chemistry ………………………………….……………….2
2. X-ray spectrum…………………………….…..…….6
3 . Emission Spectra…………………………………..8
4 . Production…………………………………………..11
5 . Properties…………………………………………..12
Guess what
these are !
1. Use of X-ray:
a. Medical diagnosis
The test is performed in a hospital radiology department or in the health
care provider's office by an X-ray technologist. The positioning of the
patient, X-ray machine, and film depends on the type of study and area of
interest. Multiple individual views may be requested
 X-ray penetrates flash but not bone. Bone will block most of the
photons, and will appear white on developed film. Structures
containing air will be black on film, and muscle, fat, and fluid will
appear as shades of gray.
For stomach or intestines, the patient must swallow barium (Ba, 鋇)
which is opaque.
For blood, the patient must swallow Iodine (I2, 錪)
b. Industry
Use the penetrating power of X-rays to examine the
inside of machine parts for cracks
c. Chemistry
Study about crystal structure by using the diffraction
of X-rays
This application relies on the fact that the wavelength
of X-rays is similar to the inter-atomic spacing in crystal.
Cryst al Diffraction
Who suggest “crystal diffraction ?
The first proof of the wave-nature of X-rays was due to Laue in 1913.
He suggested that the regular small spacing of atoms in crystals might
provide a natural diffraction grating if the wavelengths of the rays were
too short to be used with an optical line grating.
Who show “crystal diffraction ?
Experiments by Friedrich and Knipping showed
that X-rays were indeed diffracted by a thin crystal,
and produced a pattern of intense spots round a
central image on a photographic plate placed to
received them. The rays had thus been scattered
by interaction with electrons in the atoms of the
crystal. The diffraction pattern obtained gave
information on the geometrical spacing of the
a
t
o
m
s
.
X-ray Spectrum
The characteristic X-ray spectrum from a metal is
usually superimposed on a background of continuous, or socalled 'white', radiation of small intensity. Figure 33.21
illustrates the characteristic lines, Kα, Kβ, of a metal and
the continuous background of radiation for two values of p.d.,
40000 and 32000 volts, across an X-ray tube. It should be
noted that:
(i) the wavelengths of the characteristic lines are
independent of the p.d.—they are characteristic of the metal,
and
(ii) the background of continuous radiation has increasing
wavelengths which slowly diminish in intensity, but as the
wavelength diminish they are cut off sharply, as at A and B.
When the bombarding electrons collide with the metal
atoms in the target, most of their energy is lost as heat. A
little energy is also lost in the form of electromagnetic
radiation. Here the frequencies are given E = hf with the
usual notation, and the numerous energy changes produce the
b a c k g r o u n d r a d i a t i o n i n F i g u r e 3 3 . 2 1 .
From: Text Book (2) P.376
Emission Spectrum
When an energetic electron from the cathode strikes
the atom, it can knock out an inner electron, leaving a
vacancy in a lower level (line 1). An electron from a
higher energy level falls into the vacancy (line 2); the
energy difference is carried away by a photo, which is
u s u a l l y
i n
t h e
x - r a y
r a n g e .
The wavelength of the X-ray thus emitted is
obviously determined by the energy difference of the
downward transition . Downward transitions to the K
level give rise to the K α , K β ... lines; downward
transitions to the L level give rise to the Lα, Lβ... lines,
and so on., In fact, the K, L, M series are analogous
to the Lyman, Balmer and Paschen series in the
s p e c t r u m
o f
h y d r o g e n .
From: Text Book (2) P.377
What are x-rays?
 They are a type of electromagnetic
radiation produced whenever cathode
rays (high-speed electrons) are brought
t o
r e s t
b y
m a t t e r .
Production
A focused beam of
electrons is
accelerated towards
the tungsten target
( i.e. the anode)
Over 99% of the
KE of the electron
goes into heat.
On collision the
electrons decelerate
rapidly and x-rays are
produced.
The target is a highmelting point metal
such as tungsten or
molybdenum in a copper
rod, the purpose of which
is to conduct heat away
from the target.
The rod is cooled by
circulating oil through it or
by the use of cooling fans.
Maximum frequency for
given tube potential
The continuous spectrum shows a
well-defined minimum wavelength
(maximum frequency). This corresponds
to an electron losing all its energy in a
single collision with a target atom.
The longer the wavelengths (smaller
energies) corresponded to a more
gradual loss of energy, which happens
when the electron experiences several
deflections and collisions and so is
slowed down more gradually. All or some
of the K.E. of the electron is converted
into the energy of photon(s). This
radiation is called bremsstrahlung
(braking radiation). All targets show this
c o n t i n u o u s s p e c t r u m .
A continues spectrum
The kinetic energy of a bombarding
electron = eV
Where V is the accelerating p.d.
Therefore, eV = hfmax
Where fmax is the frequency of the most
energetic photon (processing all the
initial kinetic energy of the colliding
electron).
Greater current
The larger total
kinetic energy of
the electrons.
accelerating voltage
cathode
greater
accelerating
voltage
The larger kinetic
energy of each
electron colliding
on the anode
Properties
i.
They travel in straight lines at the
velocity of light.
ii. They cannot be deflected by electric
of magnetic fields. (This is convincing
evidence that they are not charged
particles.)
iii. They are electromagnetic radiation of
very short wavelength about 10-10m.
iv. They penetrate matter, penetration is
least with materials of high density
and high atomic number.They can be
reflected.
v. Refractive indices of all materials are
very close to unity for x-rays so that
very little bending occurs when they
pass from one materials to another.
They cannot be focused by lenses.
vi. They can be diffracted.
iv. They ionize gases through which
they pass.
v. They affected photographic film.
viii. They can produce fluorescence.
ix. They can produce photoelectric
emission.