3'1 The use of thermometers

1
Health Physics
by S.Reid
3.1 The use of thermometers
3.2 Using Sound 3.3 Light and Sight
3.4 Using the spectrum 3.5 Nuclear Radiation
2
Health Physics
by S.Reid
3.1 The use of thermometers
3.2 Using Sound 3.3 Light and Sight
3.4 Using the spectrum 3.5 Nuclear Radiation
3
3.1 The use of thermometers
There are many different types of thermometer
e.g. liquid in glass, clinical and liquid
crystal, to name but a few. The most commonly
used in the school lab is the liquid in glass
thermometer.
Any thermometer must have some measurable
physical property that changes with
temperature. The liquid in glass thermometer
contains a fluid which expands/contracts when it
is heated/cooled and therefore rises or falls
inside the thermometer.
4
A clinical thermometer is different from an
ordinary thermometer in the following ways
A clinical thermometer has a much smaller range
of measurable temperature. (35 - 42oC), this
makes the clinical thermometer more sensitive to
a small change in temperature and therefore more
accurate. It has a kink near the bottom of the
capillary tube which will break the thread of
mercury as it cools. This stops the flow of
fluid back into the bulb and the thermometer then
displays the maximum body temperature of the
patient. The clinical thermometer has a
specially shaped case which acts as a lens,
magnifying the scale reading.
5
When a persons body temperature is being
measured the following steps should always be
followed.
Shake the thermometer carefully. This forces all
of the mercury back into the bulb. Place the
thermometer under the patients tongue for a few
minutes. Remove the thermometer and read the
scale measurement from the tip of the mercury
level.
6
Normal (healthy) human body temperature is
approx. 37oC.
If a person falls ill this will usually have some
effect on their body temperature. If their
temperature is too high (above 37oC) we would
say that they have a fever.
If their body temperature continued to rise and
was above 43oC then the person is in danger of
losing their life!
If body temperature falls to below 34oC then the
person is hypothermic and if body temperature
falls to less than 28oC then the patient may also
die!
7
Return to start of Health Physics Advance to
next section
8
Return to start of Health Physics Advance to
next section
9
3.2 Using Sound
Sound can only be transmitted (sent) through
either a solid, a liquid or a gas. Sound can not
travel through a vacuum.
Hospitals and doctors use sound in many different
ways. The first, which we will look at, is the
stethoscope.
The stethoscope has two bells which are used to
collect sounds from inside the patients
body. The open bell - used to pick out low
frequency sounds. The closed bell (or diaphragm)
- to find high frequency sounds
10
The stethoscope is used as an instrument to check
for abnormalities within a patient without the
need for surgery.
The doctor will place one of the bells directly
onto the patients body.
If the stethoscope has been positioned correctly
the bell will pick up a vibration from within the
patient. The sound is carried up a column of air
in a tube to the earpieces. The doctor can then
hear the sounds from within the patient and
decide if various organs (lungs or heart) are
working properly.
11
Humans can usually only hear sounds with
frequencies of between 20Hz and 20 000Hz. Any
sound of a frequency higher than 20kHz is known
as ultrasound.
Ultrasound can be used in different ways, the
most common of these ways is to produce pictures
of unborn babies in the mothers womb or in the
breaking up of kidney stones.
12
The ultrasound picture is built up by sending
pulses of ultrasound into the patient.
A jelly is used to stop the pulses reflecting
from the patients skin.
The ultrasound waves reflect off the baby.
The computer takes measurements of time for each
reflection.
Using these times, it calculates distances and
can then build up a picture of the unborn baby.
13
Sound levels are measured in decibels (dB).
Some examples of typical sound levels
are Threshold of hearing 0 dB Whisper 20
dB Ordinary conversation 60 dB Heavy lorry
passing 1m away 95 dB
Pneumatic drill 100 dB Rock group at 1m 110
dB Threshold of pain 140 dB
Any sound of 90 dB or above is considered to be
noise pollution. Over exposure to a source of
noise pollution can cause permanent damage to
hearing.
14
Return to start of Health Physics Advance to
next section
15
Return to start of Health Physics Advance to
next section
16
3.3 Light and Sight
When a wave travels from one substance into
another (i.e. air to glass) it will normally
change direction. This effect is known as
refraction.
17
The normal is always at right angles to the
surface at the point where the incident beam
meets the surface.
18
The angle of incidence is measured from the
normal line to the incident beam.
19
The angle of refraction is measured from the
normal line to the refracted beam.
20
When a wave is travelling from air to glass the
wave will bend towards the normal.
21
When a wave is travelling from glass to air the
wave will bend away from the normal.
22
Another way to control the movement of light is
to use lenses. There are two types of
lens. The convex lens,
and the concave lens
23
The convex lens (sometimes known as a converging
lens) is used to bring rays of light together.
A fatter lens will be stronger. A fat lens will
have a higher power and will bring the rays of
light to a focus quicker than a thin, low power
lens. Lens power is measured in dioptres, D. A
convex lens has a positive power and focal length.
24
The concave lens (sometimes known as a diverging
lens) is used to move rays of light further apart.
A fatter, stronger lens is again more powerful
and will cause a greater spread in the rays of
light. A concave lens has a negative power and
focal length.
25
If you know the focal length of a lens, you can
calculate the power of the lens and vice - versa.
You will also be able to tell if the lens is
concave or convex. The equation that you use to
do this is
Where P is the power of the lens measured in
dioptres, D. f is the focal length of the lens
measured in metres, m. Remember convex lenses
have positive power and focal length and concave
lenses have negative power and focal length.
26
Example 1. A lens has a focal length of -25
cm. a) What is the power of the lens? b) Is the
lens convex or concave?
Solution We must first convert the focal length
into metres. -25 cm -0.25 m
a)
b) The lens has NEGATIVE power and focal length
so it is CONCAVE.
27
Example 2. A lens has a power of 5D. a) What is
the focal length of the lens? b) Is the lens
convex or concave?
Solution a)
b) The lens has a POSITIVE power and focal
length. It is CONVEX.
28
The human eye is a very complex part of the body.
It is made up of various parts
The pupil of the eye is a hole which allows
light to enter the eye. The iris of the eye (the
coloured part) can change size. The iris will
contract in darkness - this makes the pupil
larger and therefore more light enters the
eye. In bright light conditions the iris makes
the pupil smaller. This reduces the amount of
light entering the eye, thus preventing damage to
the retina.
29
Side view of the eye
30
The retina at the back of the eye is the part of
the eye that is light sensitive and collects the
information before it is sent to the brain.
31
The light falling upon the eye must be focussed
on the retina.
32
This focussing occurs because as the light passes
through the cornea and the lens, refraction takes
place.
33
The lens of the eye can change shape (known as
accommodation) which controls the focal length of
the eye in order to produce a sharp image on the
retina.
34
The image formed on the retina of the eye is
upside down and also laterally inverted (back to
front).
35
When light enters the eye from a distance we
consider the light rays to be parallel. The lens
of a healthy eye is thin in order to focus these
rays on the retina.
When light enters the eye from nearby the rays of
light are diverging. The lens of a healthy eye is
fat in order to focus these rays on the retina.
Reminder The changing of shape of the lens is
known as accommodation.
36
If a person is short sighted, their eyes can
focus rays of light from nearby objects but not
from distant objects. This means they can see
nearby objects clearly. The reason they cannot
see distant objects is that the lens of their eye
cannot be made thin enough.
Short sight
This problem can be resolved by placing a concave
lens in front of the eye.
This results in the eye focusing the rays of
light SHORT of the retina.
37
If a person is long sighted, their eyes can focus
rays of light from distant objects but not from
nearby objects. This means they can see distant
objects clearly. The reason they cannot see
nearby objects is that the lens of their eye
cannot be made fat enough.
Long sight
This problem can be resolved by placing a convex
lens in front of the eye.
This results in the eye focusing the rays of
light behind the retina.
38
The focal length of a lens is the distance from
the centre of the lens to the point where
parallel rays are focused to produce a sharp
image.
To find this experimentally
Take a convex lens and produce a sharp image of a
DISTANT object on a screen. You can then measure
the distance from the lens to the image. This
measured distance is the focal length of the lens.
39
A fibre optic is a long thin solid piece of glass
or plastic which is very flexible. Fibre optics
can be used as a transmission system for cold
light. This is because only the light from the
source travels through the fibre and not the heat
from the light source. The light travels through
the fibre by total internal reflection.
40
Endoscopes (Fibroscopes) are two bundles of fibre
optic joined together. The endoscope is used to
collect visual information from inside a
patient. The first bundle of fibre carries light
into the patient. The light then reflects off
the inside of the patient. The second bundle of
fibre then carries the light back to the doctors
eye where diagnosis can be made.
41
Return to start of Health Physics Advance to
next section
42
Return to start of Health Physics Advance to
next section
43
3.4 Using the Spectrum
A laser beam is a very intense beam of light
which carries a lot of energy.
The laser beam when used in surgery will seal
blood vessels as it cuts, reducing damage to the
patient.
The laser can be used in eye surgery to repair
the retina. It can also be used to remove birth
marks and tumours from the body.
44
If an X-ray hits a piece of photographic film it
will appear as a black area on the developed film.
X-rays can be used to find broken or fractured
bones in side a persons body.
An X-ray picture is produced by placing the
patients body between an X-ray source and a
piece of photographic film.
The bones absorb the X-rays and prevent areas of
the film being hit. These areas appear as white
patches on the developed film. Parts of the bone
which are broken allow the X-rays to pass through
the bone and appear as black parts on the
developed film.
45
A C.A.T. (Computer Aided Tomography) scan is a
special type of X-ray.
Computerised tomography take a series of X-ray
slices through the body. The X-ray tube also
rotates around the body allowing slices to be
taken in all directions.
The computer can then build up 3-D pictures
making the image more useful than a 2-D (flat)
X-ray picture. The 3-D image is also very
accurate and can pinpoint tumours while they are
very small.
46
The sun gives out ultraviolet radiation. This is
the radiation that gives us a suntan.
If you spend too long in natural sunlight your
skin will burn. Excessive exposure to
ultraviolet radiation may cause skin cancer.
47
Ultraviolet radiation is used in medicine to
treat skin diseases such as acne and
psoriasis. Ultraviolet is also used to sterilise
medical instruments as it kills bacteria.
Infrared (radiated heat) radiation is used in
hospitals to treat strained muscles. Infrared is
also used in thermograms. These are coloured
pictures in which hot areas show as a red
colour. This can be used to find tumours as they
are warmer than surrounding tissue.
48
Return to start of Health Physics Advance to
next section
49
Return to start of Health Physics Advance to
next section
50
3.5 Nuclear Radiation - Humans and Medicine
Radiation can kill or change the nature of living
cells.
This can have good effects as well as bad
ones. The fact that radiation can kill living
cells allows us to do things such as 1. Use
radiation to sterilise instruments by killing the
bacteria cells. 2. Treat cancer by
radiotherapy where the incident radiation kills
the cancerous cells within the body.
51
There are three types of nuclear radiation Alpha
(a) Beta (b) Gamma (g)
Radiation energy may be absorbed in the medium
through which it passes. Alpha particles can be
absorbed by a few mm of air or a thin piece of
paper. Beta particles can be absorbed by a few mm
of aluminium. Gamma waves can be absorbed by a
few cm of lead.
52
Absorption of alpha, a, beta, b and gamma, g
Thin piece of paper
Few mm of aluminium
Few cm of lead
Note alpha and beta travel as particles gamma
travels as a wave.
53
Radioactive material is easy to detect because of
the radiation it gives out.
Hospitals can use a radioactive tracer to study
the working of various parts of the body.
To do this they inject the patient with a
radioactive source. This source is usually a
gamma source as it can be detected outside the
body. The doctors would then use a gamma camera
to follow the tracer around the patients body.
54
The atom is made of three main components. Protons
, neutrons and electrons.
Within the atom is the nucleus which is made up
of neutrons and protons. The electrons are found
orbiting this nucleus at different
levels. Protons are positively charged. Neutrons
are neutral. Electrons are negative.
55
Electron (-)
Proton()
Neutron
This atom has as many positive protons as it has
negative electrons it is therefore overall
neutral.
56
If the atom was to lose one or more electrons, it
loses negative charge.
57
If the atom was to lose one or more electrons, it
loses negative charge.
The atom then has more positive charge than
negative. It is therefore carrying an overall
positive charge.
58
If the atom was to gain one or more electrons, it
gains negative charge.
59
If the atom was to gain one or more electrons, it
gains negative charge.
The atom then has more negative charge than
positive. It is therefore carrying an overall
negative charge.
60
An atom can be forced to gain or lose electrons
from their orbit around the nucleus when put in
the path of a radioactive particle or wave.
This change in charge due to the gain/loss of
electrons is called ionisation.
Alpha particles cause more ionisation than beta
particles or gamma rays.
An atom carrying a non-neutral charge is known as
an ion
61
Radiation will fog photographic plates. This is
used in film badges, used to monitor a workers
exposure to different types of radiation.
Top view
0.3mm aluminium
0.1mm plastic
Film badge
0.1mm aluminium
1mm lead
Side view
Windows
Photographic film
62
By studying the level of fogging underneath each
window, we can tell how much alpha, beta and
gamma radiation the badge (and the person wearing
it) has been exposed to.
Top view
0.3mm aluminium
0.1mm plastic
Film badge
0.1mm aluminium
1mm lead
Side view
Windows
Photographic film
63
Any radioactive source is said to have activity
which is measured in becquerels (Bq). One
becquerel is defined as one atom decaying per
second.
The activity of any source will reduce with
time. The rate of reduction of activity is known
as a sources Half - Life.
If a radioactive source has a half-life of, say,
two minutes. The rate at which it decays reduces
by a half every two minutes.
For example ...
64
The activity of an unknown source is measured
every minute for 10 minutes using a Geiger -
Muller tube and counter. The results are shown
below
The results show that as time passes the activity
of the source is decreasing. We could then plot
a graph of results ..
65
The graph can be used to find the half-life of
the source.
66
480
t1
Select a point on the trendline which crosses
points exactly on both axes.
At t1 the activity 480 Bq
67
480
240
t1
t2
Now find the point on the graph where the
activity is half the previous value. (240 Bq)
68
480
240
t1/2
t1
t2
The half life is the time taken for activity to
drop by half.
t1/2 t2 - t1 240 - 120 120 s 2 minutes
69
It is also possible to calculate the half life of
a source without using a graph.
Example A source has an original activity of
12000 Bq. After 24 days the activity has dropped
to only 750 Bq. What is the half life of the
source?
12000
6000
3000
1500
750
t2
t3
t4
t1
4 half lives have passed in 24 days. So, One half
life
6 days
70
When working with radioactive substances be sure
to follow these simple safety procedures
1. Never point a radioactive source towards
anyone (including yourself). 2. Always use
forceps or tongs to handle sources. 3. Store
sources in lead lined boxes. 4. Always label
radioactive sources
71
For living materials the biological effect of
radiation depends on two things. These are The
absorbing tissue itself, and The nature of the
radiation. Dose equivalent (measured in
sieverts) takes account of the type and energy
of radiation. The abbreviation for sieverts is
Sv.
72
Return to start of Health Physics
73
Return to start of Health Physics