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Transcript
LAB #77 NUCLEAR RADIATION
April 28, 2017
Figure 1. Set-up for Experiment: Instrument A
Figure 1. Set-up for Experiment: Instrument B
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LAB #77 NUCLEAR RADIATION
April 28, 2017
OVERVIEW
All matter is known to contain varying amounts of energy. This energy is stored in the
different particles that comprise the matter. This matter is composed of chemical elements,
which consist of atoms. Atoms in turn are composed of even smaller particles; protons, neutrons,
and electrons.
It is known that all atoms of the same chemical element are not always identical. Atoms
of the same element will always have the same number of protons, but may have a different
number of neutrons. This results in the atoms having different atomic weights. Atoms of the
same element that have different numbers of neutrons (and therefore different atomic weights)
are called isotopes.
Any given combination of protons and neutrons is called a nuclide. Nuclide symbols are
a sort of shorthand that is used to describe different nuclides. For example, the three isotopes of
hydrogen are described with nuclide symbols as follows.
Ordinary Hydrogen.
Tritium
1
1𝐻
Deuterium 21𝐻
3
1𝐻
The subscript in a nuclide symbol always gives the atomic number of the atom. The
symbol Z is used for the atomic number. The atomic number tells you how many protons are in
the nucleus of an atom. As mentioned earlier, the number of protons is always the same in atoms
of the same element. You can tell from the nuclide symbols for the three isotopes of hydrogen
that they all have the atomic number (Z = 1). This shows that they all have one proton in their
nucleus.
The superscript in a nuclide symbol always gives the mass number (AKA atomic weight)
of the atom. The symbol A is used for the mass number. The mass number tells you the
combined number of protons and neutrons in the nucleus of an atom. In atoms of the same
element, the number of neutrons may vary. Looking at the nuclide symbols for the three isotopes
of hydrogen above, you can tell that they have 0, 1, and 2 neutrons in their nuclei. The masses of
most isotopes are given in atomic mass units. One atomic mass unit (amu) is a very small unit
of mass equal to one-twelfth of the mass of an isotope called carbon-12. An atomic mass unit is
equal to 1.6605 x 10-27 kilograms, and is specified by the letter u.
Isotopes of certain elements are unstable and therefore are in a process of decay. As they
decay they emit unseen radiations. This phenomenon is called radioactivity, and such isotopes
are called radioisotopes. All elements with atomic numbers greater than 82 (and some with
smaller atomic numbers) possess naturally radioactive isotopes. In addition, artificial
radioactive isotopes can be created by bombarding certain stable isotopes with particles. To date,
over 2,000 radioisotopes have been discovered.
The unseen radiation emitted by unstable isotopes consists of alpha particles, beta
particles, and gamma rays. Alpha particles are helium nuclei. Alpha particles have a positive
charge and move at speeds of thousands of miles per second. They can, however, be stopped by a
thin sheet of paper or a few centimeters of air. Beta particles are electrons and also move at fast
speeds. It is possible to stop beta particles with a few hundred sheets of paper. Gamma rays are
similar to X rays, but have a shorter wavelength. Gamma rays can penetrate thick lead plating
and slabs of concrete.
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LAB #77 NUCLEAR RADIATION
April 28, 2017
As nuclear radiation travels away from its source, its strength decreases. This weakening
is inversely related to the square of the distance from the radiation source. This is known as the
inverse square law. All forms of radiation follow this law. Therefore, unless you know where a
radiation source is located, you need a sensitive device to detect it.
A common method of detecting alpha and beta particles and gamma rays is to use a
Geiger-Mueller counter (commonly referred to as a Geiger Counter). The nuclear scaler you will
use in this lab has a Geiger-Mueller tube mounted inside its cabinet. A diagram of the tube is
shown in Figure 2.
The tube consists of a metal cylinder containing two electrodes and a gas filling. The
positive electrode (anode) is a thin metal wire in the tube. The wall of the tube acts as the
negative electrode (cathode). At the sensor end of the tube there is a thin window of fragile mica,
which allows radiations to penetrate.
When a ray or ionizing particle enters the tube, it causes the gas to ionize. This causes a
pulse of electricity to be sent to the scaler, which triggers some device to indicate that radiation is
present. In the scaler for this lab the device that indicates the presence of radiation is a counter.
In other scalers, the device may be a lamp which will flash. Still other devices use a loudspeaker
which may provide the familiar clicking noise usually associated with Geiger counters.
All Geiger-Mueller tubes do not operate well at the same voltage because of differences
in the construction and gas filling of the tubes. Therefore you will need to find the operating
voltage of your tube by plotting a Geiger-Mueller plateau.
After you find the operating voltage of the tube, you will use the scaler to demonstrate
how the strength of the radiation decreases as you move the sample away from the tube (the
inverse square law). Then you will isolate (shield) the tube from alpha, beta, and gamma
radiation using samples of different materials and thicknesses. This will demonstrate the
penetrating power of the different samples, and the radiation absorption qualities of the
materials.
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LAB #77 NUCLEAR RADIATION
April 28, 2017
OBJECTIVES
1. Find a proper operating voltage of a Geiger-Mueller probe by plotting a Geiger Plateau
graph.
2. Detect nuclear radiation in the form of alpha particles, beta particles and gamma rays
using a Geiger-Mueller probe.
3. Show how the strength of nuclear radiation decreases inversely with the square of the
distance from the radiation source.
4. Show how the three different types of radiation exhibit different penetrating power.
EQUIPMENT
Radiation Demonstration Set
PROCEDURE
A. Plotting the Geiger Plateau
In this section you will find the optimum voltage
setting for your Geiger-Mueller Counter by
plotting a Geiger Plateau.
1. Plug one end of the cord into the Counter
and the other into the bench outlet. Turn
the power on.
2. Press the "TIME" button. The red light
above the button will light up to show that
the Counter is displaying the time in
seconds. Press the "UP" button until the
time reads 60 seconds (if you go too far
you can use the "DOWN" button). 60
seconds is the time interval over which
each radiation measurement will take
place. Press the "TIME" button twice to
make the red light go off. The Counter is
no longer displaying the time.
3. Press the "H.V." (HIGH VOLTAGE) button. You can tell you're in H.V. mode because
the red light above the button goes on. Press the "UP" button until you get to 260 volts.
Press the "H.V." button again to make the red light go off. The Counter is no longer
displaying the voltage.
4. Place the beta sample into the second sample holder shelf from the top. Make sure the
sample's opening is facing up (paper side down). We'll call this shelf "Sample Shelf 1".
The top shelf should not be used because it is too close to the Geiger-Mueller tube, and
any contact with the tube itself could damage the mica lens. See Figure 2. Make sure
the samples not in use are at least a half meter from the Geiger-Mueller tube, to
ensure that they will not affect the readings.
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LAB #77 NUCLEAR RADIATION
April 28, 2017
5. Press the COUNT button. The Counter is now counting. The red light above the
"COUNT" button will remain lighted until the 60 second time interval has concluded.
Record the voltage (260 V) and the count reading in the first row of Table 1.
6. Press the "H.V." button. Increase the voltage by 60 volts to 320 volts. Push the "H.V."
button again to return the display to "COUNT". Push the "COUNT" button and begin the
next count. When the "COUNT" light shuts off, record the H.V. setting and the 60
second count in Table 1.
7. Continue to increase the voltage in 60 volt increments and record the data until your 60
second count more than doubles from the previous reading. At this point the GeigerMueller probe has gone into continuous discharge and further experiments at this voltage
(or higher) could damage the probe.
8. Plot your data on the graph provided. The optimum operating voltage is somewhere
around 75 volts beyond the voltage where your graph levels off to a plateau. Use the
sample plot in Figure 3 as a guideline. Mark this point on your graph and set your High
Voltage ("H.V.") close to this value. Record this value at the bottom of Table 1 under
"Operating Voltage". The voltage should remain at this value for the remainder of the
experiment.
B. Day 1: Shelf Ratios
All types of radiation closely obey the inverse square law: the amount of radiation declines as the
square of the distance. In this section we will try to confirm the inverse square law
experimentally using the beta and gamma samples. The alpha sample won't be used, since these
particles can be stopped by just a few centimeters of air.
1. Take a 60 second count with the trays empty to determine the background radiation levels
and record the value under "Background Reading".
2. Place the beta sample in the sample tray. Slide the sample tray into the second shelf from
the top of the probe and take a 60 second count. As before, the top shelf should not be
used because it is too close to the Geiger-Mueller tube, and any contact with the GeigerMueller tube itself could damage the mica lens. Subtract the background count from your
reading and record the result for shelf one of the beta sample under "Counts per Minute"
in Table 2.
3. Move the sample tray to the next shelf down and take a 60 second count. Subtract the
background count and record the result for shelf 2 in Table 2. Repeat the process for the
remaining three shelves.
4. Repeat steps B-1 through B-3 for the gamma sample.
5. Verifying the inverse-square law: Assign a shelf ratio of 1.00 to the second shelf β€” this
will be your standard shelf. For the remaining shelves, the shelf ratio the # of counts per
minute for that shelf divided by the number of counts per minute recorded for your
standard shelf (that's why the standard shelf ratio is 1.00). In equation form it's
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LAB #77 NUCLEAR RADIATION
April 28, 2017
Shelf Ratio =
π‘ͺ𝒐𝒖𝒏𝒕𝒔 𝒑𝒆𝒓 π‘΄π’Šπ’π’–π’•π’† 𝒐𝒇 𝑺𝒉𝒆𝒍𝒇
π‘ͺ𝒐𝒖𝒏𝒕𝒔 𝒑𝒆𝒓 π‘΄π’Šπ’π’–π’•π’† 𝒐𝒇 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝑺𝒉𝒆𝒍𝒇
𝟏
6. In the column marked " βˆšπ‘Ίπ‘―π‘¬π‘³π‘­ 𝑹𝑨𝑻𝑰𝑢 ", do just as it says. Calculate the square shelf ratio
root of the inverse of the shelf ratio. If, for example, the shelf is twice as far from the
probe as the standard shelf, the count should be about one-fourth as much, and the value
in this column should be close to 2. A shelf that is three times as far away should have a
value in this column close to 3. You're using the counts and the inverse-square law to
verify the distance from the probe relative to the standard shelf
7. Repeat steps B-5 and B-6 for the gamma sample.
C. Day 2: Absorption of Radiation by Matter
Absorption of radiation by matter depends on the type of emitted radiation. In this part of the
experiment you will observe the absorption of alpha, beta and gamma radiation by different
materials.
1. Record the background reading from Table 2 onto the top of Table 3 for easy reference.
Also transfer the shelf 1 readings for the beta sample and the gamma sample to the top
row of Table 3 for the beta and gamma sample (no use taking the same measurement
twice!).
2. Place the alpha sample on the sample tray and slide it into shelf 2. Take a 60 second
count without an absorption material over the sample. Subtract the background reading
from the count and record the result in Table 3.
3. Place the first absorption material (the thinnest weight of the polyethylene) over the
sample. Make sure that the holder for the absorption material covers the entire recess of
the sample holder. Take a 60 second count. Subtract the background reading from the
count and record the result in Table 3. If the result is close to the value of the background
count, you've effectively blocked the radiation and you can move on to the beta sample.
Otherwise, try successively thicker absorption materials until you have reduced the
radiation detected to near-background levels. There are more absorption materials than
you need, so use every other absorption material. That is, try the thinnest material, then
the third thinnest, then the fifth thinnest, and so on. Record the type of material tested in
the first column of Table 3.
4. Repeat the process for the beta sample and the gamma sample. I'll warn you that nothing
stops the gamma sample from being detected so, if you're short on time, make sure you
try at least four or five absorption materials, and make sure the last one is the thickest
absorption material you have.
5. Turn off the Geiger-Mueller tube. Fill out the analysis page. Make sure you record the
sample labels on the analysis page before you turn them in.
6