BME355 Lab Listing: Ionizing Radiation

Ionizing
Radiation

Preparation

Lab Outline

Procedure

Learning more

Procedure

1     Become familiar with the scintillator. Check the descriptions of the controls and functions.

2     Get a spectrum

2.1         Settings:

2.1.1                     Place the ¹³⁷Cs source in the holder.

2.1.2                     Set the detector to:

·         range = 1MeV

·         Low limit= 0

·         High limit=100

2.1.3                     Set the timer to 1 minute.

·         You need at least 10000 counts in order to get low error, if the reading is higher, you might decrease the time; if it is lower you should increase the time.

2.2          Measure activity, record counts per minute (cpm) in table1

2.3          Increase low and high window setting in steps of 100 and repeat counting.

2.4          Plot cpm versus threshold.

2.5          Find the photopeak

2.6          Repeat 2.1 – 2.5 for another radioisotope

2.7          Repeat 2.1 -2.6 using range =2MeV

2.8          Analyze the results; explain why there are so many counts at low energies, and why decreasing window amplitude results in a lower cpm. Explain the effect of changing the range.

3                Study the effect of window size

3.1          set lower window to 100 lower than the middle value of the window where you saw the peak for the Cs source .

3.2          Set the window size to 20 and count activity.

3.3          Increase threshold 20 and count.

3.4          Repeat until you get 100 over the peak

3.5          Repeat 3.1-3.4 for the second radioisotope, range 1MeV

3.6          Repeat 3.1-3.4 for the second radioisotope, range 2MeV

TABLE 1

Radioisotope

window

Range 1MeV

Range 2Mev

Range 1MeV

Range 2Mev

0-100

100-200

200-300

300-400

400-500

500-600

600-700

700-800

800-900

900-1000

4                The photopeak of ¹³⁷Cs is 662KeV, find the theoretical photopeak fro the other radioisotope you use. Based on the theoretical values and your measurements compute the calibration factor of the scintillator at 1MeV and a 2MeV.

5                 Interaction of gamma rays with matter.

Radioactive sources and a scintillator detector will be used for this purpose.

5.1           Settings: based on your results from part 2, select the settings around the photopeak for ¹³⁷Cs, with a window size of 200

5.2          Measurements:

5.2.1        Measure the background activity.

5.2.2        Place the gamma source (137 Cs) at 5 cm from the detector. Count.

5.2.3        Place a sheet of paper between the detector and the source. Count.

5.2.4        Replace the paper by Plexiglas and count, repeating for 2 and 3 sheets of Plexiglas.

5.2.5        Replace Plexiglas by lead and count for 1, 2 and 3 sheets.

5.2.6        Repeat for the second source.

5.3          Plot thickness vs cpm for each source.

5.4          Estimate µ, the attenuation coefficient, of lead and plexiglass for 137 Cs.

5.5          Estimate µ, of lead and plexiglass for the other radioisotope

5.6          Compare your value with tables.

5.7          Compute HVL (half value layer).

5.8          Different materials have different effects in stopping radiation. The explanation for this effect is given by this equation:

where: A₀ is the initial activity,A₁ is the activity after a shielding of thickness x₁µ is the attenuation coefficient for a certain material at a given energy.

TABLE 2

material

thickness

radioisotope 1 (           )

radioisotope 2 (         )

None

plexiglass

(1)

(2)

(3)

lead

(1)

(2)

(3)

(4)

Second session:

6                Dose

In this section, the effect of irradiation time will be studied.

A "Fricke solution" will be irradiated (with assistance of authorized personnel) and analyzed with a spectrophotometer.

An x-ray machine will be employed for the irradiations.

A.      In room HRT-211:

1.A.1      Turn ON the spectrophotometer so that it starts to warm up.

1.A.2      Place 6 petri dishes on the bench and fill each of them with 6 ml of Fricke solution.

1.A.3      Let them rest for 2 or 3 minutes and discard the solution (use the vacuum system for this).This step cleans and pre-conditions the dish surfaces.

1.A.4      Label the dishes: control A, control B, and test 1 to 4.

1.A.5      Fill each again with 10 ml of Fricke solution, covering the dish after pouring the solution. This is very important since the solution is oxygen sensitive.

B.       Go to the x-ray machine room (HRT 239).

1.B.1        Warm up the x-ray machine. You will find the instructions for this on the room wall.

1.B.2        Use the collimator and the light to position the tube. For this, turn off the brake-button and move the tube until the cross-pointer coincides with the mark on the table. At this position, turn the brake on.

1.B.3        Turn off the light and remove the collimator.

1.B.4        Use the wheel to place the tube 50 cm from the table.

1.B.5        Verify that the tube is horizontal with the bubble-level.

1.B.6        Place the four test dishes around the cross-mark.

1.B.7        Keep the control dishes in the control room.

1.B.8        Leave the x-ray generator room and close the door.

1.B.9        Set voltage to 250 keV, time to 3 minutes and proceed to irradiate.

1.B.10    When the irradiation ends, open the door, remove the dish labeled Test 1 and close the door.

1.B.11    Irradiate the remaining dishes for another 3 minutes.

1.B.12    When irradiation ends, remove dish labeled Test2 and close the door.

1.B.13    Irradiate the remaining dishes for another 3 minutes.

1.B.14    When irradiation ends, remove dish labeled Test3 and close the door.

1.B.15    Irradiate Test 4 dish for another 3 minutes.

1.B.16    Remove the dish and follow the procedure for turning off the machine, also posted on the room wall.

C.      Go back to the laboratory with the spectrophotometer.

1.C.1       Measure the absorbance of each sample at 224nm and 304nm. As the absorbance is also sensitive to temperature changes, allow the temperature to stabilize at 25C before readings. For this, the spectrophotometer is equipped with a temperature controller.

1.C.2       Fill out the table and compute the doses.

Explain your results.

1                Observe the interaction of radiation with matter.
For this section, a Superheated Emulsion Chamber (SEC) will be used.

The chamber contains a gel with radiation sensitive drops. The system is pressurized. The instructor will place a seed source into the SEC and after will depressurize. The seed will be removed after few seconds.

Observe the bubble distribution. A digital camera is set to acquire images from the SEC. You will see the bubble distribution on the screen. Save the file and print it.

Analysis of the data:

Make concentric circles around the seed, spaced 0.5 cm. Each ring represents a region in your sample. Count the amount of bubbles in each region. Calculate the area of the ring and normalize your values:

counts = number of bubbles/area of the ring

Plot a profile (counts vs region). How close is the distribution to the 1/r^2?