2.4 The deflection of beta radiation in a magnetic field. Task. How does β-radiation behave in a magnetic field?

Transcription

1 Science - Physics - Radioactivity - 2 Types of radiation and their characteristics (P ) 2.4 The deflection of beta radiation in a magnetic field Experiment by: Phywe Printed: Oct 16, :04:14 PM intertess (Version B200, Export 2000) Task Task How does β-radiation behave in a magnetic field? Determine, if β-radiation is deflected in a magnetic field. Use the space below for your own notes. Logged in as a teacher you will find a button below for additional information

2 Additional Information The Lorenz force acts on β-particles which move perpendicularly to the direction of a magnetic field. At constant velocity and magnetic field strength, the β-particles move through the field in a circular path, the radius of which is dependent upon their velocity and the magnetic field strength. As β-particles exhibit a continuous energy spectrum, they are deflected by a magnetic field to different extents. This makes it possible to experimentally determine the proportions of the various energy values, for example, by evaluating the count rates C measured for pre-determined paths as a function of the magnetic flux density B. The simplified experimental set-up used here allows the following knowledge to be won: 1. β-radiation consists of electrically charged particles, as it is deflected by a magnetic field. 2. β-particles have a negative charge, as the direction of deflection is opposite to that which is to be expected form the three finger rule. 3. The stronger the magnetic field, the greater the deflection. When the direction of the field is reversed, deflection is in the opposite direction. 4. β-particles have various energies, as they are deflected to different amounts. Notes on set-up and procedure This experiment only gives satisfactory results when the set-up and procedure are carefully carried out. Pay particular attention to the following conditions: The magnetic field should be at the centre point of the angle graduation. The exit opening of the source of radiation should be in front of the magnetic poles. The distance betweeen the source of radiation and the counter tube window should on no account be changed when the counter tube is moved on the angular scale, as this would lead to large differences in the count rates. Mark the position of the counter tube in the counter holder to avoid displacement of the counter tube. The counter tube axis runs radially, when both of the counter tube holder markers point to the same angle graduation. When the suggested 60 s time of measurement is used, a measurement series takes 19 minutes. Should not enough time be available, a shorther gatetime of 10 s can be selected. If this is done, however, then the statistical uncertainty is higher, so that 3 measurements should be made at each position and the average calculated. It is emphasized to use the Geiger-Müller counter Type B (Order No ) for this experiment. The Geiger-Müller counter 45 mm(order No ) detects because of the large cross section dimension a very huge angular range, so that satisfactory results cannot be achieved

3 Attention The counter tube window of the Geiger-Müller counter Type B is not guarded. The danger that touching the counting tube window can destruct it should be particularly stressed. As a matter of principle, the students should therefore only remove the protective cap shortly before beginning the measurements and replace it directly after they are finished

4 Material Material Material from "TESS advanced Physics set Radioactivity, RE" (Order No ) Position No. 1 Material Base plate for radioactivity Order No Quantity 1 2 Cobra4 Mobile-Link Cobra4 Sensor-Unit Radioactivity Plate holder on fixing magnet Deflecting magnets for plate holder, 2pcs Additional Material 6 Source Ra-226, 3 kbq Source holder on fixing magnet Counter tube holder on fixing magnet Counter tube, type B Material required for the experiment - 4 -

5 Setup Setup Attention Although the activity of the source of radiation used is very low with 3 kbq, the source should only be removed from the storage container for the length of the experiment. The general standards for the handling of radioactive substances according to the appropriate state law which regulates this, must be followed. Setup Fix the deflecting magnets to the plate holder with the milled screws (Fig. 1-4). The distance between the two magnets should be 2 cm (Fig. 5). Fig. 1 Fig. 2 Fig. 3 Fig

6 Fig. 5 Position the plate holder on the base plate so that the centre points of the deflecting magnets are immediately above the centre point of the angular scale (Fig. 6). Fig. 6 Insert the source of radiation in the source holder (Fig. 7-8). Place the source holder in front of the plate holder (Fig. 9) and slide the source of radiation until its exit opening is in front of the deflecting magnets (Fig. 10). Now remove the plate holder from the base plate, without displacing the source of radiation (Fig. 11)

7 Fig. 7 Fig. 8 Fig. 9 Fig. 10 Fig. 11 Position the counter tube holder on the base plate so that the tip of its positioning marker points at the intersection of the 0 angle division and the semi-circle of radius r = 5 cm (Fig. 13)

8 Fig. 13 Insert the Geiger-Müller counter tube in the counter tube holder and move it until the counter tube window is directly above the tip of the positioning marker (Fig ). Fig. 13 Fig. 14 Connect the Mobile-Link to the Sensor-Unit (Fig. 15). Fig. 15 Connect the counter tube to the Sensor-Unit (Fig. 16)

9 Fig. 16 Switch on the Mobile-Link, open the menu and select under the menu item sensor the measuring range impulse per minute and change the gatetime to 60 seconds. Under the menu item datalogger select the single value measurement. Fig

10 Action Action Carefully remove the protective cap from the counter tube (Fig. 18), start the first measurement by pressing the enter button. Note the first count rate in Table 1 on the Results page. Fig. 18 Fig. 19 Move the counter tube holder to the 10 graduation on the angular scale, making absolutely sure that the distance of the counting tube from the source of radiation does not change and that the axis of the counter tube is exactly aligned along the angle graduation. Start the next measurement and enter the count rate in Table 1. Repeat this measurement with all of the angles from +90 to -90 listed in Table 1. Fig. 20 Place the plate holder with the deflecting magnets on the centre point of the angular scale (Fig. 21)

11 Fig. 21 Determine the count rates for all listed angles as before, but now with the deflecting mangets in position (Fig 22-23). Enter the count rates in Table 1. Fig. 22 Fig. 23 Repeat the measurement series after having transposed the magnets in the plate holder (Fig ). Fig. 24 Fig

12 Fig. 26 Repeat the mesurement series with the deflecting magnets at a different distance (Fig ). Fig. 27 Fig. 28 On completion of this measurement series, replace the protective cap on the counter tube and replace the source of radiation in the container

13 Results Results Table 1 Angle without magnets with magnets magnets transposed magnets closer together in degrees C in counts / 60 s C in counts / 60 s C in counts / 60 s C in counts / 60 s If you measured accurately, you should get similar values:

14 Angle without magnets with magnets magnets transposed magnets closer together in degrees C in counts / 60 s C in counts / 60 s C in counts / 60 s C in counts / 60 s

15 Evaluation Evaluation Question 1 Print the funtional diagram that you can download by clicking this link. Enter all the measured values in the functional diagram. Use different coloured pencils or markers for the values obtained with and without the deflecting magnets. Draw seperate connecting lines through the measured values of each of the measurement series. See Fig. 29. Black: without magnets; red: with magnets; blue: magnets transposed; green: magnets closer together. Fig. 29 Question 2 Draw a line in each of the resulting curves to show the direction in which the highest count rates were obtained

16 See Fig. 29. Frage 3 Which effect do the magnets exert on the movement of the β-radiation? The β-particles are deflected from their original direction of movement by the magnetic field. The direction of the deflection depends upon the direction of the magnetic field. The intensity of the β-particles is distributed over a wide angle area, which means that they are deflected to differing amounts. This indicates that they are emitted with differing energies

17 Question 4 Compare the experimental results with the behaviour of a conductor carrying current in a magnetic field. The behaviour of β-particles in a magnetic field is similar to that of a conductor carrying current, because they also represent a current of charged particles. The direction of deflection of the β-particles is opposite to that which is to be expected from the three finger rule for the deflection of conductor with electric current flow, because this rule refers to the technical direction of electric current. β-particles must therefore have a negative charge

18 Question 5 Which influence does the distance between the magnets have on the deflection of the β-radiation? Explain this observation. When the distance between the deflecting magnets is decreased, the magnetic flux density is increased. This increased strength of the magnetic field in the space between the magnets causes the β-particles to be morge strongly deflected