Studying the nuclear fission of a uranium atom from photographs of tracks
Target: Confirm the validity of the law of conservation of momentum using the example of fission of a uranium nucleus.
Equipment: photograph of tracks of charged particles (Fig. 1), obtained in a cloud chamber during the fission of nuclei of uranium atoms under the influence of a neutron, reference tables “Relative atomic mass of some isotopes”.
Read the rules and sign that you agree to comply with them .
There should be nothing foreign on the table while working.
___________________________
Student signature
Progress:
1. Repeat § 66
Questions for self-control: a) What forces act in the nucleus of an atom? b) Why does the nucleus not decay into individual nucleons? c) What happens to the uranium nucleus when it absorbs a neutron? How does nuclear fission occur? d) How is the law of conservation of momentum formulated? e) Why do the fragments of the nucleus scatter in opposite directions? f) What energy does part of the internal energy of the nucleus transform into during its division?
2. Look at the photograph (Fig. 1).
|
3. Complete the tasks: 1) Using the law of conservation of momentum, explain why the fragments formed during the fission of the nucleus of a uranium atom scattered in opposite directions. To do this, answer the questions: a) what was the momentum of the nucleus of a uranium atom before a neutron hit it? __________________ b) what should be the total momentum of the fragments formed during fission? ________________ _______________________________________________________________________________________ c) what should be the magnitude and direction of the impulses of the fragments? ___________ _______________________________________________________________________________________________________________________________________________________________________________
2) It is known that fragments of the uranium nucleus are the nuclei of two different atoms chemical elements from the middle of D.I. Mendeleev’s table. One of the possible fission reactions of uranium 235 U can be symbolically written as follows:
92 U + 0 n → 56 Ba + Z X + 2 ∙ 0 n,
where the symbol Z X denotes the nucleus of an atom of one of the chemical elements. Using the law of conservation of electric charge and D.I. Mendeleev’s table, determine what kind of element it is. _____________________________________________________________________ _____________________________________________________________________________________________________________________________________________________________________
3) Explain why the tracks of different particles in the photograph have different thicknesses? _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
CONTROL QUESTIONS:
1. Why can nuclear fission begin only when it is deformed under the influence of a neutron absorbed by it? ______________________________________________________________ ________________________________________________________________________________________________________________________________________________________________________________
2. How can you determine in which direction the particle is moving based on the type of track? ___________________ ________________________________________________________________________________________________________________________________________________________________________________
3. What determines the thickness of particle tracks? _______________________________________________ ________________________________________________________________________________________________
4. What does the length m of a particle depend on? _________________________________________________ ________________________________________________________________________________________________ _______________________________________________________________________________________
5. How does the fission reaction of uranium nuclei proceed: with the release of energy in environment or, conversely, with energy absorption? _______________________________________________________ _______________________________________________________________________________________
*Additional task
Using the photograph in Fig. 1, based on the law of conservation of momentum based on the known masses of fragments, find the ratio of the velocities of particles formed as a result of a nuclear reaction. For this:
a) Write down the formula for the law of conservation of momentum for nuclear fragments. _________________________________
_______________________________________________________________________________________
b) From the formula for the law of conservation of momentum, express the ratio of particle velocities. ____________
______________________________________________________________________________________
c) In the reference table " Relative mass some isotopes" find the masses of the resulting fragments. Write it down in the table.
d) Find the ratio of the masses of the fragments of the uranium nucleus. ___________________________________________ ________________________________________________________________________________________________
e) Write down the ratio of the velocities of the resulting fragments. _____________________ ________________________________________________________________________________________________
f) Fill out the table.
g) Conclude what is the relationship between the masses of the formed fragments and their velocities. _______________________________________________________________________________________ ________________________________________________________________________________________________________________________________________________________________________________
Grade “_______” Teacher’s signature ___________________________
Study of uniformly accelerated motion
Target: Determine the acceleration of the ball and its instantaneous speed before hitting the cylinder
Equipment: a tripod with a coupling and foot, a groove, a ball, a metal cylinder, a measuring tape, a metronome or a clock with a second hand.
Safety regulations. Read the rules carefully and sign that you agree to comply with them. .
Place equipment and materials on your workbench in such a way as to prevent them from falling. There should be no foreign objects on the table.
I have read the rules and agree to comply. ___________________________
Student signature
Progress:
1. Repeat § 5, 7. 8.
Questions for self-control: 1) What kind of motion is called uniformly accelerated? 2) What is called acceleration? 3) How to determine the displacement of a body during uniformly accelerated motion? 4) How to determine the displacement of a body moving uniformly accelerated from a state of rest? 5) How to determine the acceleration of a body? 6) How to determine the acceleration of a body moving from a state of rest?
2. Using a tripod, secure the chute in an inclined position at a slight angle to the horizontal. The inclination should be such that the ball travels the entire length of the groove in at least four metronome beats. At the bottom end of the gutter, place a metal cylinder in it.
3. Having released the ball (at the same time as the metronome strikes) from the upper end of the groove, count the number of metronome strikes until the ball collides with the cylinder (so that the cylinder does not move at the moment of impact, it must be held with your hand). It is convenient to carry out the experiment at 120 beats of the metronome per minute. In this case, the interval between impacts is Δt = 0.5 s.
4. By slightly changing the angle of the groove and making small movements of the metal cylinder, ensure that between the moment the ball is released and its collision with the cylinder there are 4 beats of the metronome (3 intervals between beats).
5. Calculate the time it takes for the ball to move using the formula t = 0.5 * (n – 1), where n is the number of metronome beats. t = ___________________________________________________________________ s
5. Using a measuring tape, determine the ball displacement modulus s (from the top edge of the groove to the cylinder)
6. Without changing the angle of the gutter, because the conditions of the experiment must remain unchanged, repeat the experiment 5 times, achieving the most accurate coincidence of the moments of impact of the metronome and the collision of the ball with the cylinder (for this, the cylinder can be slightly moved along the groove). Measure the movement of the ball each time.
CONTROL QUESTIONS:
1. Does the magnitude of acceleration depend on the time of motion of the ball? from the movement module? _______________________________________________________________________________________ ________________________________________________________________________________________________________________________________________________________________________________
2. Determine which of the following dependencies describes uniformly accelerated motion:
S = 5 + 2t, S = 2t, S = 2t + 3 t2, S = 2t – 5 t2, S = 5 t2, S = 5 + 3t + 2 t2, S = 2 – 3t + 2 t 2
_____________________________________________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________________________________________
3. How long would the ball move with the same acceleration if the length of the trench was 2 m? ______________________________________________________________________________________________________________________________________________________________________________ _______________________________________________________________________________________________ _______________________________________________________________________________________________
4. Solve the problem: A skier slides down a mountain, moving in a straight line with a constant acceleration of 0.1 m/s 2 . Write down an equation expressing the time dependence of the coordinates and projection of the skier’s velocity vector if his initial coordinates and velocity are zero. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
*Additional task
1. Change the slope of the gutter, for example, increase it.
2. Carry out experiments by repeating the actions with the ball described in paragraphs 2 – 9, find a c p 2
3. Compare a with p 2 and a with p. _________________________________________________________________________________________________________________ _____________________________________________________________________________________________________________________________________________________
4. Conclude how the acceleration of the ball’s movement changes with an increase in the angle of inclination of the chute. _______________________________________________________________________________ ________________________________________________________________________________________________________________________________________________________________________________
5. Conclude whether the acceleration of the ball depends on the angle of inclination of the chute? If it depends, then how exactly? _________________________________________________________________ ________________________________________________________________________________________________________________________________________________________________________________
Grade “______” Teacher’s signature ___________________
Student name _________________________ Class _________
Laboratory work No. 2 _____________________
The main properties of fission fragments are high kinetic energy, radioactivity and the ability to emit prompt and delayed neutrons. During the fission of uranium-235 by thermal neutrons, the specific yield of fission fragments is sharply asymmetric in mass (Fig. 8.3).
The probability of the appearance of a particular fragment is statistical in nature. The average ratio of the masses of light and heavy fragments is equal to . The probability of a nucleus dividing into three parts is 10 -2 10 -6 from the probability of division into two parts. The highest yield (6%) is for fragments with mass numbers 95 and 139. The initial velocity of a light fragment is equal to 1.4. 10 11 m/s, and heavy - 10 11 m/s.
The fission fragment yield curves of other nuclei fissile by thermal neutrons (233 U, 241 Pu) are similar. In addition, asymmetric fission is observed during the forced fission of all elements, starting with Th, if it is caused by neutrons of not very high energy, as well as during the spontaneous fission of heavy nuclei. In all cases of nuclear fission at low excitation energies, the mass curve of fragments turns out to be “two-humped.”
gi, %
70 80 90 100 110 120 130 140 A
Rice. 8.3. Specific yields of fission fragments of various atomic masses
during fission of nuclei 235 U (solid line) and 239 Pu (dashed line)
As the nuclear excitation energy increases, fission becomes symmetrized. So, when a uranium nucleus is fissioned by protons with E= 32 MeV the probability of symmetric fission increases, and at an excitation energy of 150 MeV the mass curve becomes “single-humped”.
Physical processes of nuclear fuel poisoning
Nuclear fission can occur in many ways. More than 400 different fragment nuclei were discovered during the fission of the 235 U nucleus by thermal neutrons. In addition, fission fragments in the process of - - and -decay are transformed into other nuclei. Thus, about 600 different nuclides can be counted in the reactor core. Among them there are nuclei that strongly absorb neutrons.
Short-lived radioactive fission products in a nuclear reactor, having large absorption cross sections and participating in unproductive neutron capture, are calledpoisonous products (or neutron poisons).
The kinetics of poisoning refers to the process of changing the concentration of these short-lived nuclides over time, and poisoning fuel (or reactor poisoning) is the process of their accumulation, keeping in mind that there is also a reverse process poisoning, caused by the radioactive decay of these nuclides. The most important of the poisonous products is 
, which has a very large absorption cross section for thermal neutrons. At neutron energy E= 0.084 eV
has a giant resonance in the capture cross section: 3. 10 6 barn. Xenon-135 is the strongest absorber of all known nuclides. For standard thermal neutrons (with the most probable energy E= 0.025 eV) capture cross section 
equals 2.72. 10 6 barn. With increasing neutron energy, the value
With For 
decreases rapidly. Already at E n= 1 eV, the radiative capture cross section of xenon-135 becomes approximately 300 times smaller than its maximum value. For high-energy neutrons, the capture cross section is 
insignificant. Therefore, in fast neutron reactors, poisoning is not noticeable at all.
The capture cross section for thermal neutrons of 135 Xe is almost 4000 times larger than the capture cross section of 235 U, therefore, even at a low concentration of 135 Xe it has a significant impact on the process of unproductive absorption of thermal neutrons. Fuel poisoning is a specific problem of thermal neutron reactors, which must be taken into account when addressing control issues of power reactors.
Half-life 135 Xe T 1/2 = 9.2 hours. 135 Xe is formed in the reactor (albeit in small quantities) as a direct fission product of the 235 U nucleus. In every 1000 fissions, an average of 3 135 Xe nuclei are obtained, i.e. its specific yield = 0.003 = 0,3 %.
However, a significantly larger amount of 135 Xe is formed as a result of two successive decays 
- direct fission product, the specific yield of which = 0.06 = 6% (20 times more than that of xenon-135).
The entire diagram of the formation and loss of 135 Xe in the reactor looks like this:
+ 235 U g= 0.003 135 Xe * + 
(n, ) 136 Xe *
() T = 9.2 h
g = 0.06 (g = 0.06) () T= 6..7 h
135 Te * () T 18 With 135I*+ 
(n, ) 136 Ba
Rice. 8.4. Scheme of formation and loss of iodine and xenon
The half-life of 135 Te T 1/2 18 s, which is many times less than the half-life of 135 I (T 1/2 = 6.7 h), therefore it is believed that 135 I is formed as a direct fission product with a specific yield of 6%. Strictly speaking, not all 135 I turns into 
. Part of it burns up (i.e., interacting with a neutron, gives 136 I), as shown in Fig. 8.4. But the microscopic absorption cross section of 135 I is negligible, and this effect is usually not taken into account (the rate of loss of 135 I due to decay is hundreds of times greater than the rate of its burnup).
The decrease in the concentration of 135 Xe occurs due to its radioactive decay (T 1/2 = 9.2 hours) and burnup with the formation of 136 Xe. The absorption cross section of 136 Xe is small ( With= 0.16 barn), and changes in its concentration have virtually no effect on the conditions for neutron multiplication.
From the moment the reactor is started, the accumulation of 135 Xe increases, then an equilibrium occurs between the generation and loss of 135 Xe, and from this moment onwards its concentration does not change over time. This poisoning of the reactor with xenon is called hospital poisoning.
After the reactor is shut down, the formation of 135 I stops completely, and the concentration of 135 Xe first begins to increase (due to radioactive decay of a large amount of 135 I accumulated in nuclear fuel and, having passed through a maximum, decreases, since the parent nuclei of 135 I are no longer formed. Graphs of changes in the concentration of iodine and xenon nuclei depending on the time after turning off the reactor are shown in Fig. 8.5.
At t = 0 (at the moment of reactor shutdown), N 0 Xe 0, because a certain amount of Xe-135 had already accumulated during reactor operation at the time of shutdown (most often this is the stationary xenon concentration).
Rice. 8.5. Change in 135 I and 135 Xe concentrations after reactor shutdown
The time to reach the maximum concentration of 135 Xe is 610.5 hours and depends on the neutron flux density in the reactor core before shutdown, that is, on the power level at which the reactor operated. The phenomenon of the current xenon concentration exceeding its stationary value after reducing the reactor power or shutting it down is called “ iodine pit.”
After the reactor is shut down, a situation may arise in which starting the reactor is difficult or even impossible for some time due to uncompensated xenon poisoning of the reactor.
Nuclear fuel slagging processes
Long-lived and stable fission products with a noticeable capture cross section are called slags.
When the reactor operates at constant power, the concentration of slag monotonically increases, and after shutdown it does not decrease. Among the fission products of 235 U by thermal neutrons, there are over 60 different varieties nuclei, which are slags. For convenience of calculations, all slags are divided into 3 groups depending on the value of the absorption cross section.
The first group includes strong slags, the absorption cross sections of which are many times larger than the absorption cross section 
. Among them, the main contribution to slagging is made by samarium 149 Sm, therefore, when calculating the slagging of fuel, its accumulation is especially taken into account. Samarium-149 is formed in the reactor core, mainly not as a fission fragment (the specific yield of 149 Sm does not exceed 10 -4), but as a result of the radioactive decay of another fission fragment - 149 Nd, which has a specific yield = 0.0113. The chain of main transformations leading to a change in the concentration of 149 Sm has the form:
Promethium concentration 
decreases only due to its radioactive decay at a rate Pm N Pm . As a result, the concentration of 149 Sm increases at the same rate, and the rate of decrease of 149 Sm is determined solely by the rate of absorption of thermal neutrons by its nuclei (burnup).
The state of an operating reactor in which the concentration of 149 Sm does not change over time is called stationary slagging. In this case, the rates of formation and loss of samarium are compared.
After the reactor shutdown, samarium, being stable, accumulates in the core. Moreover, its concentration increases until all promethium-149 accumulated before the shutdown disintegrates. The process of increasing the concentration of 149 Sm after the reactor shutdown as a result of the decay of the 149 Pm accumulated before the shutdown with its transition to 149 Sm is called “ promethium failure».
Co. second This group includes slags whose absorption cross-section is commensurate with the absorption cross-section of 235 U ( a a 5), and to third group - slags, in which a a 5.
In the thermal region, the macroscopic cross section of the fuel is much larger than the average macroscopic cross section of the slag, that is, the absorption of neutrons in the fuel is of primary importance.
For intermediate reactors, the harm from slag increases, because in this region, the macroscopic absorption cross sections of slags increase.
(1)
from here
(2)
Knowing the acceleration, you can determine the instantaneous speed using the formula:
(3)
If you measure a period of time t from the beginning of the ball's movement to its impact on the cylinder and the distance s traversed by it during this time, then using formula (2) we calculate the acceleration of the ball a, and using formula (3) - its instantaneous speed v.
Time interval t measuredusing a metronome. The metronome is set to 120 beats per minute, which means the time interval between two successive beats is 0.5 s. The beat of the metronome, at the same time as the ball begins to move, is considered zero.
A cylinder is placed in the lower half of the groove to brake the ball. The slope of the chute and the position of the cylinder are experimentally selected so that the impact of the ball on the cylinder coincides with the third or fourth beat of the metronome from the beginning of the movement. Then it's time to movet can be calculated using the formula:
t = 0,5 P,
Where P- the number of metronome beats, not counting the zero beat (or the number of time intervals of 0.5 s from the beginning of the movement of the ball to its collision with the cylinder).
The initial position of the ball is marked with chalk. Distance s The distance traveled by him to the stop is measured with a centimeter tape.
Directions for use
1. Assemble the setup as shown in Figure 178. (The slope of the chute should be such that the ball travels the entire length of the chute in at least three metronome beats.)
font-size:10.0pt">2. Copy Table 4 into your notebook.
Table 4
font-size:10.0pt">3. Measure the distance s , traveled by the ball in three or four metronome beats. Enter the measurement results in table 4.
4. Calculate the timet the movement of the ball, its acceleration and instantaneous speed before hitting the cylinder. Enter the measurement results in Table 4, taking into account the absolute error, assuming
font-size:10.0pt; color:black;letter-spacing:-.4pt">Lab No. 2
Determination of gravity acceleration
Goal of the work:calculate whisker rooting of free fall from the formula for the oscillation period of the mate matic pendulum:
font-size:10.0pt; letter-spacing:-.5pt">To do this you need to measureoscillation period and suspension lengthpendulum. Then from the formula(I ) you can calculate the acceleration of freedoms a long fall;
font-size: 10.0pt">Equipment : clock with second hand,measuring tape (Δl = 0.5 cm),
a ball with a hole, a thread, a tripod with a coupling and a ring.
Directions for use
1. Place on the edge of the table tripod. At its upper end strengthen the ring using a coupling and hang a ball on it threads The ball should hang on at a distance of 3-5 cm from the floor.
2. Tilt the pendulum away from the polobalance by 5-8 cm and release it.
3.Measure the length of the hanging measures noah tape.
4.Measure time Δ t 40 complete oscillations (N).
5. Repeat Δ measurements t (not changing the experimental conditions) and findaverage value Δ t avg.
6. Calculate the averageoscillation period T avg by average value Δ t avg.
7.Calculate the value gcp using the formula:
font-size:10.0pt;letter-spacing:-.3pt"> 8. Results obtained forput in the table:
Experience number | l, m | Δt, s | Δ t avg, s | T av = Δ t av / N | gcp, m/s2 |
|
9. Compare the resulting average value for gcp with value g = 9.8 m/s2 and calculate relativesignificant measurement error according to the formula:
font-size:10.0pt">Laboratory robot No. 3
Study of the dependence of period and frequency free vibrations thread pendulum on the length of the thread
Goal of the work:find out how the period and frequency of free oscillations of a thread pendulum depend on its length.
Equipment: a tripod with a coupling and a foot, a ball with a 130 cm long thread attached to it, pulled through a piece of rubber1, a watch with a second hand or a metronome.
Directions for use
1. Draw Table 7 into your notebook to record the results of measurements and calculations.
Table 7
2. Fasten a piece of rubber with a pendulum hanging on it in the tripod leg, as shown in Figure 183. In this case, the length of the pendulum should be 5 cm, as indicated in Table 7 for the first experiment. Lengthl
measure the pendulum as shown in the figure, i.e. from the suspension point to the middle of the ball.
3. To conduct the first experiment, tilt the ball from its equilibrium position by a small amplitude (1-2 cm) and release. Measure a period of timet, for which the pendulum will complete 30 complete hesitation. Record the measurement results in table 7.
4. Conduct the remaining four experiments in the same way as the first. In this case, the lengthl Set the pendulum each time in accordance with its value indicated in Table 7 for this experiment.
5. For each of the five experiments, calculate and write down the period values in Table 7 T pendulum oscillations.
_____________________
1 A piece of rubber (for example, an eraser) is used to ensure that the thread does not slip out of the tripod foot and so that the desired length of the pendulum can be quickly and accurately set. The thread is pulled through the rubber using a needle.
6. For each of the five experiments, calculate the values of the frequency ν of the pendulum oscillations using the formula: ν = 1/T or ν = N/t . Enter the results obtained in Table 7.
7. Draw conclusions about how the period and frequency of free oscillations of a pendulum depend on its length. Record these findings.
8. Answer the questions. The length of the pendulum was increased or decreased if: a) the period of its oscillations was initially 0.3 s, and after changing the length it became 0.1 s; b) the frequency of its oscillations was initially equal to 5 Hz, and then decreased to 3 Hz?
Laboratory work No. 4
Study of the phenomenon of electromagnetic induction
Goal of the work:study the phenomenon of electromagnetic induction.
Equipment: milliammeter, coil-coil, arc-shaped magnet, power source, coil with an iron core from a dismountable electromagnet, rheostat, key, connecting wires, generator model electric current(one per class).
Directions for use
1. Connect the coil to the clamps of the milliammeter.
2. Observing the readings of the milliammeter, bring one of the magnet poles to the coil, then stop the magnet for a few seconds, and then bring it closer to the coil again, pushing it into it (Fig. 184). Record whether an induced current arose in the coil while the magnet was moving relative to the coil; while it is stopped.
font-size:10.0pt"> 3. Write down whether the magnetic flux F passing through the coil changed while the magnet was moving; while it was stopping.
4. Based on your answers to the previous question, draw and write down a conclusion about the condition under which an induced current appeared in the coil.
5. Why did the magnetic current passing through this coil change when the magnet approached the coil? (To answer this question, remember, firstly, on what quantities does magnetic flux F and, secondly, is the module of the induction vector B the same? magnetic field permanent magnet near this magnet and far from it.)
The milliammeter needle deviates from the zero division
Check whether the direction of the induction current in the coil will be the same or different when the same magnet pole approaches it and moves away from it.
7. Bring the magnet pole closer to the coil at this speed
so that the milliammeter needle deviates by no more than half the limit value of its scale.
Repeat the same experiment, but at a higher speed of the magnet than in the first case.
At a higher or lower speed of movement of the magnet relative to the coil, did the magnetic flux F passing through this coil change faster?
With a rapid or slow change in the magnetic flux through the coil, did a current of greater magnitude arise in it?
Based on your answer to the last question, draw and write down a conclusion about how the modulus of the induction current depends, arising in the coil, from the rate of change of magnetic flux F piercing this coil.
8. Assemble the setup for the experiment according to Figure 185.
9. Check whether an induced current occurs in coil 1 in the following cases:
A) when closing and opening the circuit in which it is included
coil 2;
b) when flowing through the coil 2 direct current;
V) as the current flowing through the coil increases and decreases 2, by moving the rheostat slider to the appropriate side.
10. In which of the cases listed in paragraph 9 does the magnetic flux passing through coil 1 change? Why is it changing?
11. Observe the occurrence of electric current in the generator model (Fig. 186). Explain why an induced current appears in a frame rotating in a magnetic field.
font-size:10.0pt">Lab No. 5
Studying the nuclear fission of a uranium atom from photographs of tracks
Goal of the work:apply the law of conservation of momentum to explain the motion of two nuclei formed during the fission of the nucleus of a uranium atom.
Equipment:photograph of tracks of charged particles (Fig. 187) formed during the fission of the nucleus of a uranium atom.
font-size:10.0pt"> Explanations. In this photograph you see the tracks of two fragments formed during the fission of the nucleus of a uranium atom that captured a neutron. The uranium nucleus was at the pointg, indicated by the arrow.
The tracks show that the fragments of the uranium nucleus scattered in opposite directions (the kink in the left track is explained by the collision of the fragment with the nucleus of one of the atoms of the photographic emulsion in which it was moving).
Exercise 1.Using the law of conservation of momentum, explain why the fragments formed during the fission of the nucleus of a uranium atom scattered in opposite directions.
Task 2.It is known that the fragments of the uranium nucleus are the nuclei of atoms of two different chemical elements (for example, barium, xenon, etc.) from the middle of the table.
One of the possible fission reactions of uranium can be written symbolically as follows:
92 U + 0 n 56 Ba + z X + 2 0 n,
where symbol Z X the nucleus of an atom of one of the chemical elements is indicated.
Using the law of conservation of charge and Leu's table, determine what kind of element it is.
Physics lesson in 9th grade
Fission of uranium nuclei. Chain reaction. Laboratory work No. 7
“Studying the fission of the nucleus of a uranium atom from photographs of tracks”
Tishchenko E.V., teacher
physics municipal educational institution "Setsishchenskaya oosh"
Type – a lesson in learning new material.
Target :
Introduce the concept of a nuclear chain reaction,
Find out the conditions for its occurrence,
- verify the validity of the law of conservation of momentum using the example of fission of uranium nuclei.
Equipment: photograph of charged particles formed in a photographic emulsion during the fission of the nucleus of a uranium atom under the influence of a neutron (from a textbook); measuring ruler.
During the classes
I . Organizing time.
II . Updating knowledge . Frontal conversation:
The structure of the atom according to Rutherford (At the center of the atom there is a positively charged nucleus, around which negative electrons rotate)
Why is this structure called the planetary model of the atom? (The structure of an atom is similar to the structure of a star system).
What particles make up the nucleus of an atom? (From protons and neutrons (nucleons))
Which of these particles has a charge, and which one? (Proton. Positive.)
How do protons in the nucleus interact with each other electrically? (Since they are charged with charges of the same name, therefore protons repel)
What forces then hold the nucleons in the nucleus? (Nuclear forces attraction. They act between nucleons and are hundreds of times stronger than the electrical repulsive forces).
Chemical element in general view is written like this:X. What they mean and what they showZ And N? (The number of neutrons is indicated by the letter N , number of protons - Z , also the number of electrons in an atom, also serial number in the periodic table)
What is a mass defect? (The difference between the mass of nucleons and the mass of the nucleus).
What is binding energy? (Minimum energy that must be expended to completely split a nucleus into individual nucleons E = Δ m c 2)
III . Learning new material.
In 1938, Irene Curie among the decay products formingwhen bombarding uranium with neutrons, she discovered a radioactive isotope whose properties are those of lanthanum. Irene Curie standingwas on the verge of discovering uranium fission, but no one believed her, not Bohr,nor Rutherford. They all considered such a disintegration impossible. Otto Hahn and Fritz Strassmann irradiated uranyl nitrate with neutrons and obtainedradioactive barium.
They actually discovered the separation of the uranium nucleus, their article wasfixed December 22, 1938.
In 1939, German scientists Lise Meitner and Otto Frisch wrotepublished an article in which they showed that such a reaction is possible. In the same year, the Russian scientist J. Frenkel and N. Bohr developed the theory of nuclear fission of the uranium atom.
2. Getting to know the theory of nuclear fission.
The uranium nucleus captures a neutron and, like a liquid drop, begins to deform and takes on a dumbbell shape. KuloNova repulsion becomes stronger than nuclear attraction, and the nucleusbreaks into two unequal parts, the fragments are radioactive, and as a result of a series of β-decays they turn into stable isotopes.
An example of a nuclear fission reaction of a uranium nucleus
IV . Performing laboratory work. Occupational safety briefing.
Look carefully at the photo of the tracks.
N 
and it shows the tracks of two fragments formed during the fission of the nucleus of a uranium atom that captured a neutron. The uranium nucleus was located at point g, indicated by the arrow.
The tracks show that the fragments of the uranium nucleus scattered in opposite directions (the kink in the left track is explained by the collision of the fragment with the nucleus of one of the atoms of the photographic emulsion in which it was moving).
It is known that conservation laws play a special role in nuclear physics. Let's remember the basic conservation laws that we will need to successfully write today's work.
Law of conservation of momentum: The vector sum of the impulses of the bodies that make up a closed system does not change over time for any movements and interactions of these bodies.
Law of conservation of electric charge: IN nuclear reactions the total electric charge in the input channel is equal to the total electric charge in the output channel.
Law of conservation of the number of nucleons: In nuclear reactions the sum mass numbers before the reaction is equal to the sum of the mass numbers after the reaction.
Execute laboratory work
1 task: Using the law of conservation of momentum, explain why the fragments formed during the fission of the nucleus of a uranium atom scattered in opposite directions.
Reply in writing: Are the charges and energy of the fragments the same? Please indicate in your answer, By what signs can we judge this?
It is known that the fragments of the uranium nucleus are the nuclei of atoms of two different chemical elements (for example, barium, xenon, etc.) from the middle of the table of Dmitry Ivanovich Mendeleev. One of the possible fission reactions of uranium can be written in symbolic form as follows: where by the symbol Z X the nucleus of an atom of one of the chemical elements is indicated.
(Answer option: When a neutron is captured, the uranium nucleus is divided into approximately two equal parts, which are called fission fragments. In this case, the fragments fly away in opposite directions. This can be explained on the basis of the law of conservation of momentum. The momentum of the uranium nucleus before the neutron is captured is practically zero. During capture neutron, the nucleus, receiving some momentum from it, splits into two flying parts with masses m 1 and m 2. If we write down the law of conservation of momentum: 
)
Task 2: Using the law of conservation of charge and the table of Dmitry Ivanovich Mendeleev, determine what this unknown element is.
By virtue of the law of conservation of charge, we write: 92 + 0 = 56 + Z + 2 * 0. From here we get Z = 36. According to the table of D.I. Mendeleev determines that this is the nucleus of krypton.
At the end of the work, do not forget to draw a general conclusion about the work done.
V . Lesson summary.
VI . Homework. § 74.75, answer questions.
Used Books:
Peryshkin A.V. Gutnik E.M. Physics 9th grade: textbook for general education. institutions, M.: Bustard, 2009.
Maron E.A. Basic notes and multi-level assignments for the textbook by A.V. Peryshkin “Physics 8th grade” St. Petersburg LLC “Victoria Plus”, 2009