alpha_beta_gamma by liwenting

VIEWS: 16 PAGES: 46

									Alpha,beta and gamma radiation
    Warm-up
1   Which of the following is more important
    for finding out the rush-hour of the tunnel?
                          The number of cars
                          entering it since the
                          start of the day (count).

                         The number of cars
                          entering it per minute
                          (count rate).
    Warm-up
2     Below is the time record of the total
      number of cars entering the Cross-
      harbour tunnel since time zero.
                          What is the flow rate of
                          cars? 10 cars/min
      1      Origin and nature of nuclear
             radiation
• Alpha (), beta () & gamma () radiation
  all come from the nucleus
• Their nature:
    a stream of helium nuclei (positively charged)
        a stream of negatively charged electrons
        an EM wave of very high frequency, whose
          property is similar to X-ray
   1    Origin and nature of nuclear
        radiation
 radiation and X-rays are both highly
penetrating EM waves.
                                               ray


                                         X-rays

But  radiation has higher frequency and
more energy
 more dangerous than X-rays
1   Origin and nature of nuclear
    radiation
      2      Ionizing power
  a Ionization
      When nuclear radiation passes through a gas,



uncharged
molecule                                            -
                                          gains      ve ion
      losses an e                        an e

                +ve ion   +
       removes e from some gas molecules
       gas molecules are ionized
   a Ionization
uncharged
molecule                                    -
                                  gains   ve ion
      losses an e                an e

                +ve ion   +
     • +ve & ve ions always form in pairs.
     • Ions are free to move
        ionized gas can conduct electricity
     • If intensity of radiation 
        more ion pairs form
        gas is more conducting
   Example 1
    Detector making use of ionization
    Below shows the structure of an earlier radiation
    detectors used by Marie Curie.
    Explain why the meter detects an electric current
    when some radioactive substance is placed
    between the 2 conducting plates.

                                        G
                               sensitive galvanometer
radioactive
sample
     Example 1
Detector making use of ionization

                                    G
radioactive             sensitive galvanometer
sample

              +


     Radioactive substance emits radiation
      ionizes air molecules between the plates
      +ve ions are attracted to ve plate &
       ve ions are attracted to +ve plate.
      Current forms in the circuit.
    2      Ionizing power
 b Ionizing power




We can study the ionizing power of radiation with a spark
 counter (火花計數器) .
When radiation is detected, sparks are produced.
Amount of sparking shows: ionizing power of radiation
         2          Ionizing power
        b Ionizing power
(3) radiation passes
through the air, removes         (2) a radioactive source is
e from gas molecules &          brought near the gauze
ionizes air                  radioactive source
        gas molecule                                                (1) a high
             electron                                               voltage is
                                                                    applied to
                                             gauze ( electrode)    the gauze

                                           spark                   air      400 V d.c.
        spark
                                                      gauze (+ electrode)
                (4) the air can now conduct
                electricity & a spark is produced
 Experiment 18b
 Ionizing power of radiation

Investigate the ionizing effects of nuclear radiation & compare
the ionizing power of ,  &  radiation.



                                               radioactive source
             EHT supply




                                             spark counter
 2        Ionizing power
  b Ionizing power
 radiation produces lots of sparks,
 radiation produces only a few sparks,
 radiation hardly gives any.

Ionizing power of nuclear radiation: >  > 
              (strong)   
                         (weak)
                                       
radioactive
source


                             (the weakest)   
3 'Seeing' nuclear radiation
a Diffusion cloud chamber
Nuclear radiation is invisible, but we can observe their tracks
using the diffusion cloud chamber (擴散雲室).
  a Diffusion cloud chamber
 (1) A weak radioactive source inside the chamber
     emits radiation that produces ions along its path.
 (2) The alcohol vapour which diffuses downwards from
     the top condenses around the ions.
 (3) The resulting tiny alcohol drops show up as a track
     in the light.                     felt ring soaked in alcohol
                          plastic lid
 full of alcohol
 vapour
weak radioactive
source                                                     lamp

     insulation
                            foam                dry ice
Experiment 18c
   Diffusion cloud chamber
  Observe the tracks of  particles in a diffusion cloud chamber.




                      felt ring
                lid

 radioactive source




                                  lamp to illuminate
                                  cloud chamber
3 'Seeing' nuclear radiation
b Cloud chamber photographs

                                         Cloud
                                         chamber
                                         tracks



                                         (Photo credit: Lord
                                         Blackett’s Estate)



The tracks are produced randomly along different
directions & are of slightly different lengths.
 shows that the radiation is emitted at random (隨機)
    from the source.
3 'Seeing' nuclear radiation
b Cloud chamber photographs

                 radiation:
                Having a strong ionizing
                power, the heavy  particles
                give straight and thick
                tracks of about the same
                length.


                 (Photo credit: Lord
                 Blackett’s Estate)
3 'Seeing' nuclear radiation
b Cloud chamber photographs
 radiation:




                                       (Photo credit: Lord
                                       Blackett’s Estate)




Having a weaker ionizing power, the light particles
give thin and twisted tracks.
They are twisted because the particles are small in
mass and bounce off from air molecules on collision.
3 'Seeing' nuclear radiation
b Cloud chamber photographs
 radiation:




                                       (Photo credit: Lord
                                       Blackett’s Estate)




Tracks of  rays can hardly be seen.
The tracks are due to some other particles produced
by a complicated mechanism involving  rays.
4 Penetrating power
How far radiation can penetrate into a material
relates to its ionizing power?
Use Geiger-Müller counter (G-M counter, 蓋革彌
勒計數器) to study penetrating power.




                          A device for measuring the
                          counts or the count rate of
                          radiation reaching a location.
 How G-M counter works?
          (1) a high voltage is applied to the central wire
                                               central wire (+)
    radiation        aluminium tube ()


                                                                   400 V
mica end-window
admits nuclear       argon gas at
radiation into the   low pressure
tube                                                     counter
                       argon atom
 (2) when a                               e
                                                            (3) a pulse is
 particle from                                        created due to the
 radiation      central                                attraction of e to
 enters the     wire                                   the central wire &
 tube, it pulls                                       can be counted by
 an e from an                                           the G-M counter
 argon atom                                pulse
Experiment 18d
Penetrating power and range of radiation
Set up a G-M counter to compare the penetrating power and
range of radiation of ,  and  radiations.


                                  scaler




                                                      source held in
                                                      lifting tool


                             G-M tube
                                           absorber
                             in holder
4 Penetrating power
a Range in a material
When radiation passes through a material (e.g. air
or metal), it gradually loses energy to ionize the
atoms on its way and stop at last.

In other words, it is gradually absorbed.

The distance it travels before it is completely
absorbed is called the range (射程).
4 Penetrating power
a Range in a material
Radiation with a strong ionizing power:
has a short range in a material,
as the radiation quickly gives all its energy to ionize
the atom in the material over a short distance.


Radiation with a weak ionizing power:
has a long range in a material.
4 Penetrating power
 a Range in a material
  radiation:
   strong ionizing power
  can only travel a few cm in air

  radiation's range in air: several m

  radiation's range in air: > 100 m

 Range of radiation in air:  >  > 
a Range in a material

Range in denser                          water
materials is 
because ionization           concrete
                      iron
occurs  in equal
distance.                    lead         radioactive
                                          source
                     Shielding of equal effect
4 Penetrating power
b Penetration through various materials

Radiation with strong ionizing power:
has a low penetrating power
 easier to be stopped or absorbed

Radiation with a high penetrating power:
more difficult to be stopped or absorbed.
b Penetration through various materials
 radiation is stopped by 1-2 sheet of paper.
 radiation is stopped by 5 mm of aluminium.
 radiation is never fully stopped but 25 mm lead
   halves its strength.
                                      

radioactive
source
                          
              



                  paper   aluminium (5 mm)          lead (25 mm)

Penetrating power of nuclear radiation:  >  > 
   Example 2
Distinguish radiation with a spark counter
An unknown radioactive source held above a spark
counter produces many sparks.
When a sheet of paper is put between the source &
the spark counter, no. of sparks drops to only a few.
What is/are likely the radiations emitted by the
source?
 The source likely emits  &  radiation, because:
  radiation is absorbed by the paper,
 but  radiation can still pass through to
 produce a few sparks.
    Q1 Which of the following...
Which of the following radiation detectors CANNOT give result
immediately?


A      Photographic film.
B      Spark counter.
C      Diffusion cloud chamber.
D      G-M counter.
     Q2 Which types of radiation...
Which type(s) of radiation can be stopped by a sheet of 5 mm
aluminium?


A        only.
B        only.
C        and  only.
Q3 What types of charge are...
What types of charge are carried by  and 
radiations?
         radiation     radiation
   A    Positive       No charge
   B    Negative       No charge
   C    No charge      Positive
   D    No charge      Negative
Q4 A block of 25-cm thick lead...


 A block of 25-cm thick lead can reduce
 ____________ radiation(s) to nearly
    ,  and 
 background level.
Q5 True or false: If an object...
True or false: If an object closed to a spark
counter makes no spark, it is not radioactive.


                                       (T/F)
Q6 The count rate at 3rd min. is...
Figure below shows the number of counts
recorded by a G-M counter at different times.




The count rate at the 3rd minute is about
  100 counts per min
___________________.
5 Deflection in electric & magnetic
  field
a In electric field
                         electric field       (+)
radioactive
source
                                                 


                                          +


                           (-)
mass of  >> mass of 
 deflection:  << 
  5 Deflection in electric & magnetic
    field                     F

  b In magnetic field                                            B


                                                             I
                                          
radioactive
source
                                              


                                      B-field (into paper)
                          


 mass of  >> mass of         deflection:  << 
5 Deflection in electric & magnetic
  field
b In magnetic field




  radiation tracks in
 a very strong B-field
        (Photo credit: Lord Blackett’s Estate)
Experiment 18e
Magnetic deflection of  radiation
Investigate the charge that the  radiation carries, if any, by its
deflection in a magnetic field.



                                      scaler




                                          magnet                 source
                           G-M tube                lead sheet
Q1 True or false: All nuclear...
True or false: All nuclear radiations deflect in
the same direction in a magnetic field.


                                          (T/F)
Q2 True or false: The tracks of...
True or false: The tracks of nuclear radiation
become visible by using photographic films.


                                        (T/F)
Q3 True or false: For X-rays...
True or false: For X-rays and rays,
(a) both of them can be deflected by a         (T/F)
    magnetic field;

(b) both of them carry charges;                (T/F)
                                               (T/F)
(c) both of them are transverse waves;
                                               (T/F)
(d) both of them have strong ionizing power;
                                           (T/F)
(e) both of them can travel through a vacuum.
Q4 What are X, Y and Z?
Radiation from a radium source is split into X,
Y & Z by a strong magnetic field.
What are X, Y & Z?
                                      
  radioactive source              Z



                                                  Y   


 lead shield                              X
                       magnetic
                        field                 
Q5 State which type of nuclear...
State which type of nuclear radiation

(a) carries a positive charge,     radiation

(b) has characteristics similar to X-ray,  radiation

(c) is made up of fast-moving electrons.  radiation

								
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