Physics 15: Radioactivity, Semiconductor Physics Homework Assignment

This physics assignment solution addresses key concepts in radioactivity and semiconductor physics. It begins with calculations of half-life, followed by analysis of beta decay processes. The assignment then delves into semiconductor physics, covering doping, band gaps, and charge carriers. It explains P-type and N-type semiconductors and the formation of depletion regions in diodes. The solution also examines forward and reverse bias conditions in diodes. The assignment concludes with an analysis of different regions in radiation detectors, including recombination, ionization, proportional, Geiger-Muller, and continuous discharge regions, and the application of these regions in current and pulse modes. The solution includes relevant diagrams and references to support the analysis.

Running head: PHYSICS 0
PHYSICS
Name of Student
Institution Affiliation

PHYSICS 2
1.
a).
b).
Time Activity Ln (Activity)
0 30000 10..309
1 11900 9.84

PHYSICS 3
2 4570 8.427
3 1820 7.506
4 720 6.579
After using the above values the graph of Ln activity against time was plotted as below;
c).
The activity increases from 5000 to 10000 within 1.5 seconds, therefore the half-life for this
decay is 1.5 seconds (Avanzi, 2012).
d).
i. 3.0 seconds
Activity A= A02 - t/T1/2

PHYSICS 4
A= 30000× 2-1.5/3
A=30000× 2 -0.5
A=30000× 0.07071
A=21213.2
ii. 10.5 seconds
Activity A= A02 - t/T1/2
A= 30000× 2-1.5/10.5
A=30000× 2 -0.1428
A=30000× 0.905
A=27171.7
e).
Mean life = Half life
ln 2
Mean life = 1.5
0.693
Mean life = 2.164

PHYSICS 5
f).
i. This is the ratio of the number of counts recorded by the detector to the number of
rays which are emitted by the source (Chen, 2011).
ii. The true activity for Bq is given by
A= 30000× 88
100
A= 26400
1.
a)
i. 116C + 01 H+ 115B
ii.
This process is a Beta decay process. Beta decay involves two processes where a proton can
be added (Beta Minus decay) or where a proton can be reduced by one (Beta plus
Decay). Fort this decay in I above, we can say that it is Beta plus decay.
Iii
These particles in Beta decay have a specific and well-defined energy value. This is because, in
Beta plus decay, there is a specific and a well-defined energy which is always the same for all
other Beta plus decay losing a proton (Correll, 2011).
b).

PHYSICS 6
i. spectrometer is a device employed for recording and measuring spectra, especially as a way of
analysis.
ii. After a single decay, Strontium-90 will have a bigger height as compared to Polonium-210.
This is because Polonium-210 is a heavier atom than Strontium-90, hence Strontium-90 will
move higher height because of its low weight (Floerkemeier, 2012).
Iii,
When energy spectrum for the both Strontium-90 and polonium-210, the Strontium-90 will show
a decaying exponential graph which is a perfect decaying substance as shown in the figure
below:
While that of polonium-210 will show a growth and then shows a decay as shown in the figure
below;

PHYSICS 7
2.
a)
i. Doping
This is the process of adding impurities to a pure semiconductor (intrinsic semiconductor) to
form extrinsic semiconductor (impure semiconductor/ doped semiconductor) (Kiritsis, 2012).
ii. Band gap
This refers the difference in energy in electron volts amid the lowest point of the
conduction band and the topmost point of the valence band in semiconductors and
insulators (Ruiz, 2013).
iii. Charge carrier

PHYSICS 8
These are particles which carry electrical charges, they can either be minority charge carrier
or majority charge carriers depending on their numbers.
b).
c).
In the P-type region, the majority charge carriers are holes while in the n-type the majority
charge carriers are the electrons. These concentrations will make the holes to diffuse from
the region where they are highly concentrated (p-type) to a region where they are lowly
concentrated (n-type). The electrons will also move from a region where they are highly
concentrated (n-type) to the region where they are lowly concentrated (p-type). At the
center. There will be neutralization of holes (positively charged) and electrons (negatively

PHYSICS 9
charged) creating a space charge layer known as a depletion region as shown in the figure
below;
This is can be then employed to construct a good diode which will only allow the movement
of electric current in one direction only when positive is connected to p-type and negative is
to the n-type ( Forward biased) (Paulo, 2013).
d).
i.
If positive terminal of external voltage is connected to p-type and the negative is connected to
the n-type. The negative terminal will inject electrons to the n-type of the semiconductor. This
act will replace those `` lost`` because of the migration across the junction. The positive terminal
of the external voltage removes the electrons from p-type of semiconductor generating more
holes to replace those holes which were lost during recombination. When the external voltage
applied exceeds the barrier voltage of the semiconductor, it will fully replace the depletion layer
(Nkuka, 2012). If this occurs the migration of electrons across the junction will resume. The
results are that when this connection is done, it reduces the barrier voltage and when the applied

PHYSICS 10
external voltage applied is more than that of the barrier voltage, there will be current flow
generated through the semiconductor. This is clearly shown in the diagram below;
And when the diode is connected in the circuit, it will appear as below;
Ii. Reversed Biased
When p-type is connected to the negative terminal of a battery while the n-type of the
semiconductor is connected to the positive battery terminal. So the external source is applied in

PHYSICS 11
the same direction of the potential barrier. If the applied voltage is taken as V and the barrier
potential is taken as Vx, then the overall voltage in the PN is V+Vx. S The electrons in n-type
will be moved from the depletion layer region to the terminal region of n-type and the same for
the holes at the p-type depletion layer will be moved towards the terminal location of the p-type.
This will lead to an increased depletion layer. The diagram below clearly illustrates the motion of
charge carriers in reverse biased.
And when the diode is connected in the circuit, it will appear as below;
4.
a).
Region A: Recombination region:

PHYSICS 12
In this region, Vdc is comparatively low such that rejoining of the cations and anions.
Due to this, not all ion pairs will be collected and the voltage height pulse is somehow
low.
Region B: Ionization region
Vdc is adequately high in this region such that only negligible amount of recombination
will take place. In this region a type of detector known as ionization chamber works.
Region C: Proportional region
Vdc is adequately high such that electrons moving towards the center wire achieve
enough energy amid collision with the gas anions to generate new ion pairs Therefore the
amount of the electrons improves such that the electric moving via the resistor, R, more
than thousands times more the charge generated initially by the interaction of radiation.
Region D: Geiger-Muller Region.