20.2-20.3: Nuclear Bombardment Reactions-Radiations and Matter

Transuranium Elements:

DOES NOT INCLUDE URANIUM!!!

Includes elements with an atomic number greater than that of uranium (Z = 92), the naturally occurring element of greatest Z.


Radiation Counters:
Because nuclear radiations can ionize molecules and break chemical bonds, they adversely affect biological organisms. So it's good to know when they're there.

Geiger Counter: a kind of ionization counter used to count particles emitted by radioactive nuclei, consists of a metal tube filled with gas, such as argon. Can detect both alpha and beta particles, and under special circumstances, neutrons.

¹₀n + ¹⁰₅B → ⁷₃Li + ⁴₂He



Scintillation Counter: a device that detects nuclear radiation from flashes of light generated in a material by the radiation. A phosphor is a substance that emits flashes of light when struck by radiation. Can detect both beta and gamma particles using a photomultiplier tube.




One e^– can produce 10⁶ e^–.

This vintage reel on Scintillation Counters is way better than a diagram:


Radiation counters measure the number of nuclear disintegrations in a radioactive sample.

Activity: (activity of a radioactive source) is the number of nuclear disintegrations per unit time occurring in a radioactive material.

Curie: Denoted Ci, is a unit of activity equal to 3.700 x 10¹⁰ disintegrations per second.

Nuclear disintegrations / second = nuc / s = Ci


Biological Effects of Radiation Dosage:

Radiation dosage effects can be quite damaging. DNA is especially affected, which interferes with cell division.

rad → (radiation absorbed dose) the dosage of radiation that deposits 1 x 10^-2 J of energy per kg of tissue.

Depends not only on the amount of energy deposited in the tissue, but also on the particle. Neutrons are more destructive than gamma rays of the same radiation dosage measured in rads.

rem → a unit of radiation dosage used to relate various kinds of radiation in terms of biological destruction. It equals the rad times a factor for the type of radiation, called the relative biological effectiveness, or RBE (basically, how efficient it is at killing you).

rems = rads x RBE → total damage

RBE = 1 for γ (gamma) and β (beta).
RBE = 5 for n^o
RBE = 10 for α (alpha)

Here's that movie mentioned in class:


It really is an awesome movie, go rent it!


Rate of Radioactive Decay:

The rate equation for radioactive decay has the same form as the rate law for a first order chemical reaction.

R = kN_t

R = Rate = Activity
N_t = the number of radioactive nuclei at time t
k = radioactive decay constant, the rate constant for radioactive decay

Example:

A 2.50 mg sample of Tc-99 has an activity of 2.70 x 10^-5 Ci and is decaying by beta emission. What is the decay constant?

R = kN_t

First convert the activity to nuc/s:

R = 2.70 x 10^-5 Ci x (3.700 x 10¹⁰ nuc/s / 1s) = 9.99 x 10⁵ nuc/s

Now convert the given sample to N_t:

2.50 mg Tc-99 = 2.50 x 10^-3g Tc-99
(2.50 x 10^-3g Tc-99) x (1 mol Tc-99 / 99g Tc-99) x (6.023 x 10²³ nuclei / 1 mol Tc-99) = 1.52 x 10¹⁹ nuclei = N_t

k = Rate / N_t
k = (9.99 x 10⁵ nuc/s) / ( 1.52 x 10¹⁹ nuclei)
k = 6.57 x 10^-14 s^-1

Units for k will always be inverted seconds because it is first order.