Radioactive Decay Calculation

Radioisotopes are in a continual state of decay.

True

The rate of decay is constant and unique for each isotope

True

Because a radionuclide is constantly decaying at a specific rate, the activity is constantly decreasing.

True

The amount of decrease per unit time is dependent upon what?

The half-life of the radionuclide

If you know the activity and half-life of a radionuclide, you can calculate the activity for any time in either the future or the past using the Universal Decay Law

True

What is the decay of radioactivity formula?

N=Noe^Yt

What is disintegration rate?

dN/dt, is termed the radioactivity or simply the activity of a radionuclide and denoted by A.

What does the same amount of radioactivity means?

the same disintegration rate for any radionuclide, but the total number of atoms present and the decay constant may be different for different radionuclides

What is the different for each radionuclide?

The total of atoms present and the decay constant

What is No and Nt mean?

the number of radioactive atoms present

What does t=0 and t mean?

Time

What does N=Noe^yt represents?

the exponential decay of any radionuclide

The exponential formula can be written as?

A=Aoe^yt

What does At represents?

Activity at specified time

What does Ao represent?

Original activity

What does E represent?

Euler’s number 2.718 which remains a constant in the equation

What does λ represent?

decay constant which describes the fraction of atoms that are likely to decay per unit of time (second) for a particular radionuclide=0.693/t1/2

What does t represent?

elapsed time

what does t1/2 represent?

half-life

Every radionuclide is characterized by a half-life, which is defined as the time required to reduce its initial activity to one half.

True

What does this t1/2=ln2/y=0.693/y simplify to?

y=0.693/t1/2

What is the one hour decay factor?

e-(0.693/6hr)(1 hr)=0.891

How to calcaulate the decay factor when time is moving forward? What is the activity of 25mCi of Tc-99m after one hour of decay?

multiply the activity by the decay factor: A = 25 mCi (0.891) = 22.3 mCi

How to calculate the decay factor when moving backward in time? At 0900, a syringe of Tc-99m assays at 25mCi. How much activity was in the syringe at 0800?

divide the activity by the decay factor: A = 25 mCi / 0.891 = 28.1 mCi

What is the fraction remaining at 1 hour?

0.891

What is the fraction remaining for 2 hours?

.794

What is the fraction remaining for 3 hours?

.707

What is the fraction remaining for 4 hours?

.630

What is the fraction remaining for 5 hours?

.561

What is the fraction remaining for 6 hours?

.500