Cracking the SAT Physics Subject Test

(a) Strontium-90 ; β− decay

(b) Argon-37 ; electron capture

(c) Plutonium-239 ; alpha decay

(d) Cobalt-58 ; β+ decay

You Got Chemistry

In My Physics

Don’t be alarmed should

a rare question like this

show up on the Physics

test. You are being tested

on Physics, and if there’s

something you need to

know about Chemistry—

such as the Periodic

Table—it will be provided

to you.

Here’s How to Crack It

(a) daughter = yttrium-90

(b) daughter = chlorine-37

(c) daughter = uranium-235

(d) daughter = iron-58

Radioactive Decay Rates

Although it’s impossible to say precisely when a particular radioactive nuclide
will decay, it is possible to predict the decay rates of a pure radioactive sample.
As a radioactive sample disintegrates, the number of decays per second decreases,
but the fraction of nuclei that decay per second—the decay constant—does not
change. The decay constant is determined by the identity of the radioisotope.

Boron-9 has a decay constant of 7.5 × 10^17 s−1 (rapid), while uranium-238 has a

decay constant of about 5 × 10−18 s−1 (slow).

The activity (A) of a radioactive sample is the number of disintegrations

it undergoes per second; it decreases with time according to the equation