Juliet started working with a radioactive substance. in 1997, she started with 200 g. when she measured again in 2002, she had 100 g left. in 2007, she had 50 g. which substance is she most likely measuring?
a. rubidium-91
b. iodine-131
c. cobalt-60
d. carbon-14

Cobalt-60, 60Co
, is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisotopic and mononuclidiccobalt isotope 59Co

C. cobalt-60

Explanation:

Given: The sample remains half of its original sample after 5 years (2002-1997)=5 years.

The radioactive isotope rubidium-91 used for radiotherapy has a half-life of 58.4 seconds. A sample originally containing 200 g of rubidium-91 would contain only 100 g of rubidium-91 after 58.4 seconds.

The radioactive isotope iodine-131 used for radiotherapy has a half-life of 8.02 days. A sample originally containing 200 g of iodine-131  would contain only 100 g of iodine-131  after 8.02 days.

The radioactive isotope cobalt-60, used for radiotherapy has a half-life of 5.26 years. A sample originally containing 200 g of cobalt-60 would contain only 100 g of cobalt-60 after 5.26 years.

The radioactive isotope carbon-14, used for radiotherapy has a half-life of 5,730 years. A sample originally containing 200 g of carbon-14 would contain only 100 g of carbon-14 after 5,730 years.