Question

   When he received the Nobel Prize in chemistry in 1960, Willard Libby gave a lecture about radioactive dating to the king of Sweden and assembled guests in Stockholm.
a)   It was suspected that cosmic rays from space hit the Earth. What evidence confirmed this hypothesis?

b)   What did Libby have to assume to reach the conclusion that the expected specific radioactivity of living matter would be 2.0/8.5 disintegrations per second per gram of C? What evidence was there to confirm his conclusion?
  

c)   Libby concluded that we humans are in steady state with respect to our own concentrations of carbon-14 while alive but not after death. How did he justify this conclusion?
  
d)   On what basis did Libby conclude that humans are radioactive? What evidence did he collect to test his conclusion?

And here is the link provided for answering this question: http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1960/libby-lecture.pdf

Answer 1

a.)It was suspected that cosmic rays from space hit the Earth. What evidence confirmed this hypothesis?

These comic rays are high energy particle which travel with high velocities and bombard the earth's atmosphere. When cosmic particles strike the earth's atmosphere, collisions with air molecules initiate cascades of secondary particles. These showers often containing many millions of particles, fall to earth over a large area. These can be detected by various kinds detectors located at various places around the globe. Based on the interaction with the detector material on can calculate the type of radiation and energy of radiaton.

b) What did Libby have to assume to reach the conclusion that the expected specific radioactivity of living matter would be 2.0/8.5 disintegrations per second per gram of C? What evidence was there to confirm his conclusion?

Living organism consume food in the form of carbohydrates which contain carbon. One of the isotope of carbon is 14C, which is radioactive and has a half life of 5700 years. When we consume the carbohydrates the carbon-14 also enters our body and become part of the body after metabolsm of the food. The C14 ratio to C12 is constant for a given organism. The dps may vary as suggested by Libby from 2.0 to 8.5 dps per gram of carbon.

c) Libby concluded that we humans are in steady state with respect to our own concentrations of carbon-14 while alive but not after death. How did he justify this conclusion?

The C-14 concentration is in steady state composition when the humans are alive, but after death since there is no consumption of food in the form of carbohydrates, the C-14 decay and since the decay of C-14 is first order, there is a exponential decay of C-14.

d) On what basis did Libby conclude that humans are radioactive? What evidence did he collect to test his conclusion?

our bodies are radioactive, because we eat, drink, and breathe radioactive substances that are naturally present in the environment. These substances are absorbed by our bodies, into our tissues, organs, and bones, and are constantly replenished by ingestion and inhalation. K40 and C14 are present in a very minute quantites which can be detected by whole body detector which will register gamma radiation coming from these radiation. On can also take a few mL of blood or urine and check in the high sensitive detector(for beta and gamma radiation.