125I has largely replaced 131I as the label in radioimmunoassay. The main reason is that 125I:

Has a longer half-life, so kits keep longer

Needs no radiation safety precautions

Emits beta particles that are easier to count

Binds the antibody more strongly

Radioisotopes and radioactivity

14 Kerala PSC Lab Technician Gr II practice questions on Radioisotopes and radioactivity, part of Biochemistry. The first 14 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Tritium (3H) and carbon-14 emit weak beta particles. Which instrument counts them best?
1. ALiquid scintillation counter
2. BGeiger-Muller counter
3. CSodium iodide gamma counter
4. DPocket dosimeter
Show answerHide answer
Answer: A · Liquid scintillation counter
Weak beta particles travel only a very short distance and are absorbed by the sample itself or by a counter window. In liquid scintillation counting, the sample is mixed into a cocktail containing a fluor; each beta particle excites the fluor, which gives off light pulses that photomultiplier tubes count. Gamma emitters such as 125I are counted instead in a well-type counter with a thallium-activated sodium iodide crystal.
- B: The Geiger-Muller tube window stops most weak beta particles, so counting is poor.
- C: A sodium iodide crystal counter is used for gamma emitters such as 125I.
- D: A pocket dosimeter monitors staff exposure; it does not count samples.
Question 2 · Medium
125I has largely replaced 131I as the label in radioimmunoassay. The main reason is that 125I:
1. ANeeds no radiation safety precautions
2. BEmits beta particles that are easier to count
3. CHas a longer half-life, so kits keep longer
4. DBinds the antibody more strongly
Show answerHide answer
Answer: C · Has a longer half-life, so kits keep longer
The half-life of 125I is about 60 days, against about 8 days for 131I, so labeled reagents can be used for weeks to months. 125I emits low-energy gamma rays and X-rays that a well-type gamma counter detects efficiently, and it is less hazardous than 131I, which also emits beta particles. Labeled reagents still need shielding, monitoring and proper disposal.
- A: Every radioisotope needs safe handling, monitoring and disposal.
- B: It is 131I that emits beta particles; 125I emits low-energy gamma rays and X-rays.
- D: The isotope labels the antigen; it does not change antibody binding.
Question 3 · Hard
A 125I tracer (half-life 60 days) had an activity of 80 µCi on its reference date. What activity remains 180 days later?
1. A10 µCi
2. B20 µCi
3. C40 µCi
4. D26.7 µCi
Show answerHide answer
Answer: A · 10 µCi
180 days is three half-lives (180 / 60 = 3). The activity halves with each half-life: 80 to 40 to 20 to 10 µCi, which is 80 x (1/2)^3. Dividing 80 by 3 wrongly treats decay as linear. Old tracer gives low counts and poor standard curves, so kit expiry dates must be respected.
- B: 20 µCi is the activity after two half-lives (120 days).
- C: 40 µCi is the activity after only one half-life (60 days).
- D: 26.7 µCi divides by the number of half-lives instead of halving each time.
Question 4 · Easy
What is the SI unit of radioactivity, equal to one disintegration per second?
1. AGray
2. BSievert
3. CCurie
4. DBecquerel
Show answerHide answer
Answer: D · Becquerel
One becquerel (Bq) is one nuclear disintegration per second. The older unit, the curie (Ci), equals 3.7 x 10^10 disintegrations per second, so 1 microcurie is 37 kBq. The gray measures absorbed dose (1 joule per kg), and the sievert measures equivalent dose, the unit used in staff radiation monitoring.
- A: The gray is the unit of absorbed radiation dose.
- B: The sievert is the unit of equivalent (biological) dose.
- C: The curie is the older non-SI unit, 3.7 x 10^10 disintegrations per second.
Question 5 · Easy
Which type of radiation has the least penetrating power?
1. ABeta particles
2. BAlpha particles
3. CX-rays
4. DGamma rays
Show answerHide answer
Answer: B · Alpha particles
Alpha particles are helium nuclei with two protons and two neutrons; being heavy and doubly charged, they lose energy quickly and are stopped by a sheet of paper or the outer skin. Beta particles (electrons) travel further and need a few millimeters of plastic or aluminum, while gamma rays and X-rays need lead shielding. Alpha emitters are dangerous mainly when swallowed or inhaled.
- A: Beta particles penetrate further than alpha and need plastic or aluminum shielding.
- C: X-rays, like gamma rays, are penetrating electromagnetic radiation.
- D: Gamma rays are highly penetrating and need lead shielding.
Question 6 · Medium
What does a thermoluminescent dosimeter (TLD) badge worn by laboratory staff measure?
1. AContamination on bench surfaces
2. BActivity of the stock isotope
3. CThe cumulative radiation dose received
4. DHalf-life of the isotope in use
Answer and explanation in LearnFree account
Question 7 · Easy
Isotopes such as 12C and 14C have the same atomic number but differ in their:
1. APlace in the periodic table
2. BChemical reactions
3. CNumber of electrons
4. DNumber of neutrons
Answer and explanation in LearnFree account
Question 8 · Medium
Radioimmunoassay tubes labelled with 125I are best counted in a:
1. AWell-type gamma counter
2. BGeiger-Müller survey meter
3. CPocket ionization dosimeter
4. DLiquid scintillation counter
Answer and explanation in LearnFree account
Question 9 · Easy
Which radioisotope is used to label red cells for measuring red cell survival and red cell volume?
1. AIodine-125
2. BPhosphorus-32
3. CChromium-51
4. DCarbon-14
Answer and explanation in LearnFree account
Question 10 · Hard
A 14C sample gives 2,450 cpm in a liquid scintillation counter. Background is 50 cpm and counting efficiency is 80%. What is its activity in disintegrations per minute (dpm)?
1. A3,063 dpm
2. B3,000 dpm
3. C2,400 dpm
4. D1,920 dpm
Answer and explanation in LearnFree account
Question 11 · Medium
In a radioimmunoassay for thyroxine, a patient sample gives much lower bound radioactivity than the low standard. This means the patient's thyroxine is:
1. AHigher than the low standard
2. BEqual to the low standard
3. CNot measurable by RIA
4. DLower than the low standard
Answer and explanation in LearnFree account
Question 12 · Medium
Short-lived radioactive laboratory waste is commonly held in storage before disposal as ordinary waste. For how long is it usually stored?
1. AExactly 1 half-life
2. BAbout 10 half-lives
3. CAbout 100 half-lives
4. DAbout 2 half-lives
Answer and explanation in LearnFree account
Question 13 · Easy
One curie (Ci), the older unit of radioactivity, is equal to:
1. A3.7 x 10^10 disintegrations per second
2. B1 x 10^6 disintegrations per second
3. C1 disintegration per second
4. D3.7 x 10^10 disintegrations per minute
Answer and explanation in LearnFree account
Question 14 · Easy
The detector crystal of a gamma counter used for radioimmunoassay is made of:
1. APure quartz
2. BSodium iodide activated with thallium
3. CSilver chloride activated with copper
4. DLithium fluoride
Answer and explanation in LearnFree account

Radioisotopes and radioactivity

Tritium (3H) and carbon-14 emit weak beta particles. Which instrument counts them best?

125I has largely replaced 131I as the label in radioimmunoassay. The main reason is that 125I:

A 125I tracer (half-life 60 days) had an activity of 80 µCi on its reference date. What activity remains 180 days later?

What is the SI unit of radioactivity, equal to one disintegration per second?

Which type of radiation has the least penetrating power?

What does a thermoluminescent dosimeter (TLD) badge worn by laboratory staff measure?

Isotopes such as 12C and 14C have the same atomic number but differ in their:

Radioimmunoassay tubes labelled with 125I are best counted in a:

Which radioisotope is used to label red cells for measuring red cell survival and red cell volume?

A 14C sample gives 2,450 cpm in a liquid scintillation counter. Background is 50 cpm and counting efficiency is 80%. What is its activity in disintegrations per minute (dpm)?

In a radioimmunoassay for thyroxine, a patient sample gives much lower bound radioactivity than the low standard. This means the patient's thyroxine is:

Short-lived radioactive laboratory waste is commonly held in storage before disposal as ordinary waste. For how long is it usually stored?

One curie (Ci), the older unit of radioactivity, is equal to:

The detector crystal of a gamma counter used for radioimmunoassay is made of: