An instrument which measures concentration of `C-14` directly is

Options

A. Accelerator mass spectrometer
B. Spectroscopy
C. Geiger counter
D. All the above.

Answer

To determine the instrument that measures the concentration of Carbon-14 (C-14) directly, we can analyze the options provided:

1. **Understanding Carbon-14 Measurement**:
Carbon-14 is a radioactive isotope of carbon used in radiocarbon dating. The measurement of C-14 concentration is crucial in various fields, including archaeology, geology, and environmental science.

2. **Evaluating the Options**:
- **Accelerator**: This term is too vague and does not specifically refer to a device that measures C-14.
- **Mass Spectrometer**: This is a device that can measure isotopes, including C-14, but it does not do so directly in the context of radiocarbon dating.
- **Spectroscopy**: This technique studies the interaction between matter and electromagnetic radiation and is not specifically used for measuring C-14 concentration.
- **Giga Counter**: This instrument detects and measures ionizing radiation but is not specifically designed for measuring C-14 concentration.
- **Accelerator Mass Spectrometer (AMS)**: This is a specific type of mass spectrometer that accelerates ions to high kinetic energies before mass analysis. AMS is particularly powerful in separating rare isotopes like C-14 from more abundant isotopes.

3. **Conclusion**:
The correct instrument for measuring the concentration of C-14 directly is the **Accelerator Mass Spectrometer**.

### Step-by-Step Solution:
1. Identify the need to measure C-14 concentration.
2. Review the options provided.
3. Analyze each option to determine its relevance to measuring C-14.
4. Conclude that the **Accelerator Mass Spectrometer** is the most suitable instrument for this purpose.