Thread: 1027 CRM function

According to Sun Nuclear, the 1027 uses a Diffused-junction photodiode to measure radon. They don't describe how this Diffused-junction photodiode actually does it's job. I searched the web and am still looking for an understandable answer.

What specifically does the Diffused-junction photodiode do to get the job done?

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Originally Posted by John Dirks Jr

According to Sun Nuclear, the 1027 uses a Diffused-junction photodiode to measure radon. They don't describe how this Diffused-junction photodiode actually does it's job. I searched the web and am still looking for an understandable answer.

What specifically does the Diffused-junction photodiode do to get the job done?

Are you looking to understand how silicon-based photodiodes work, how photodiodes detect radiation sources, how radiation works, or ?? And do you have an EE or physics background or are you looking for a layman's explanation?

I'm not sure why they include it in their marketing (other than it sounds impressive) but diffuse-junction photodiodes are used for a number of reasons, including low cost (there is a ready supply of silicon etching equipment on the market) and, as far as diffused-junction silicon photodiodes are concerned, they are relatively low noise and therefore can be constructed without biasing circuits (further lowering the cost). Photodiodes are generally used for detecting alpha, gamma, and photon emission.

The way these devices work is by applying a filter to the collection mechanism that allows radon gas diffusion but blocks other noise (UV radiation, polonium and other decay products, etc). Because radon is present in very small quantities, the existence of other radioactive daughter particles can lead to schottky noise, and contribute to an unreliable reading. This photodiode avoids that problem and detects the radon gas signatures and translates it into a series of electromagnetic pulses which are amplified and measured over time to create a standard Pc/L reading.

Any more specific and I might have to include diagrams and math equations. Any less specific and I'd have to say, "so, it like sees the radon but none of the other stuff that might confuse it, so it gives a more accurate reading than some other types of photodiodes."

What he said.

Corn,

You siad it detects radon gas signatures. How does it detect them? The electrically charged qualities of the radon? What?

Originally Posted by John Dirks Jr

Corn,

You siad it detects radon gas signatures. How does it detect them? The electrically charged qualities of the radon? What?

Photodiodes work essentially by measuring the voltage change across an P-N semiconductor bridge. Diffused junction photodiodes allow for custom calibration of the sensitivity of the diode through the use of specific dopants in the creation of the diode. This allows you to tailor a photodiode to react to specific energy levels.

Different semiconductors will detect different wavelengths and excitation levels effectively. When radon 222 decays it emits an alpha particle with an embedded energy of 5.590 MeV. In a photodiode this creates a voltage potential across the P-N junction which can then be amplified and recorded. Through specific doping agents the response level of the photodiode can be tailored to register only above a specific energy threshold and within a specific wavelength.

When Rn-222 is present, so are practically all of the products of the radium decay series. Most of the decay products have a lower energy value than the Rn-222 alpha particle except for those emitted by polonuim decay, thus the filter to block polonium detection.

So, the alpha particle has an electrical charge that gets detected and recorded. Is that correct? If so, is the charge in the alpha particle positive or negative?

Originally Posted by John Dirks Jr

So, the alpha particle has an electrical charge that gets detected and recorded. Is that correct? If so, is the charge in the alpha particle positive or negative?

Yes and no. An alpha particle has a positive charge (usually, although the ion can pick up an electron pair, in which case it is identical to a helium atom). But electrical charge, in the sense that we think about it, is not how you should think about energy levels of radiation particles.