List page: Ionizing radiation

Modifying a radiation meter for (radioactive) rock collecting:

The Ludlum Model 3 is quite a nice radiation meter, as long as you like analog displays and don’t mind it weighing one and a half kilograms:

Afternoon project: Observing X-ray fluorescence

When an atom’s inner electron absorbs an X-ray ray, it gets knocked out it’s atom, and then falls back into place after a few nanoseconds. In the process, the electron emits a new X-ray photon, with an energy dependent on the charge of the atom’s nucleus, in other words, the atomic number.

Making a simple ionization chamber radiation detector:

To build the ion chamber, I soldered some wire to a steel can as one electrode, drilled a hole in the closed end and inserted another wire as the other electrode. I then covered the opening with metal mesh to keep out static and stray fingers. Aluminum foil can be used for better shielding from static electricity, but at the cost of blocking alpha particles from entering¹.

How (ionization) smoke detectors actually work:

Most sources describe smoke detectors as using an ion chamber, with a strong alpha source inside. The alpha radiation ionizes the air inside the chamber, creating a small current flow, measured by the detector. When smoke enters through vents in chamber, it blocks the alpha particles, reducing the current and activating the alarm. A video on the Wikipedia article even gives a rough schematic¹:

Generating a spectrum from the PIN diode X-ray detector.

The pulse shaping amplifier outputs short pulses with around 10 mV/keV:

.2 ms/50 mV per division

These pulses have a similar amplitude to audio from a microphone preamp, so they can be digitized by feeding them into a computer’s soundcard. If the computer has dedicated audio input, connect the circuit as the headset microphone using a TRRS plug. Generally, the sleeve is ground, and the ring next to it is the microphone signal. The soundcard will apply a bias voltage to power the microphone, so add a 1 uF MLCC DC blocking capacitor.

Cleaning up the signal from the PIN X-ray detector

Yesterday, we demonstrated detecting soft X-rays using a BPW-34 PIN photodiode, and a charge sensitive preamplifier. Unlike a conventional G-M tube, the photodiode can measure the energy/wavelength of X-ray photons, which is characteristic of the source. However, the raw output from the preamplifier is not very usable; It is very weak, noisy and the pulses have long tails:

(Ab)using PIN photodiodes as soft X-ray detectors

PIN photodiodes are diodes with a thick undopped/intrinsic layer between the P and N-type regions. This layer is typically around .1 mm, creating a similarly thick depletion layer. When light shines into the depletion layer it liberates electron-hole pairs, which, if reverse biased drift into the dopped regions, allowing a small current to flow.