Answer to Question #8921 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
I have disassembled various consumer-grade digital cameras with CMOS (complementary metal-oxide semiconductor) and CCD (charge-coupled device) image sensors and (after removing the plastic and glass optics) experimented with holding the image-sensing silicon wafer up to various sources. When I held it ~1 cm from a smoke detector's 241Am source foil, I see the tons of randomly flickering white dots. But a piece of copy paper fully blocks this effect, and a bottle of 40K containing salt substitute (KCl) fails to excite the CCD sensor at all. So, apparently this CCD system only works as an alpha scintillator. My question is, why? Americium-241 should also be giving off a 60 keV gamma photon that would penetrate a sheet of copy paper, and the beta particle from 40K-decay in the salt substitute has a pretty high peak energy as I recall. But my CCDs don't register even a single "blip" for ether of these scenarios. Are CCD sensors somehow insensitive to ionizing photons and betas?
P.S. It goes without saying, but all of my experiments are conducted with careful attention to safety, e.g., double gloves, respirator, personal dosimeter to check for source leakage onto work surfaces, etc. I verified with the smoke detector manufacturer's consumer affairs that as long as I fully reassemble the detector housing and do not puncture or separate the foil from its steel block, there would not be any problem accepting the detector for recycling.
Sounds as if you're having some fun with these imaging devices. As you have observed, it is true that both the charge-coupled silicon devices and the metal-oxide semiconductors may produce visible light scintillations in response to ionizing radiation. Just as visible light will set free electrons through photoelectric interactions in silicon, higher-energy radiations such as alpha, beta, and gamma radiation also have the ability to free electrons in the material. The subsequent light emission normally occurs when free electrons combine with the holes that represent the positive charge carriers; this recombination process sometimes results in the emission of energy as visible light photons. In silicon the process is relatively inefficient so that relatively few of the ionization events that set free electrons lead to light emission. If the frequency and density of recombination events is sufficiently high, however, some of the light pulses may be sufficiently intense to be visible to the naked eye, especially in a darkened environment.
I believe the major reason you see the flashes when the detector is exposed to alpha radiation from the 241Am but not from the gamma and beta radiations has to do with the nature of the energy deposition process by the alpha radiation compared to the other types of radiation. As you have demonstrated, using a piece of paper as a shield, the alpha particles have a very short range in condensed materials. Most alpha particles from 241Am have individual energies of about 5.5. million electron volts (MeV), and when one of these alpha particles enters the silicon, it will deposit all of its energy within approximately 20 micrometers (this is not much different from the thickness of silicon used in many CCDs), thus producing a track of very dense ionization, a single alpha particle producing likely between four and five million ionization events. The electrons (and holes) produced may recombine, and this provides the potential to produce a burst of light photons in sufficient intensity to be visualized.
The situation is quite different for beta and gamma radiation. The maximum energy beta radiation from 40K is about 1.3 MeV, and the approximate distance such a beta particle could travel in silicon is about 3 mm, a dimension much greater than the thickness of the active material in a CCD or CMOS. The average beta particle traversing the same 20 micron path length as the alpha particle would produce approximately 5,000 to 10,000 ionization events. Thus, the ionization density, as well as the total ionization per particle, would be much less for the beta radiation than for the alpha particle. This makes it much less likely that one would be able to see the light emission with the naked eye. There are optical microscopy enhancement techniques that might make these weaker scintillations visible.
Gamma rays will also interact in silicon to set free electrons that may lead to recombination and light emission, but the situation with respect to the gamma radiation is even more restrictive as regards the possibility of viewing the light emission. The probability that a 60 keV gamma ray will interact within a 20 micron thickness of silicon is about 1.4 x 10-3. This compares to the probability of 1.0 for the alpha particle (and the beta particle). Additionally, the photon energy is much less than the alpha or beta energy and only about half of the photon energy, on average, would be deposited in the silicon per interaction. Thus, one would have an even more difficult time trying to visualize light emissions from gamma-ray interactions.
Other factors, such as the intensity of the various sources, also play some part in the likelihood of producing enough interactions to yield visible light output. You don't specify the thickness of the bottle holding the KCl or whether the CCD was exposed to the bare KCl (the bottle walls could produce considerable attenuation of beta radiation). In any event, the rate of incidence of beta particles on the CCD would likely be considerably less than the alpha particles from the 241Am source. The 40K also emits 1.46 MeV gamma rays in about 11 percent of its disintegrations, but again you would not have sufficient source intensity to yield an interaction density sufficient to produce light pulses visible to the unaided eye.
It's good to hear that you are aware of potential hazards associated with some radioactive sources, and I expect that you will continue to invoke appropriate safety measures and to seek guidance as necessary if your experimentation expands to include other radioactive sources. Good luck.
George Chabot, PhD, CHP