Friday, June 09, 2017

Radiacmeter & Dosimeter: #UnknownEquipment no more!

I thought I had another #UnknownEquipment post for @ThePTSOS but I managed to identify this one! While cleaning out a common prep room I came across an old cardboard box that has been sitting out on a table for literally a year (since the last time I cleaned the room). While opening it with my colleague Matt I said, "What is a dos-i-meter?" (Imagine me saying it like a kindergartener learning to read.) 

He got all excited since he is a Navy veteran that worked on a nuclear sub. "I used one of these," he said, "But it didn't look like this!" The larger box had smaller boxes with a cylindrical radiacmeter and wooden dosimeter adapter kits. 

After reading the instructions for both we were able to get a better idea of what they did and how they worked. Well Matt already knew, but was very excited to see the "antique" version. Note the box is stamped April 1953! My school is old but we don't usually find equipment this old. 

The cylindrical radiacmeter measures radiation exposure and would have been worn in the pocket of servicemen in the 1950s. This page has images of different models. The cost of this one is labeled as $26.00 in March 1951, which is approximately $243 in 2017! The instructions said: 

A pocket radiacmeter which measures and indicates the accumulated dose of X and gamma radiation to which the wearer has been exposed. At one end of the radiacmeter is an optical eye-piece and at the other end is the charging contact. The radiacmeter contains an ionization chamber into which is mounted a small electrometer. A radiac-detector charger is required to charge the electrometer. A scale is calibrated in milliroentgens is mounted in such a manner that the amount of radiation to which the wearer was exposed since the charging of the electrometer can be read directly by holding the radiacmeter up to a source of light and looking into the eye-piece.

The dosimeter is the device that was needed to measure the radiation exposure after the fact which surprised me. I know some modern dosimeters give live alerts when radiation levels reach dangerous or lethal levels. These older models would only alert wearers if they looked through it after their suspected exposure. The Roentgen unit is actually pretty complicated, its scale depends on the type of radiation its exposed to and it has fallen out of use. This gives it some scale: "An exposure of 500 roentgens (~5 Sv) in five hours is usually lethal for human beings." 

The dosimeter adapter instructions provided some interesting information. The adapter was really a holder to make it easier to charge the radiacmeter as part of a larger circuit using a 1 megaohm resistor and two 45-V batteries. The Wikipedia link below says that the radiacmeter would need 150-200 V to charge! The huge resistor is added at the battery for safety. To check your radiation levels you look through the end of the radiacmeter towards a bright light. This allowed you to see the scale inside (at left). There was a warning on the instructions not to use the dosimeter near AC wiring as the "alternating field surrounding the probe will make the fiber vibrate 120 times per second and appear as a nearly invisible blur against the scale."

That gave me some more information to research. "Radiacmeter" didn't give me many results about this particular device as the label is outdated. It seems more appropriate to call it a "Radiacmeter-Dosimeter" as at the top of the instructions. The more common "Dosimeter" was too broad of a category but "pocket dosimeter" or "quartz fiber dosimeter" on Wikipedia yielded something  more like what I had: "A quartz fiber dosimeter, sometimes called a self indicating pocket dosimeter (SIPD) or self reading pocket dosimeter (SRPD), is a type of radiation dosimeter, a pen-like device that measures the cumulative dose of ionizing radiation received by the device. It is usually clipped to a person's clothing and worn to measure one's actual exposure to radiation."

Continuing on in the article I learned that a dosimeter essentially acts as an electroscope! The quartz fiber is inside a sealed cylinder and attached parallel to an electrode. By connecting the circuit as describe above the electrode becomes negatively charged. By conduction the quartz fiber is as well and it repels from the electrode due to an electric force according to Coulomb's Law. Below is a schematic I found that shows the internal components:

This image shows the quartz fiber laying parallel to the hook of the electrode (looks just like a Leyden Jar huh?) and as it is repelled it is pushed away. A more modern image is here and very similar. If you were exposed to radiation then neutral atoms inside the cylinder would become positive and negative ions. The positive ions would be attracted to the negative electrode decreasing it's charge, decreasing the magnitude of the electric force as per Coulomb's Law and the quartz fiber would move closer to the electrode. 

This image below from the Nondestructive Testing Resource Center shows the view through the eyepiece as well as the internal components of the dosimeter. The vertical line around 100 milliroentgen is the quartz fiber. With this orientation I assume that the fiber would be all the way to the left (at 0 milliroentgen) when fully charged and thus max deflection but there is no radiation present. If there was radiation exposure then the fiber's repulsion to the electrode will decrease and it would move closer. That means that the electrode would be on the right side (at the 200 milliroentgen side) I think.

Several times in the instructions it was mentioned that the dosimeter has to be recharged. So I worried that you might think you had been exposed to radiation as the quartz fiber drifted back towards the electrode (and the high side of the milliroentgen scale) as the charge leaked off it over time. How would you know if you'd actually been exposed?? The NDE had the details though: a dosimeter is expected to lose 2% of its charged in 24 hours. Whew! So if you charged it and recorded base levels at the beginning of each shift it would be fairly accurate.

So this tiny little thing left somewhere in my school for going on 65 years turns out to be pretty cool! And what can I do with it? Well now that I know how it works and what its used for I think it could be brought up a couple of times:

1. During electrostatics after students are introduced to traditional electroscopes I can show students how this is charged and how the quartz fiber moves along the scale. We can talk about which parts of the device should be an insulator and which should be a conductor and why.

2. During current electricity we could discuss the basic albeit super dangerous circuit required to charge this thing the size of a pen.

3. During our light unit as students are introduced to the electromagnetic spectrum we can discuss why only X or gamma (and sometimes beta) radiation would trigger this device. I want to ask students why looking at a bright light source wouldn't move the scale of the dosimeter. (And of course not let them try to look at the sun through it to test out any theories!)

4. During an optics unit we could look at the arrangement of the lenses inside the device and talk about their shape and their position.

Basically I want to use it all year. I would love to get it charged up but I don't know if I have a DC power source that can go that high. It may have to become my new favorite useless accessory just so people ask me what it is.

1 comment:

Dan Burns said...

Cosmic rays were discovered with a device like this. They knew radiation caused the charge on an electroscope to dissipate quicker. They suspected the main source of background radiation was the Earth. To show evidence for this they took a charged electroscope up in a balloon. It should have discharged less quickly. It discharged even faster, showing that radiation was coming from space. Makes you have a lot more respect for electroscopes knowing this.