The first CSL article in this series has introduced you to X-Ray Fluorescence, the method of exciting an element's atoms and measuring the Characteristic X-Rays given off when the atom de-excites back to a stable state.
Any number of methods may be used to excite ordinary atoms to fluoresce in this manner. Just about any energetic charged particle will do it, as well as ionizing electromagnetic photons, such as Gamma Rays and X-Rays. The first article in this series used small radioactive beta sources to provide that excitation.
The only requirement is that the energy imparted to the target atom exceeds the binding energy of the particular electron shell involved. This binding energy is unique for each and every element and each and every electron shell within that element.
For an atom to be in a stable condition, quantum theory states that orbital electrons must exist in discrete energy levels (called shells) and that shells must be filled.
XRF involves an inner electron being first ejected from the atom by the added energy, then being replaced in it's shell by a donor electron. When we measure the resulting XRF, what we see is the difference energy between the kinetic energy state of the donor electron, minus the binding energy of its new orbital shell. Therefore each and every element has a set of "Characteristic X-Ray" signatures for each of its electron shells.
When a pure beta source is used to excite XRF, the beta particles penetrate the target's surface only slightly, therefore betas are used in such applications as coating thickness analysis, and surface elemental analysis.
If we were to use X-Rays, they of course penetrate deeper, causing elements beyond the surface of the target to become excited. Sometimes, if those atoms are no too deep in the target, their resulting fluorescent X-Rays will break through the surface, allowing them to be measured. A rule of thumb is, the higher the target's Atomic Number the more self-shielding is an issue (element density also plays a part in this).
Introducing APXS, a form of XRF using alpha particles to excite the target atoms. Traditionally element analysis used heavy lab equipment, large X-Ray generators and liquid nitrogen cooled sensors. Today's world requires portable, lightweight equipment designed to be used away from the Home Lab, out in the field. No better example of remote site operation is Mars! Mars Rover Curiosity uses XRF with excitation provided by alpha particles, a relatively new technology
Curiosity uses some pretty hefty, military grade radioactive sources, like Curium, which are undesirable and unobtainable to Citizen Scientists and school labs.
NASA REFERENCES: http://mars.jpl.nasa.gov/msl/mission/instruments/spectrometers/apxs/
We do however have a pretty potent alpha particle source not only readily available but downright cheap! Ionization type Smoke detectors, presently available at Wal-Mart for less than $4 USD will supply the only excitation needed. They contain less than one microCurie of Am-241, in a safe configuration, that can be owned without license and disposed of in any trash can when finished.
Still we caution young students to seek adult supervision. We should note that devices containing these radioactive elements are NRC controlled, and cannot be sold or resold without a specific license, so don't try to make and market the apparatus described below, just make one for your personal use.
IMPORTANT NOTE: Depending on the country you live in, these ionization smoke detectors may or may not be available, plus other restrictions might apply.
Am-241 gives alpha particles and some low energy photons, both of which combine to make an excellent excitation source, exciting the surface with the alpha particles and deeper layers with the photons. This is a distinct advantage as we will see in later experiments. From one to eight smoke detectors are required, but no more are ever needed.
Before we go into the adaptor containing the exciters, let's talk about the sensors and the one selected for use in this project.
Sensors specifically designed to measure very low energy photons have to have some pretty unique qualities. First they have to have a housing, or at least a "window" made of very lightweight material. Anything too robust will simply block the desired photons by absorption. Next they have to be sensitive to the energy range in question, and finally they must give a signal output that is proportional to the amount of energy detected. After all, we are judging our results by the energy given off an atom, so we must be able to measure that energy.
Geiger Counters are well known, and some even will respond to low energy photons, but DO NOT give any energy information. All the "clicks" heard in the speaker are the same, no matter what sort of radiation caused them. Geiger Counters are ruled out for XRF.
Another sensor called a Gas Flow Proportional Detector DOES give energy information, and have been used in commercially made XRF machines for a long time. They have some disadvantages, namely:
1) They are VERY expensive.
2) They require THOUSANDS of Volts to operate.
3) A large bottle of nuclear counting gas- usually argon/methane is required.
4) Their output signals are VERY WEAK, requiring expensive and fragile preamplifiers to be used before the MCA.
Â On the plus side, they do operate at room temperature, so don't need liquid nitrogen. For this reason alone proportional detectors have been used before superior but not more affordable technology has recently become available.
Next we will examine the scintillation detector. These types of sensor work by emitting a flash of light from a crystal when radiation strikes it. These small flashes are routed to a Photo Multiple Tube (PMT) which converts them to photoelectrons, then greatly amplify those photoelectrons into an electrical pulse. Some have an amplification factor of one million or more. Once amplified, these electrical pulses are analyzed by our Multi Channel Analyzer (MCA), and their height determined individually. The height of these pulses is what contains the originating energy information- the higher the pulses in milliVolts, the higher was the originating radiation energy. This whole process is called PHA or Pulse Height Analysis, and is the basis for all the scans that I show related to radiation energy. On the scan, the horizontal axis increases as per energy, while the vertical axis increases by number of pulses. Later on we will publish many tutorial that explain each and every aspect.
Traditional scintillation detectors used a crystal of sodium iodide, activated with a small percentage of thallium, These are very efficient, but sodium iodide is hygroscopic, which means it absorbs moisture from the air like a sponge. Sodium iodide left in the open will turn to slush overnight.
Recent developments in crystal technology have allowed a new crystal to become available, but at a cost. This one uses Cesium Iodide, also thallium activated. It does not share the same moisture issues as its sodium iodide cousins, and to add to its efficiency, it is also denser.
Therefore a really good scan can be made with a 1 mm thick CsI(Tl) scintillator sensor so long as the "window" allows the photons to enter the sensitive volume.
S E International has created an example of this technology , in the embodiment of their RAP-47 LEG Probe. The RAP-47 uses a select grade PMT, a 1" diameter X 1mm thick CsI(Tl) crystal and a thin aluminum window. At less than $1000, these are a bargain, but check eBay for them at less than half that. Other suitable LEG probes are made by Ludlum Measurements and Technical Associates, but none of those use CsI(Tl) yet.
THE RAPCAP APPARATUS
The RAPCAP is a home built XRF exciter made to clip on to the front of a RAP-47 or other LEG probe.
Radioactive sources removed from up to 8 $4 ionization type smoke detectors provide the alpha particles and low energy X-Rays. These are arranged so they point away from and perpendicular to the face of the sensor, and have a small layer of lead shielding immediately behind them to avoid direct rays for the source hitting the probe window.
This first version shown uses 8 such sources, mounted on a brass washer with 1/8" holes drilled around the periphery to accept the sources, to completely surround the target as it is placed immediately in front of the RAPCAP, with a space of 1`/4" allowed. This small space is required so the excitation rays and particles have enough room to fully illuminate the target. Subsequent XRF rays FROM the target are collimated through a hole in the center of the RAPCAP and back to the sensor window.
This "flat" version is best for flat targets, which need be no larger than 1" X 1".
For tiny specimens, another version was tried and deemed successful, it has the excitation sources positioned at 22 1/2 degrees from perpendicular, thereby focusing the excitation onto a spot, just in front of the collimation hole. This version can analyze sub-gram sized samples, held in place by a fixture( tweezers). Over a few week period of time, I have used the RAPCAP to analyze all applicable elements on the Periodic Chart from Iron through Bismuth. Since a few elements within that range are naturally radioactive, these have been analyzed by the probe but without excitation (self-excited).
RAPCAP STEP BY STEP CONSTRUCTION
Step 1) Have your mentor purchase several ionization smoke detectors and remove the sources, using approved safety methods.
Step 2) Prepare a washer like base from brass, aluminum or other metal that is easy to work. If using a large brass washer, drill up to 8 evenly spaced 1/8" holes surrounding the central hole. The sources have a protrusion on the back that will fit perfectly into these holes. Secure them with epoxy, then place an undrilled similar washer on the back to further shield the sensor.
Step 3) Attach the now modified probe to the class MCA* (set to read from 2 through 50 keV) , and begin sampling elements!
I will be available for questions from teachers and mentors, and over the next few months will go through the Periodic Table of the Elements with you, one at a time. Some pretty cool elements can be found in everyday household items.
HINT: Next time a battery goes dead in a device of yours, keep it because we will dissect it later!!
* If your classroom does not own an MCA, don't worry, we will be showing in weeks to come how to use a freeware program on an ordinary computer with a soundcard to fulfill all the functions of an expensive MCA.
Glossary of terms used:
Beta Particle: an electron, whose origin is the nucleus of an atom. Can be positively or negatively charged.
Alpha Particle: a mass bundle consisting of two protons and two neutrons, originating in and expelled from the nucleus of an atom. Carries a double positive charge.
Gamma Ray: A photon created within the nucleus of an atom.
X-Ray: A photon created within the electron shell area of an atom.
XRF: X-Ray Fluorescence.
LEG: Low Energy Gamma.
MCA: Multi Channel Analyzer.
PHA- Pulse Height Analysis, analyzing and displaying pulse heights (amplitude), a function of an MCA.
keV: kilo electron Volt (a measurement of energy).