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April 06, 2022
How To Irradiate Your Samples
Following some of the successes and pitfalls at the DCF facility we now have all the irradiations completed thanks to the dedicated staff at the Dalton Cumbrian Facility. Since the energies in this round of experiments were sufficiently below the threshold of activation, samples were dispatched in the post.
The next stage will be to clean and mount sample for cross-sectional imaging, hardness testing, EBSD and TEM but that will be discussed in a later entry since much of that work will be for publication.
So how does one get to irradiate samples in the first place? And what has this specific experiment got to do with the first goal of the Radiation Dense Materials concept?
First of all as with any new science experiment, you have to talk to people first, in person, online or by email. This is critical to ensure that you can flesh out the proposed experiment and see how feasible it is or isn't. By discussing the planned irradiation schedule with the staff at the Dalton Facility we were able to acertain that ambient and non-ambient irradiation was possible with the proton beam and the gamma irradiator.
One of the inherent difficulties of current irradiation facilities is that generally samples can only be irradiated by one type of radiation at one time, unless placed in a test reactor. This is similar to reproducing a painting but with only one brush and one pigment. Hence, it is important to state the limitations of irradiation testing on samples with this in mind.
So, how are samples actually irradiated? This depends on the method and the radiation used. For the gamma irradiator it is simply a case of tracking the face closest to the 60Co source and placing it in a low-activation borate glass. The 60Co irradiator is shown below:
For low-temperature (77K) irradiations, samples were placed in a dewar full of LN2. Since oxygen liquifies at 85K this forms a pool in the dewar which under gamma irradiation transforms to O3 which is solid at LN2 temperatures. Solid O3 is an explosion hazard and thus nescessitates emptying and renewing the LN2 every 5 kGy. The low-temperature high-dose gamma sample had 90 kGy in total.
For proton beam irradiation, sample preparation is more involved. Samples need to be mounted with a heat-resistant conductive adhesive (silver paste) and need alignment such that the beam energy and radiation time can be computed, the exact figures depending on the dosage and the target temperature. Samples mounted for proton-beam irradiation are shown below:
Since protons are charged particles, total proton dosage can be computed from the charge accumulated from the beam current. Alongside sample irradiation, other data from samples such as emissivity can be obtained from proton irradiation at elevated temperatures.
Data analysis is at the earliest stages with the following planned sequence (1) Cross-sectional imaging (2) microhardness measurements (3) EBSD and (4) TEM (specifics to be determined following from EBSD and HV measurements).
Ultimately, this work will form the outline for subsequent radiation work as this is a first for both cWC and RSB materials.