Film Change Guide

Introduction

The XRF Core Scanner is designed in such a way that the source and detector both move together down the length of the track. The components are held at fixed geometries relative to the (ideal) sample surface by a hollow piece of plastic that we call the prism. On one side, the source window sits nearly up against the prism, only separated by the cross-core slits and filters. On the other side, the nose of the detector fits inside of the prism, allowing the detector window to sit ~1 cm off of the base (Figure 1). During each measurement, the prism lands on the sample surface, meaning that the distances between the detector and the sample and the sample and the source are fixed (approximately).

In order to improve the measured intensities of light elements, we fill the prism, and therefore all of the space between the sample and the detector and most of the space between the source and sample, with helium. Helium absorbs a lot fewer x-rays than air, so by displacing the air from the beam path, we get a lot more x-rays reaching the sample and the detector. More information about the impact of air can be found in the quick start guide, but suffice it to say, your signals from elements such as Al, Si, K, and even Ca will be a lot smaller if you don't keep the air out. 

In order to maintain a relatively gas-tight yet x-ray transparent system, pieces of very thin (4 micron) polyethylene film (Ultralene) are used as x-ray windows at the back (source side) and base (sample side) of the prism. Ultralene is strong enough to hold back the positive pressure of the helium gas, and it has one of the lowest x-ray absorption tendencies of any solid material on the market.

Unfortunately, 4 micron thick plastic film isn't very strong. When the prism lands on the sample, the film at the bottom comes in direct contact with whatever is in the way. In an ideal scenario, the core surface is flat and covered itself with a protective layer of film. Ultralene is designed to handle those sorts of stresses. But in a non-ideal scenario, the sample surface may be rough, it may contain sharp objects, or the prism may land in an unintended location, such as on the sharp plastic of an end-cap. In those non-ideal scenarios, the bottom film is likely to be punctured or torn.

When the bottom film breaks, then the prism is no longer gas-tight, and air starts getting into the chamber. The bubbles will stop appearing in the outflow bottle and the intensities of the light elements will start to drop. If you're not paying attention, your data quality will begin to degrade. That means that once you stop seeing bubbles, you'll want to change the film.

There are other scenarios where you'll want to change the film, as well. Sometimes you may suspect (or know) that you've gotten contamination on the bottom film. Every time that the prism lands to make a measurement, it makes contact with whatever is on the sample surface. If the film on the core is dirty, or if you forget to put the film on, the prism will pick that dirt up, and you'll see that dirt's x-ray signal in every point that you measure. If you suspect contamination, you'll want to change the film.

Typically, the Lab Manager or Technician will change the film for you. But, if they are not available, you may (in some cases) be given permission to try changing it on your own. Follow the instructions below very carefully! Failure to perform certain steps (such as closing the slits) may introduce significant risk of instrument damage! If the instructions are properly followed, then changing the film is a simple (if not always easy) task that just requires some practice.

Preparing the Instrument

Once you're ready to change the film, you'll want to remove any core or set of standards that you have loaded. Slide the clamps off to the sides of the track so that you can create space in the middle for working. Also, make sure you turn the helium off before you begin. It doesn't hurt anything if you leave it on, but it just wastes the gas.

Before you change the film, you'll need to make some adjustments to the instrument. First, you'll close the down-core slits. That will protect the sensitive end of the detector while you're working on the prism. Second, you'll move the source + detector assembly to the middle of the track so that you can access it much more easily.

To close the slits, you'll want to follow the instructions in Figures 2-6. Set the instruments to "Local" mode using the control panel software. If you don't see the option in the menu on the new instrument, you'll need to toggle the menu at the top left (Figure 2). Once you're in local mode, switch to down-core slit control mode (Figure 3). Click on "Destination" (Figure 4) and enter the value "0.1" using the keypad that pops up (old instrument) or the slider (new instrument) (Figure 5). Then, click "GoTo" and the slits should start to close (Figure 6).

It is absolutely critical that you perform this step! The end of the detector inside the prism is protected by a very thin (12.5 micron) piece of beryllium. That piece of metal is extraordinarily fragile, and if it gets broken, it costs tens of thousands of dollars to repair. The down-core slits sit on the base of the prism, so by closing them, we prevent errant tools and fingers from coming into contact with the detector.

It's difficult to change the film when the prism is at it's home position on the track, so you'll want to move the prism to a more convenient location. Follow the instructions in Figures 7-10. First, switch the instruments over to "Servo" mode (Figure 7). If you need help getting back to the main menu on the old instrument, please see Figure 6. Second, click on "Destination" (Figure 8) and enter "800", or a similar high number, using the keypad (old instrument) or keyboard (new instrument) (Figure 9). Then click "GoTo" and the entire measurement assembly should move down the track to an easier-to-access location (Figure 10).

Changing the Film

Now that you've closed the down-core slits and you've moved the prism to an easily-accessible location, it's time to change the film. Start by prying the red metal ring off the base of the prism using a spatula or similar tool (Figure 11). Carefully insert the spatula between the ring and the plastic and twist to pop the ring off. Make sure you don't slide the spatula too far past the corner, or you'll potentially damage the O-ring that provides the seal.

Next, peel the old film off the bottom of the prism (Figure 12). If the film is torn, you'll want to make sure to remove all of the fragments.

Now you'll need to fit a new piece of film over the ring. We have two metal holders with rolls of film that will make this a bit easier for you. Those are the rolls of the film that we typically use for film changes, so please try to keep them clean. You'll want to pull a bit of film out from the roll, cut it off, and discard to make sure that you're getting fresh plastic. The film is slightly wider than the longest dimension of the metal ring (Figure 13), so the easiest way to cut a new piece is to roll the plastic over the ring and make a single cut. Once you've got a piece of film that is slightly larger than the ring in all dimensions, you'll want to gently hold the plastic against the sides of the ring to keep it tight (Figure 13). Make sure that the film is laying on the flat side of the ring, because that's the side that sits flush against the prism.

Now you'll need to get the ring and the new film back on the base of the prism. This is the hardest step in the process, and it can take some practice. Sometimes, you'll get it right on your first try, and sometimes you'll get it right on your fifth try (or more). Be patient, and ask for help if you need it. Once you get the feel for this step, it becomes easy.

Before you start trying to snap the ring back in place, you'll need to find a way to hold the prism still. The prism slides up and down on a track to allow for some movement when it lands on the sample. When the prism slides all of the way to the top of the track, the source-side film gets very close to the cross-core slits and may break (Figure 14). You won't be able to change that film yourself, so you're going to want to prevent that from happening! Fortunately, it's fairly easy to hold the prism in place. Use your right hand to push down on the plastic "arm" sticking out to the right of the prism (Figure 14). Do NOT hold on to any other components during this step! If you want, as an extra precaution, you can fold up a Kimwipe and place it between the back metal ring and the filter + slits assembly. That will provide a cushion and prevent the gap between the back film and the slits from closing all the way.

Once you've stabilized the prism, you can snap the metal ring back in place with the fresh piece of film on top (Figure 15). Line the ring up, and hold the film in place very loosely. Make sure the flat side of the ring is facing upwards, and make sure the film is between the prism and the ring. As you begin to press the ring back into place, allow the film to slip a bit from your fingers. The film needs to slip into the space between the metal ring and the O-ring, so if you hold it too tightly, it will tear. Make sure you press the metal ring all the way back onto the prism. It doesn't require much force, but it should "snap" into place. Once you're finished, check your work. You'll want to make sure the film doesn't have any large, obvious wrinkles or tears. You'll also want to make sure that the ring is actually flush with the plastic all the way around (Figure 16). If you tore the film during this step, remove the ring, get a new piece of film, and try again.

Now it's truly time to test your work. Turn the helium back on and watch the outflow bottle for bubbles (Figure 17). It may take a few seconds to re-pressurize the system, but if you don't see anything soon, then you either tore the film or you didn't get the ring all the way back on the prism. The new instrument utilizes a high-rate flush procedure, so you should see a very vigorous bubbling at first. Wait for ~1 minute to see if the bubbling continues after the rate goes down to the normal steady-state value. Sometimes you'll see bubbles at the high rate and not the low rate, which tells you that you that you have an imperfect seal somewhere (tiny hole, ring slightly out of place, etc.). On the old instrument, the rate will start at the normal steady-state value, so if you see bubbles right away, you're good to go!

Getting Ready to Scan Again

Once you're confident that you've successfully replaced the film, you can get the instrument ready to scan again. First, move the servo back to the "0" position (see Figures 7-10 for a reminder of how to use the menus). That will move it out of the way so you can load standards and/or cores. Second, return the instrument to "Remote" mode (see Figure 2 for a reminder of where to find the menu options). This step is critical, because if you forget to do this, you'll won't be able to run your next set of analyses. The acquisition software will give you an error, and it won't tell you anything useful such as "instrument is in local mode, can't control it from here". You don't need to open the down-core slits back up at this stage, because they'll be opened automatically when you start your next analysis. 

Finally, you'll need to let the helium flow for ~8-10 minutes on the old instrument and ~4-5 minutes on the new instrument to make sure all of the air is purged from the prism (Figure 18). At that point, we recommend that you run the standards again just to make sure that everything looks OK. If you need help remembering what to look for, check out the Quick Start guide.