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Fifth Week

This week, Nick and I learned a lot about how the new project assigned last Friday.

The experimental setup is something like this (Figure 1).

Figure 1: We used energetic proton beams (1MeV or 4MeV) to ionize and dissociate target molecules.  The ions produced in these breakup interactions are drawn to a detector via an electric field.  The box in Figure represents our target volume.  In that area, protons are colliding with target molecules until the proton has lost its energy or has vacated the target volume.  During the collisions, the target molecules have a chance to either dissociate or ionize.  Then, the ion fragments are accelerated toward the detector to be measured.  Neutral and electron fragments are not detected because there are no means to measure the neutral fragments in this experiment.

We can tell the difference between particles that have different mass/charge ratios.  Ions with more charge are accelerated more while more massive fragments are accelerated less.  The proton beam that causes the breakups is pulsed so that we can identify events that originate from certain pulse.  For example, we know our proton pulses a certain time T1.  At T2, we might receive one distinguishable hits at the detector.  If T2 is close to T1, then we figure that the hit was caused by the pulse at T1.  We can also receive two or three hits from one pulse.  As long as the fragments arrive at different times, we can resolve the hits.  Consider an H2O breakup.  H2O can breakup into an H+, an H+, and an O+.  At our detector, the two H+ cannot be resolved as separate events (both H+ have the same mass/charge ratios), but the O+ can be resolved from the H+ because it has a different mass/charge ratio from H+.  At our detector we will see H(+) + O(+).

One difficulty in this experiment is that the detector only has a about a 1/3 chance of measuring an actual ion hit.  Consequently, there are some breakup channels that are difficult to exclusively identify.  Consider two H2O breakups.  One is the H(+) + H(+) + O(+).  This channel can appear on the detector as H(+) + O(+) or H(+) or O(+) or nothing.  The second breakup channel is H + H(+) + O(+).  The other can appear as exactly the same things.  Therefore, when our detector sees those events, it is difficult to know which breakup caused the hits.

The targets that Nick and I are looking at are H2O, D2O, HDO, and normal air.  In the experiments, the target volume is evacuated and then the target is introduced.  The proton beam had 4MeV of kinetic energy.

Why is this experiment important?

It allows us to study the physics of water molecules' breakup.

So...

Well, it's important to understand those physics.  There's one cool thing that this experiment helps.  It's believed that cancer can start when our DNA is broken apart by some kind of ionizing radiation.  I didn't say the radiation needs to actually hit the DNA.  It is far more likely that the water molecules that are near DNA will be ionized and then harm the DNA in turn.  The more we understand these problems on a basic level, the better we can solve them.  Cool, huh?