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This is a diagram of the Moessbauer apparatus.
In order to produce a spectrum, we must be able to create a range of photon energies. This is accomplished with a Doppler shift. Naturally, if a decaying photon source is moving towards the sample at the time it emits a photon, this emitted photon will have slightly more than the energy of the nuclear transition it originated from. (And vice versa, it will have less if the source is moving away.) Thus, if we oscillate the photon source, we can produce a range of photon energies. Doing this is the job of the Mossbauer driver and linear motor components.
The Cobalt 57 spectrum contains multiple peaks, but we only want to study the interactions of the 14.4 keV photons, and not photons from other peaks. The single-channel analyzer (SCA) picks out only 14.4 keV photons, and tells the DAQ when a 14.4 keV photon is observed. The DAQ also constantly receives information about the velocity of the source.
Of course, any spectroscopy setup must include a detector. In this setup, photons emitted from the Cobalt 57 source interact (or not) with the absorber and are detected (or not) by a silicon x-ray detector Amptek XR-100CR. This detector outputs an analog signal where the amplitude is proportional to the energy of the detected photon. The detector's output is sent to a Amptek PX2T amplifier made specifically to work with this detector. The amplified signal is then sent to the Ortec 850 SCA. If the detector observes a 14.4 keV photon, the SCA will output a TTL pulse that acts as a trigger signal for the DAQ. When the DAQ (and associated LabVIEW program) receives this trigger signal, it then reads in the velocity signal at that instant. The LabVIEW software uses this information to compile a histogram, and will write data to a file at intervals decided by the user.
The Detector and Its Amplifier
In this experiment, an Amptek XR-100CR silicon detector is used. When a high-energy photon interacts with the detector, it outputs a pulse for which the amplitude (voltage) is proportional to the energy of the detected photon. Silicon is used for this experiment because it has a high efficiency in the energy range (keV) that we want to study. The output pulses are very small in amplitude, however, so the detector has its own amplifier (Amptek PX2T) to amplify all of the signals coming out of it. The gain setting on this component is adjustable.
The PX2T amplifier for the Silicon detector:
The Mossbauer Driver and Mossbauer Linear Motor
Oscillating the source is carried out by the Mossbauer driver and the Mossbauer linear motor. A small but highly potent (on the order of a few mC) photon source (Cobalt 57) is mounted on the spring-like coil of the linear motor. This coil is similar to what is found inside of a loudspeaker. When the coil is driven by the linear motor it oscillates back and forth, providing the Doppler shift. An HP 33120A function generator supplies a 14Hz reference frequency (which is close to the natural frequency of the spring-like coil) to the Mossbauer Driver (Austin Science Associates' Model S3). The driver is connected to the linear motor (see image below) via a feedback loop. The driver's "goal" is to maintain a constant acceleration of the source as it travels towards the sample, and then quickly reverse its direction and provide a constant acceleration away (and then repeat). So, the component of the feedback loop between the driver and the motor labelled 'velocity' should appear to be a very clean, flat-sided triangle wave. This results in the oscillating source spending equal amounts of time at each velocity in its range. Deviations from a pure triangle shape will influence count rates and distort a resulting velocity vs. count rate histogram. The component of the loop labelled 'drive' is representative of the voltage supplied by the driver to the motor to maintain constant acceleration - larger spikes indicate more drastic corrections and appear near the peaks/valleys of each triangle.
The Mossbauer driver, front panel (back panel has inputs for 'velocity,' 'drive'):
The Mossbauer linear motor. The spring-like copper coil is visible at one end, and also note the 'velocity' and 'drive' inputs. A small button source of Cobalt 57 is mounted on the spring-like coil during operation (not shown). THE SOURCE IS EXTREMELY STRONG. BE SURE TO COMPLETE AND ADHERE TO RADIOACTIVITY SAFETY TRAINING.
Making the triangle wave as clean as possible is important. Use BNC tees to split both the 'velocity' and 'drive' signals so that one part of each signal can be viewed on the oscilloscope. If the triangle wave is not already very clean-looking, gently and slowly adjust the fidelity knob on the Mossbauer driver until you have a near-perfect triangle wave. Oscilloscope screen captures from the Mossbauer driver are shown below: the triangular 'velocity' signal (left), the roughened square wave 'drive' signal (center), and both signals overlaid (right).
In the final setup, the 'velocity' signal is split using a BNC tee. The one part goes to the linear motor, and the other part goes to a Stanford Research Systems Model SR560 low-noise pre-amplifier ('Source A'). The output from the pre-amp (an analog signal that describes the current velocity of the source) is constantly input into a DAQ (using the NI PCI-6036E card) running a custon-made LabVIEW program.
The SCA and Counter
The SCA upper- and lower-limit thresholds allow for windowing on a specific energy range. If the SCA receives a signal that falls into the window, it outputs a TTL pulse. If it receives a signal that falls outside the window, it outputs nothing. In this case, the window is centered on the 14.4 keV peak of Cobalt 57, since these photons are the ones involved in resonant absorption. Properly calibrating the SCA will be described in a seperate section.
The Ortec 550 SCA and the Ortec 994 counter:
The spec sheets for the SCA and counter:
The DAQ and LabVIEW Program
The DAQ (which interfaces with the NI PCI-6036E card) is what brings the data together and communicates it to the PC. We will use two of the DAQ's many channels. Channel ACH0 receives the velocity signal at all times - but it doesn't read the signal at all times. The DAQ will only read the current velocity at the moment it recieves a TTL pulse in the PFI0 channl. Each time the DAQ is triggered by a PFI0 pulse it samples the velocity, and lots of samples create a histogram of how often each velocity showed up in a sample. A custom-written LabVIEW program is what makes this happen. The LabVIEW code can be viewed here:
MossbauerWorkingRewrite.vi: download the LabVIEW code for Mossbauer Data Collection
Next, Setup and Operation
-- Main.pfei0142 - 06 Jun 2013
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