Untitled Document

The Noble Gas Mass Spectrometer Lab

Fig 1: The mass spectrometer source. A nice feature of the source is that alignment is quite easy even after completely disassembling the source.Currently, our sensitivity for He at 300 uA trap current is 2.25 x 10^-4 Amps/Torr

Fig 2: The mass spectrometer source.

Fig 3: Back ground count rates on the three multipliers. The above figure was taken at the start of the 7th cycle, with each cycle being 50s in duration. The background count rates on the three multipliers varies between 1-2 counts in 100s. The detectors are in fixed position and multicollection is achieved with the two quad lens. The quad lens can be used to rotate the focal plane, vary the beam dispersion, as well as move the beam. The axial multiplier has a retardation filter. For two of the multipliers (the high mass and axial) the beam can be steered after the detector slit to hit the multiplier 'sweet spot'. To hit the sweet spot on the low mass multiplier the quad lens can be tweaked.

Fig 4: Peak flat on IC1. Each point represents 3 x 0.1s integrations. Peak flat on IC1 is 1 part in 1000 over ±300 ppm. This is comparable to Faraday peak flats. Click here for the peak flat results.

Fig 5: 3He-HD peak. The energy filter on the axial multiplier removes scattered ions and photons. As a result the background at mass 3 is equivalent to the electronic noise on the mulitplier. While making 3He measurements, we resolve 3He only partially from HD. The advantage is higher sensitivity and better reproducibility. However, because this is technique is quite different from other labs, where 3He measurements are made at mass resolution of ~600-660, it is necessary to prove that tailing from HD does not in any way compromise the 3He measurements. Below are a set of experiments performed to prove that there is no tailing from HD onto the region where 3He measurements are made.

Fig 6: The first set of experiment consists of isolating the mass spec and partially closing the getter valve to vary the amount of HD in the mass spec. A run is setup to look at the tailing where 3He would normally be present. Each cycle consists of measuring the HD signal and then jumping to the 3He peak. Three measurements are made on the 3He peak, corresponding to the actual offset from HD, offset - 0.001amu, and offset + 0.001 amu. For a 400cps HD signal (the normal amount of HD while running standards and blanks) we measure a signal between 0.02-0.04 cps. No corrections were made for background multiplier noise. In addition to the run, scans where also run across the 3He peak (figure on the right). The gridlines are 2 cps. Each point represents 108 x 0.1s integrations. Click here for larger image. The 0.02-0.04 cps range is valid for HD signals of 400 - 1000 cps.

Fig 7: Tailing from HD signal onto 3He. The HD signal is ~3500 cps. The gridlines in the image on the right are 2 cps. The tailing from HD is 0.05 cps when HD is 3500 cps. Each point represents 100x 0.1s integrations. Click here for larger image.

Fig 8: Tailing from HD signal onto 3He. For a 7800cps HD+H3 signal the tailing on 3He is 0.12 cps. Each point represents 100x 0.1s integrations. The gridlines are 2cps. In the peak scan, the count rate integrated over the 3He region, was 0.14 cps. Click here for larger image.

The above set of experiments have been repeated multiple times and they prove that in the absence of 4He, tailing from HD onto the region where 3He will be measured is absent. Only when HD reaches unusually high levels (7800 cps) is there a hint of a tail. It is, however, conceivable that tailing could be a problem in the presence of a large 4He beam. Below are a set of experiments done to show that even in the presence of a large 4He beam, tailing from HD is not present in the region where 3He is measured (~0.003 amu peak flat available without tailing).

Fig 9: Peak flat on a 10 cps 3He signal with 1.1 volt of 4He in the instrument. If there is significant tailing from HD onto 3He a slope would be imparted on the 3He peak. A 20 cycle measurement was setup across the peak. Four different masses where measured (20 s each) in a cycle and the following sequence was followed in each cycle: Mass1, Mass2, Mass 3, Mass4, Mass4, Mass3, Mass2, Mass1. The 4 arrows in the figure correspond to the 4 masses. The results show no evidence of tailing from HD with 1.1V of 4He (corresponding to ~120 ncc) and 500 cps of HD. If tailing is present, it is equivalent to the signal noise (<0.3 cps). We have repeated the experiment 4 times and have reached the same conclusion each time. For large 3He signals (>100 cps) the uncertainity is negligible. For small signals (say 5 cps), Poisson stats will completely dominate the uncertainty. Note that 0.3 cps tailing uncertainty is not applicable to blanks since no 4He is present. For blanks an upper limit is 0.1 cps although a more reasonable value would be 0.05 cps (see figures above).

Fig 10: We have also measured the 3He signal by varying the amount of HD in the presence of a 1.1V 4He signal. If there is a tail from HD than the 3He signal should vary as the amount of HD in the instrument changes. We have varied the amount of HD from 450 cps (normal run condition is ~220 cps) to 9000 cps. Initially, the getter was partially closed and the mass spec isolated so that the HD signal reached an equilibrium value of 3800 cps. Then commercial tank He was introduced into out extraction system, cleaned, separated from Ne with a cryo trap, and let into the mass spec. The 3He signal was measured for 2x100 secs. Then the getter was opened (no variations were seen in the 4He beam by opening and partially closing the getter valve) HD decreased to 650 cps and 3He was re-measured. As can be seen (Points 1 and 2) there is no difference in the measured 3He. Because the 3He measurement with 650 cps HD was made after the 3800 cps measurement, if anything, 3He should be lower due to decay. The getter was isolated, almost completely this time, until HD reached 9000 cps. For such a large HD signal there appears to be a tailing of about 0.5 cps (Point 3). However, unless the getter is partially isolated, such high levels are never achieved in any instrument even after bakeout. The getter was then completely opened and 3He re-measured (Point 4). Over the course of the experiment 4He decayed from 1.15V to 1.10V. Even correcting for decay, there is no change in our conclusions that tailing from HD does not in any way comprise 3He measurements.

The above set of experiments proves that the Harvard lab is capable of measuring 3He/4He ratios with an accuracy that is better than, or comparable to the accuracy obtained in other labs. Additional information regarding reproducibility and linearity is given below

Fig 11: Instrument linearity and reproducibility of our high 3He/4He standard. The mass spectrometer is linear between He partial pressure of 5x10^-8 Torr and 2x10^-10 Torr. While the instrument is likely to be linear at higher pressures as well (based on runs done in the factory), we have not yet run standards to prove this.

Fig 12: Reproducibility of 4He and 3He/4He ratio. The helium is UHP helium out of a commercial tank. The standards were run over a period of two days.

Facilities • Group • Publications • EPS Home