MSBS (Mass Spectrometer-Beam System)

The MSBS (Mass Spectrometer-Beam System) was a gas expansion system combined with a mass spectrometer developed at the University of Minnesota in middle 1970's decade. The instrument was designed for mass spectrometry investigations in the stratosphere.

In the image at left we can see a scheme of the instrument in flight configuration (click for more details). The design of the instrument included a gas inlet system composed of two large spheres connected by a smaller central sphere. These spheres housed liquid helium pumps that provided the necessary pumping speed. A diaphragm within the center sphere created two separately pumped chambers. Ambient stratospheric gas particles entered through a small orifice, encountering a rapid pressure reduction facilitated by the liquid helium pumps. Molecules aligned with a second orifice passed into a subsequent chamber, where further pressure reduction occurred before the particles reached the ionization region of the mass spectrometer. The system's construction enabled a decrease in background pressure by more than four orders of magnitude, ensuring effective gas analysis in the low-pressure environment.

A key feature of the system was a movable flag positioned in front of the entrance to the mass spectrometer ion source. This flag could either block the beam entirely or retract to allow its passage. With the flag engaged, only background gas was analyzed, while in the retracted position, both beam and background gases were measured. The signal difference between these two states represented the contribution from the direct beam, ensuring that the majority of analyzed particles had traveled from the ambient atmosphere without interacting with the walls of the instrument. This setup resulted in a beam-to-background ratio exceeding 30 for nonreactive gases such as nitrogen, argon, and carbon dioxide.

The liquid helium pumps played a crucial role in the instrument's functionality by providing continuous high-speed pumping. At liquid helium temperatures, nearly all atmospheric gases adhered effectively to the pump surfaces, with the exception of helium, which did not adsorb significantly due to its low atmospheric abundance. Each pump contained a 2-liter capacity of liquid helium, allowing for a holding time of approximately 6-7 hours under stratospheric conditions. The evaporated helium was designed to be vented away from the experiment to minimize potential disturbances. These pumps demonstrated effective pumping speeds exceeding 3000 liters per second, yielding exceptionally low residual gas levels.

The mass spectrometer itself featured a compact magnetic design based on the Mattauch-Herzog geometry, similar to instruments used in satellite applications. It comprised an ion source, electric and magnetic analyzers, and an ion collector system. Ionization of the neutral beam occurred through electron impact, with an electron beam intensity of approximately 100 microamperes and an electron energy of 75 electron volts. The mass spectrometer covered a range from 7 to 100 atomic mass units. Detection sensitivity extended over a dynamic range exceeding nine orders of magnitude through the combined use of an electrometer and a counting multiplier. The counting multiplier provided a low-noise background and high gain stability, while the electrometer detected major atmospheric gases.

For balloon flights the beam system, telemetry, and batteries were mounted inside a pressure tight, spherical aluminum gondola that had a diameter of 80 cm, and contained internal thermal insulation. The crash protection was a collapsible framework of vented aluminum rods surrounding the gondola. All exterior surfaces were cleaned with methyl alcohol, and the entire experiment was sealed in a bag which was purged with dry nitrogen. The bag was removed just prior to launch. To further insure cleanliness during the flight, the experiment was suspended by a nylon line separating it from the parachute and balloon by 120 meters. The protective cap over orifice 01 was removed after descent was established and only then was stratospheric air permitted to enter the beam system.

Balloon launched on: 10/30/1984 at 15:58 utc
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon SF370.34-050-NSCHR-01
Balloon serial number: W18.35-2-01
Flight identification number: 1385P
End of flight (L for landing time, W for last contact, otherwise termination time): 10/30/1984 at 23:30 utc
Landing site: 20 Miles SE of Jackson, Mississippi, US

• A highly sensitive mass spectrometer detector system International Journal of Mass Spectrometry and Ion Processes, vol. 76, issue 1, pp. 85