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`High flux beam source of thermal rare-gas metastable atoms
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`INTEL 1105
`
`

`

`Apparatus and techniques
`
`minimises shock overloads. This simple technique ensures
`rapid application of an accurate load. The rate of penetration
`of the indenter is thus essentially material-dependent and
`therefore enables comparisons to be made with results from
`other hardness tests, e.g. Brinell and Vickers toBritish Standards
`specification. The load is maintained for a known time (eg.
`15 5) prior to removal; recovery is allowed for a second fixed
`period. The transducer readings corresponding to the unde-
`formed sample surface, the depth of penetration under load
`and the recovered depth of indentation are fed into a computer
`which calculates the hardness and elastic modulus of the
`material, as well as parameters such as the diameter and
`volume of the indentation and the radius of the recovered
`surface. The tester can also be used to obtain data on time-
`
`dependent properties such as creep compliance: the load is
`maintained for the desired period and data are collected
`continuously.
`
`5 Conclusions
`This indentation tester has already been used successfully in
`the investigation of pharmaceutical tablets (Aulton et al 1974,
`Aulton and Tebby 1976). The versatility of the tester can be
`considerably increased by changes in indentation load, type
`and size of indenter
`tip, and transducer characteristics.
`However, we consider the realistic applications of this instru-
`ment to be limited to materials with hardness values between
`5 and 1000 MPa.
`We suggest that this indenter could find wide use in the
`characterisation and routine quality control of materials such
`as polymer films and plastics, foodstuffs (e.g. fruit, chocolate,
`ice—cream), pharmaceuticals, and soft metals such as lead.
`
`References
`Aulton M E and Tebby H G 1976 Time-dependent
`deformations of tablets during indentation testing
`J. Pharm. Pharmac. 28 Suppl. 66P
`Aulton M E, Tebby H G and White P J P 1974
`Indentation hardness testing of tablets
`J. Pharm. Pharmac. 26 Suppl. 59F
`
`
`
`J. Phys. E: Sci. Instrum., Vol. 13, 1980. Printed in Great Britain
`
`High flux beam source of thermal
`rare-gas metastable atoms
`
`D W Fahey, W F Parks and L D Schearer
`Physics Department, University of Missouri, Rolla,
`Missouri 65401
`
`Received 9 April 1979, in finalform 5 October 1979
`
`Abstract A high-flux beam source has been constructed for
`the production of helium, neon and argon metastable atoms.
`The source is a DC electric discharge maintained in an
`expanding gas. A metastable flux of 3-5 x 1014, and 7-2 x 1013
`atoms 8‘1 st—1 has been achieved with most probable
`energies of 66, 72 and 74 meV for the helium, neon and
`argon sources, respectively. Time-of—flight measurements
`showed the widths of the respective velocity distributions to
`be 45 f’/,,, 27 ‘34, and 27 °/,;.
`
`Introduction
`1
`A novel metastable beam source was recently described by
`Leasure et a1 (1975), whose design employed a weak, high-
`voltage corona discharge between a sharp needle and a
`cone-shaped anode. The discharge was maintained across a
`substantial pressure gradient. This source provided up to 1014
`metastable helium and argon atoms s—1 sr'1 with beam
`energies between 5 eV (helium) and 74 eV (argon). The
`attractive features of this source were its relative simplicity
`and high flux.
`We report here modifications to the Leasure et al design
`which result in a further simplification, enhanced beam flux,
`species-independent energies, and importantly, beam energies
`only slightly in excess of thermal energies. The source is
`capable of providing very stable thermal energy beams of
`helium, neon, and argon metastable atoms with flux values
`near 101‘1 metastable atoms s—1 sr—l.
`
`2 Source design and operation
`The source is essentially a low-voltage discharge between a
`sharp needle and cone-shaped skimmer electrode. The dis-
`charge is maintained across a pressure gradient created by
`differentially pumping a gas nozzle. The source design is
`shown in figure 1. A vacuum fitting is mounted in a vacuum
`
`Source region
`
`Reaction
`region
`
`Vacuum woll
`
`B
`
`015 mm
`C
`D
`/ r—l-O mrr
`.iiommh-y- y‘-
`5mm
`
`Figure 1 Beam source schematic showing Pyrex tube (A),
`boron nitride nozzle (B), skimmer (C), and needle or needle
`array (D).
`
`0022-3735/80/040381 +02 $01.50 © 1980 The Institute of Physics
`
`

`

`Apparatus and techniques
`
`wall and seals around a 7 mm 0D Pyrex glass tube (A) that
`extends into the vacuum chamber. A machined piece of boron
`nitride (B) is attached with epoxy to form a cap for the end of
`the glass tube. A small hole drilled in this cap serves as the
`nozzle opening. The skimmer
`is a cone-shaped piece of
`stainless steel (C) with a small hole at the apex. Inside the glass
`tube behind the nozzle, several steel hypodermic needles are
`supported to lie near the axis of the tube. The skimmer piece
`is attached with an aluminium gasket to a vacuum wall to
`allow differential pumping of the source. Gas is admitted to
`the glass tube by a micrometer leak valve mounted outside of
`the vacuum chamber. The source region is contained inside a
`a 10cm Corning Pyrex glass cross which is evacuated by a
`300ls*1 oil diffusion pump. The reaction region is a 971
`stainless-steel chamber in which the pressure is maintained
`below 1-3 X 10—4 Pa (10‘6 Torr).
`The needles behind the nozzle are the cathode of the
`electric discharge and hence are maintained at a negative
`potential with respect to the skimmer which is kept at ground
`potential. Since the needle electrode is a cold—cathode type,
`the application of the voltage necessary to sustain the dis-
`charge is not generally suflicient to initiate a discharge. It is
`therefore necessary to apply an initiating high-voltage pulse
`simultaneously with a negative DC sustaining voltage in order
`to turn on the source. The DC source discharge after initiation
`by the high voltage pulse is maintained at 3 mA and 400 V.
`The optimum nozzle pressures for the three source gases
`used were measured with a Wallace and Tiernan dial gauge to
`be 6-7 kPa (50 Torr) for He, 11-3 kPa (85 Torr) for Ne, and
`60 kPa (45 Torr) for Ar. The background pressure in the
`source region at these operating pressures was measured with
`an ionisation gauge at low emission current to be between
`0-13 and 020 Pa (1-0 and 15 mTorr). At 0-13 Pa helium
`background pressure the mass flow rate was determined to be
`66 Pa 15—1. The beam flux is a slowly varying function of
`operating nozzle pressure. At
`2-7—6-7 KPa (20—50 Torr)
`below optimum pressure the source discharge cannot be
`sustained, and at 6-7—13-3 kPa above optimum pressure the
`beam flux is reduced to zero after slowly decreasing from the
`optimum value.
`The stability of the source output at 3 mA emission was very
`good. Degradation in source yield from use results primarily
`from wearing of the nozzle opening. After a run period of a
`month at several hours a day, the nozzle diameter is virtually
`unchanged as measured with a travelling microscope.
`
`3 Beam diagnostics
`The beam was characterised using two very different detection
`methods. The first method of detecting the beam made use of
`particle detectors. The second method used the detection of
`optical emission resulting from the interaction of the beam
`components with a strontium vapour target. Two types of
`particle detectors were used to observe all beam components
`and to estimate the absolute flux and energy of the corn-
`ponents. For all diagnostic measurements, the beam was kept
`free of charged species by maintaining an adequate voltage on
`a set of parallel sweep plates mounted after the skimmer.
`
`3.1 Particle detection
`For direct particle detection, both a coppereberyllium particle
`multiplier and a specially designed metastable detector were
`used. Copper—beryllium dynodes have sh own better than 50 3/0
`efficiency for secondary electron ejection by slow metastables
`and up to 20 % efliciency for photons below 200 nm (Dunning
`er a] 1975, Smith 1972). The surface is also sensitive to fast
`neutral ground state particles and ions of suflicient kinetic
`energy. The particle multiplier was used with a chopper wheel
`
`382
`
`(TOF) spectrum for different
`to analyse the time of flight
`component
`species and their
`respective velocities. The
`multiplier was mounted in the chamber such that it could
`undergo displacements of up to 60cm in order to allow
`accurate measurements.
`
`The TOF spectrum for the helium, neon and argon beams as
`observed with the particle multiplier revealed only two peaks.
`The first is identified as the detection of resonant photons from
`the source discharge and the second as the detection of the
`respective metastable species of the source gas. A sample TOF
`spectrum is shown in figure 2 for the helium source. The
`photon peak is established by the fact that its shape matches
`the aperture function, as it must for photons, and by the fact
`that the position of the peak in time with respect to the chopper
`wheel reference signal and the shape of the peak remain
`unchanged for multiplier displacements. The velocity of the
`slower peak is established equally by its separation in time
`from the photon peak and from its displacement in time
`resulting from a spatial displacement of the multiplier.
`
`
`
`
`Relativeintensny
`
`0
`
`0-2
`
`0-1.
`Time (ms)
`
`0-6
`
`08
`
`1-0
`
`Figure 2 Tor spectrum for the helium source for a detector-
`to-chopper wheel separation of 85 cm.
`
`The most probable velocity and velocity distribution of the
`metastable species were obtained by assuming a weighted
`Gaussian for the distribution (Anderson and Fenn 1965). It
`was determined by numerical
`integration that,
`for
`these
`measurements, the detector current was well approximated by
`the ideal case in which the Chopper wheel aperture was open
`for a time negligible compared to the time of flight. In this
`ideal case the velocity distribution, f(v), is proportional to the
`time-of-flight spectrum, [(2‘), multiplied by t2, where or is the
`detector-to-chopper wheel separation. The most probable
`velocities of the calculated distribution for the three metastable
`species were 1-8 ><103 m 5—1. 8-3 X 102 m s—l and 6-0><102 m
`s—1 for helium, neon and argon,
`respectively, for source
`conditions of 3 mA discharge current and optimum nozzle
`pressures. These velocities correspond to energies of 66 meV,
`72 meV and 74 meV, respectively. The half-widths of the
`velocity distributions were 45 3g for helium and 27 "/3 for neon
`and argon.
`The specially designed metastable detector incorporates the
`principal features of a gas cell used by Dunning and Smith
`(1971) to measure secondary emission coefficients. With this
`detector absolute flux measurements of
`the metastable
`component of the beam were obtained. The sensitive surface
`was a disc of chemically cleaned stainless steel or copper from
`which secondary electrons ejected by the beam were measured.
`The secondary electron ejection coefficients for the target
`surface for the three metastable species lie very close to unity
`(Dunning and Smith 1971). The flux at 3 mA emission and
`optimum pressure for the helium, neon and argon metastables
`
`

`

`Apparatus and techniques
`
`5“1 sr"1,
`was 35 x10”, 15 x1014, and 7-2 x1013 atoms
`respectively. The flux values obtained for the different target
`surfaces agreed within the coefficient uncertainty. The photon
`flux was shown to contribute much less than 1 f’/., of the total
`secondary current.
`
`3.2 Strontium optical emission
`The metastable character of each beam was established
`independently of particle detectors by observing the optical
`excitation produced in strontium vapour titrated into the beam
`path. The metastable beam components react with strontium
`in Penning ionising collisions which can leave the strontium ion
`in an excited state. The resultant excited state emission is
`monitored as evidence of the metastable beam component. A
`spectrum of the strontium emission for each metastable beam
`revealed the SrII 52P3/2-5251/2 emission line as the most
`intense in the visible region. This result agrees with flowing-
`afterglow emission studies that have been performed in our
`laboratory. Thus, with the added information that the beam
`energies are near thermal energy,
`the observed strontium
`emission is sufficient evidence of the metastable character of
`the beam.
`
`4 Source performance
`The beam source of metastable helium, neon and argon atoms
`has proved to be highly reliable and very stable, operating
`many hours per day over several months without attention.
`The high beam flux and the high stability have permitted us to
`observe the formation of coherently excited ions in Penning
`ionisation (Fahey et a] 1978).
`We have also successfully extended the operation of this
`source to include molecular systems such as nitrogen. In this
`system a significant flux of A32 metastable nitrogen molecules
`is obtained. Nozzle seeding techniques to increase the beam
`velocity have also been successful. In argon, for example, the
`velocity is enhanced by a factor of 2 by the addition of
`hydrogen. In the neon source the addition of helium results in
`a 20 % velocity enhancement.
`
`Acknowledgment
`This research is supported by the Office of Naval Research.
`
`References
`Anderson J B and Fenn J B 1965 Velocity distributions in
`molecular beams from nozzle sources
`Phys. Fluids 8 780—7
`
`Dunning F B and Smith A C 1971 Secondary electron
`ejection from metal surfaces by metastable atoms. II
`Measurements of secondary emission coefficients using a gas
`cell method
`J. Phys. B: Atom. Molec. Phys. 4 1696—710
`Dunning F B, Rundel R D and Stebbings R F 1975
`Determination of secondary electron ejection coefficients for
`rare gas metastable atoms
`Rev. Sci. Instrum. 46 697—701
`
`Fahey D W, Parks W F and Schearer L D 1978 Aligned,
`excited ions from Penning ionization
`Bull. Am. Phys. Soc. 23 1088
`Leasure E L, Mueller C R and Ridley T Y 1975 ‘Hot’
`metastable atom, molecular beam source
`Rev. Sci. Instrum. 46 635—7
`
`Smith A B 1972 Notes on the performance and application
`of EMI windowless particle detectors
`EMI Electronics Ltd Document R/P034
`
`
`
`
`J. Phys. E: Sci. Instrum., Vol. 13, 1980. Printed in Great Britain
`
`An apparatus for the
`measurement of initial magnetic
`permeability as a function
`of temperature
`
`E Cedillofi J Ocampofir V Rivera: and R Valenzuela:
`1' Departamento de Ciencia de Materiales, Escuela Superior
`de Fisica y Matematicas, Instituto Politécnico Nacional,
`México
`
`3: Centro de Investigacién de Materiales. Universidad
`Nacional Autonoma de Mexico, Mexico Apartado Postal
`70—360, México 20, DF
`
`Received 29 .May 1979, in finalform 31 October 1979
`
`Abstract A simple apparatus for the study of the quasi-static
`initial magnetic permeability as a function of temperature is
`presented. It is based on the Faraday law of electromagnetic
`induction and is especially suited for toroidal samples of
`ferrimagnetic compounds. The measurements can be
`performed at frequencies from 1 to 100 kHz, and
`temperatures from 80 to 900 K. In addition to the initial
`permeability and the Curie temperature, this apparatus
`provides a qualitative determination of the chemical
`homogeneity of the samples.
`
`Introduction
`1
`The initial magnetic permeability is a very microstructure-
`sensitive property. The study of the thermal variations of
`initial permeability of polycrystalline ferrites has contributed
`to significant improvements in our knowledge of the magneti-
`sation mechanisms (Globus and Duplex 1966). It can also be
`used as a quality test (Globus and Valenzuela 1975) in the
`preparation of ferrite samples.
`Measurement of initial permeability as a function of tem-
`perature is generally performed by measuring the inductance
`of a toroidal sample on an impedance bridge at different
`temperatures. However, this technique can be tedious if the
`temperature intervals are very narrow and an interesting
`phenomenon can be missed if they are very widely spaced. At
`the Curie point the initial permeability falls from a high value
`(generally its maximum) to near 1. The verticality of this drop
`provides an evaluation of the sample’s chemical homogeneity.
`Clearly, it is difi‘icult to estimate the homogeneity using this
`technique.
`In this paper we describe a simple apparatus for recording
`continuously the initial permeability as a function of the
`temperature, from 80 to 900 K. This apparatus is especially
`suited for toroidal samples of ferrimagnetic oxides.
`
`2 Basis of the apparatus
`The toroidal sample is used as a transformer core, and a
`sinusoidal current ip is established in the primary coil. As a
`consequence of the induction a voltage Vs appears at the
`secondary coil:
`
`Vs=ns qu/dt
`
`(1)
`
`where its is the number of turns in the secondary coil.
`As
`
`ip = i0 6‘1“”,
`
`3*
`
`0022-3735/80/040383+04 $01.50 © 1980 The Institute of Physics
`
`