**<" coA

ilcr

*<"*AU O* NBS TECHNICAL NOTE 646

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Status Report on

Primary Frequency Standards

.U5753

no. (0^(0

1373

C2.
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (I) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences " — Applied Radiation " — Quantum Electronics 3 — Electromagnetics 3 — Time

and Frequency ° — Laboratory Astrophysics " — Cryogenics ". THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety * — Building Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

a Located at Boulder. Colorado 80302.

« Part of the Center for Building Technology.
-~ 1 1974

)V

00

74

Status Report on Primary Frequency Standards

V

Helmut Hellwig

Time and Frequency Division

Institute for Basic Standards

,$, National Bureau of Standards

Boulder, Colorado 80302

19 "»t»u o«

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

Issued September 1973
National Bureau of Standards Technical Note 646

Nat. Bur. Stand. (U.S.), Tech Note 646, 15 pages (Sept. 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office , Washington, D.C. 20402.

(Order by SD Catalog No. C13.46:646). $0.30
STATUS REPORT ON PRIMARY FREQUENCY STANDARDS

Abstract

This report surveys the state-of-the-art in atomic

frequency standards with exclusive regard to their use

as primary frequency reference; i.e., only accuracy is

discussed. The report covers operational standards as well as devices which are still in the research or ex- ploratory development phase. It, is predicted that

accuracies of better than 1 x 10 will be achieved

within a few years, and that, as a consequence of new

techniques, accuracy may be treated statistically in

the not too distant future. Also, clocks may become

available which remain accurate continually.

Key words : Atomic frequency standards; Accuracy;

Cesium beam tubes; Clocks; Hydrogen

masers; Lasers; Primary frequency

standards.
INTRODUCTION

Since the advent of quantum electronic frequency

standards in 1948 [1] , research has been done on a great

variety of different devices, techniques, and methods.

The majority of existing standards is based on the cesium

atom. Cesium hyperfine resonance standards or clock devices were developed in the early 1950 's, and the first

routinely used clock was built in 1955 by the National

Physical Laboratory of the United Kingdom [2] . Th^ subsequent refinement of these devices in manv laboratories

in the world and the increasing usage of cesium standards

for the generation of time scales led in 1967 to the international agreement at the General Conference on Weights

and Measures to adopt the cesium resonance frequency for

the definition of the second. As a result of this

agreement, the Universal Coordinated Time (UTC) has

been based since 1971 [3] on these atomic seconds. Time

scales which refer to the rotation of the earth are approximated by inserting or leaving out seconds at certain specified dates during the year, as necessary.

Other devices which have been and are being used as primary frequency references include the hydrogen maser oscillator. This device has reached some technical perfection,

and is being used by several laboratories not only as a primary frequency reference but also as a clock for time scale

generation. Other promising devices which have been investigated, or are presently under investigation, include beam

tubes based on the thallium atom, the barium oxide molecule,

and the magnesium atom. Also studied are ion storage devices

using mercury or barium, as well as devices in the infrared

and visible region of the electromagnetic spectrum where

lasers are locked to resonances in molecules such as methane,

iodine, sulphur tetrafluoride and others.

The principle laboratories currently engaged in research, development and maintenance of primary frequency

standards are the National Physical Laboratory of the United

Kingdom, the Physikalisch-Technische Bundesanstalt in West

Germany, the Laboratoire de 1 ' Horloge Atomique in France, the

National Research Council in Canada, the National Research

Laboratory for Metrology, and the Radio Research Laboratory

in Japan, the Istituto Elettrotecnico Nazionale in Italy,

the Australian National Standards Laboratory, and the

National Bureau of Standards in the United States. In

addition to the work at these laboratories, a significant

amount of work has been done in other government agencies

of the various countries and at universities.
Present Status of Primary Frequency Standards

I. Cesium

At the present time , there are four operating primary laboratory type cesium beam frequency standards located at the PTB in Germany [4] , NRC in Canada [5] , and

NBS in the United States [6] . These standards have been

evaluated with respect to most parameters affecting their

output frequency; i.e., experiments and theoretical studies

have been performed which yield knowledge about the biases

which cause the output frequency to differ from the unperturbed atomic resonance frequency. The accuracy which is then

ascribed to the standards is a statistical combination of

the uncertainties associated with biases. These four frequency standards have been built independently; they have

considerably different designs, and the methods of evaluation are quite different although the same parameters are being evaluated. Most recently, a comparison has been made

between the corrected output frequencies of these three primary standards using the International Atomic Time (TAI)

Scale as a common reference: the corrected frequencies of

the three standards agree to within a peak-to-peak variation

of 1 part in 10 . Each of the laboratories has preliminarily and independently claimed an evaluated accuracy between, 1 and 2 parts in 10 . The frequency is about 1 part

in 10 lower than the current frequency of TAI [5,7,8], Thus, we may conclude at this point in time that TAI runs at a frequency too high with respect to the definition of the

second by about 1 part in 10

Similarly designed cesium beam frequency standards

are nearing completion and evaluation in Japan and in the

United Kingdom. In addition to the traditional way of

building a special, usually long, cesium beam machine in

order to be able to evaluate the biases, new techniques have

been developed. They may permit an evaluation of most existing cesium beam standards, whether they were designed for

evaluation or not, including commercial beam tubes [9,10].

Thus, many more laboratories could join the group of laboratories owning primary frequency standards provided they go

through certain electronic test procedures with existing

cesium beam standards found suitable for the application

of these techniques. This may increase the confidence in

the value of the unperturbed cesium frequency.
II. Hydrogen Devices

The hydrogen maser has been under study since 1960

[11] . It has been presently developed to be one of the most

stableoscillators , with a fractional frequency stability of

2 x 10 and better [12] which exceeds that of a cesium standard for measurement times of the order of an hour. However, the

stability of the hydrogen maser is not better than that of

cesium for measurement times much longer than one day. In

other words, the present principal utility of hydrogen maser

oscillators lies in highly precise measurements of relatively

short duration but its performance as a clock is not superior

to that of cesium beam tubes. The accuracy of the hydrogen

maser is mainly limited due to the uncertainty in the knowledge

of the frequency shift caused by the collision of the hydrogen

atoms with the walls of the storage bulb used inside of the

microwave cavity. The traditional evaluation methods have been

shown by several laboratories (NBS, NRC.-NPL) [13,14,15] to

yield an accuracy of about 1 part in 10 . These methods

are based on testing a series of storage bulbs of different

size in the same hydrogen maser. In an operational sense,

this method also suffers the disadvantage of not allowing an easy and operational re-check of the accuracy of a given device because its operation has to be interrupted for a considerable length of time for any reevaluation procedure.

The use of the hydrogen atom in devices other than the

hydrogen maser oscillator has been proposed. Two experimental devices have been built and the results obtained are encouraging. One device (at NBS) is a hydrogen storage bulb

system in which a crystal oscillator is locked to the hydrogen

resonance via atomic hydrogen detection or via receiving the

microwave hydrogen dispersion [20,21], The other device (at

NASA) is a free hydrogen beam device using a Ramsey type microwave cavity and atomic hydrogen detection in order to slave

a crystal oscillator to the hydrogen resonance [22]

.
Ill . Other Devices

Atomic beam tubes based on the thallium atom have

been developed and have been tested [16,17]. The experimental results indicated that accuracies may be obtained

which are comparable with those of cesium devices. Their

operational and functional aspects are very much like

those of cesium beam tubes. From this work it may be con- cluded that the use of an atom as similar to cesium as thallium does not lead to devices which are fundamentally superior to the cesium beam tube. In fact, certain aspects in

the practical operation of such a beam tube are inferior as compared to cesium beam tubes. Therefore, to our knowledge,

no work is presently performed on thallium devices.

Other atomic beam devices such as barium oxide [18] or magnesium devices [19] have been considered and have been

or are presently being tried. Indications are that these

devices have some potential; e.g., reduced sensitivity to

magnetic fields, possibility of very high beam intensities,

exceptional line-Q due to higher transition frequencies, etc.

However, further research is necessary to evaluate the practicality of such advantages.

Ion storage devices in which charged atoms are stored

within containing electromagnetic fields have been proposed

for a long time and research has been done to a considerable

extent [23,24] using mainly the He ion. Recently, actual

frequency standard devices based on ion storage have been

experimentally built and are currently under test [2 5] using either mercury or barium ions. Experimental results on their capability are not available at this time.

The whole class of stabilized lasers, where molecular

resonances are used to slave a laser oscillator to a fixed

frequency, have been tested over the past 5 years with con- considerable experimental success in stability and promise in accuracy [26,27,28]. Present. devices have not yet

demonstrated better than 1 part in 10 in accuracy. However, no real physical limit down to 10 appears to be

known. The most promising candidate is methane (several

transitions in methane could be utilized) . In a practical sense, the drawback of these optical devices, of course,

is the difficulty to obtain precise second ticks. Further

refinement of frequency multiplication schemes from the

microwave region into the infrared and visible radiation

region are therefore necessary both in terms of precision

and in technical perfection.
Future Outlook of Primary Frequency Standards

For the next several years it can be assumed with

reasonable certainty that cesium beam tubes will remain

the basis of the legal definition of the second, as well

as the practical sources for the generation of time scales

and the unit of time interval . Further improvement in

cesium devices_^.s likely which may push the accuracy to

better than 10 ~ within the next few years. These improvements will be based on new designs allowing quicker

and easier evaluation of the most significant frequency

biases as well as improvements in the stability of these

devices which allow faster evaluation and measurement procedures. It will be difficult in the near future for any

other device to exceed the cesium atomic beam machine in

terms of accuracy. Thus, for practical devices we can be

reasonably certain that for the next several years cesium

beam devices will remain the workhorse of primary frequency

and time standards laboratories. We can also be sure that

for the foreseeable future the cesium beam frequency standard will remain the basis of the legal definition of the

second.

If one considers accuracy as the primary objective,

then beam machines appear to be the best choice since they

approximate the ideal of the free, unperturbed atom to the

highest degree. In contrast, storage devices such as the

hydrogen devices, (maser oscillators or hydrogen storage

beam tubes) , suffer from the unavoidable interaction of the

atom with the storage container. The future of the hydrogen

maser as an accurate device hinges crucially on better evaluation procedures. Presently, very little work is going on in

this direction. The most notable effort is the big-box

storage bulb maser with a variable storage bulb surfaceto-volume ratio [29,30]. It is the intention of at least

three other laboratories to initiate investigations with

hydrogen devices using new techniques [31,32] for measuring the wall shift. In this connection, hydrogen beam or hydrogen storage beam devices appear to be attractive since

they offer the use of the known techniques with greater ease

(and hopefully precision) , than the maser oscillator. New

results in this direction may be expected within the next

several years, and accuracies rivaling that of the present

day cesium beam standards are likely to be achieved in the

future.
Ion srorage devices are in the initial phase of

being investigated in terms of their capability as frequency standards. Again, first experimental results may

be expected within the next two years. In ion storage

devices, however, it is difficult to know precisely the

velocity of the stored ions; the magnitude of the velocity

is crucial to the evaluation of the second-order Doppler

effect. Also, an increase in the number of stored

particles in the ion storage trap is necessary in order

to make it competitive with hydrogen or cesium devices.

Laser devices are, of course, promising, and are being investigated in an increasing number of laboratories

in the world. As far as primary frequency standard applications are concerned, the value of these laser-based devices is crucially coupled to the multiplication from the

microwave into the infrared and visible radiation regions.

Without a practical* and precise multiplier or syntnesizer,

these devices are not attractive as primary frequency

standards or clocks in the classical sense. Again, new results on tne accuracy and on the future

potential of multiplication techniques may be expected

within the next few years allowing a more final evaluation

of the potential of laser based devices. They are, of course,

in their own right of great interest as secondary frequency

standards and wavelengtn references for the optical region.

In connection with new developments in the multiplication from tne microwave region upwards , not only tne saturated absorption tecnniques become even more attractive but

also beam devices become of interest again: As was mentioned

above, they approximate in the best way tne ideal of the free

and unperturbed atom; thus, beam devices in the far infrared

or at even higher frequencies may be of considerable interest.

Spectrally pure and convenient excitation sources may be found

which match with convenient transitions sucn as in magnesium

or calcium.

*At present, such systems are based on tne fragile, fairly

unreliable catwnisker type diode and several lasers (H-,0,

C0

2

, HCN) which serve as intermediate oscillators in tne

chain [33]

.
Summary

It may be predicted that within^ few years accuracies of better than one part in 10 ' will be realized with

cesium machines. Cesium will remain the workhorse of the

primary frequency and time standards for many years to come. Beam devices, including cesium but including also other

atoms have the highest probability of reaching even higher

accuracies. Storage devices such as those based on hydrogen or storage cells as used in laser stabilization have

the potential of equalling or possibly surpassing cesium

and are therefore of great interest.

Several developments may be expected to influence the

field for years to come. (1) An increasing availability of

standards (cesium as well as others) with evaluated accuracies. (2) Several laboratories will compare via TAI

their accurate frequencies. (3) The possibility —due to

new techniques— to automatically control a standard to re- main accurate, and (4) the trend towards improved accuracies

which are comparable with long term stability values. Thus,

operational clocks and corresponding time scales may well

be stable and accurate continuously without resorting to

occasional (elaborate) calibration and evaluation procedures,
TABLE

PRESENT STATUS AND FUTURE OUTLOOK FOR ACCURACIES

Class of Devices

Accuracy Confidence

Present Projected for "Projected

(Cs)1.5xl0~13 [5,7,8] < 1x10 13

Storage Beams

(H)

Ion Storage

(Hg, Ba, He)

(He)lxl0" 9 [23]

.-11 Stabilized Lasers (CHjlxlO [27]

(CH I C0

2

, SF

6

, etc.

)

1x10

-14

Microwave Beams

(Cs, H, Tl, BaO)

Submillimeter Beams

(Mg, Ca)

Maser Oscillators (H) lxlO

-12

[13 ,14, 15] < lxlO -13

(H)

<; 1x10

< 1x10

^ 1x10

-13

-12

-13

90%

10%

80%

80'

10%

60%

This table lists seven types of frequency standards and

their corresponding accuracies at the time of this writing as presently achieved with at least one of the listed molecules.

The figures for the "projected" accuracy are based on published or public statements by the workers closely associated

with these devices. The last column gives the author '

s

con- fidence estimate (condition: unlimited resources and time to

reach objective) for an actual realization of the "projected"

accuracies using one of the possible atoms.
REFERENCES

1. H. Lyons "The Atomic Clock," NBS Tech. News Bull. 33

p. 17 (1949).

2. L. Essen and J. Parry "An Atomic Standard of Frequency and Time Interval" Nature 176, p. 280 (1955).

3. H. M. Smith "International Time and Frequency Coordination" Proc. IEEE 60, p. 479 (1972).

4. G. BecKer, B. Fiscner, G. Kramer, and E. K. Muller

"Neuentwicklung einer Casiumstrahlapparatur als

primares Zeitund Frequenznormal an der PhysikalischTecnniscnen Bundesansrait, " PTB-Mitteilungen 2,

p. 77 (1969) .

5. A. G. Mungall, R. Bailey, H. Daams , D. Morris, and

C. C. Costain "The New NRC 2.1 Metre Primary Cesium

Beam Frequency Standard, Cs V," Metrologia 9, p. 113

(1973) .

6. D. J. Glaze, H. Hellwig, S. Jarvis Jr., and A. E

Wainwright "Recent Progress on tne NBS Primary Frequency Standard" Proc. 27th Ann. Freq. Contr. Symp.

,

Ft. Monmourn, NJ, p. 347 (1973).

7. D. J. Glaze, H. Hellwig, D.W. Allan, S. Jarvis, Jr.,

and A. E. Wainwrignt "Accuracy Evaluation and Stability

of tne NBS Primary Frequency Standards," IEEE Trans.

on Instr. and Meas. IM-23 , (1974) to be published.

8. G. BecKer "Frequenzvergleicne mit dem primaren Zeitund Frequenznormal CSI der Pnysikalisch-Tecnniscnen

Bundesanstalt zwiscnen 1969 und 1973, PTB-Mitteilungen

83, p. 319 (1973)

.

9. H. Hellwig, S. Jarvis Jr., D. J. Glaze, D. Halford,

and H. E. Bell "Time Domain Velocity Selection

Modulation as a Tool to Evaluate Cesium Beam

Tuoes" Proc. 27th Ann. Freq. Contr. Symp., Ft.

Monmouth, NJ, p. 357 (1973).

10. H. Hellwig, S. Jarvis Jr., D. Halford, and H. E.

Bell "Evaluation and Operation of Atomic Frequency

Standards Using Time Domain Velocity Selection

Modulation" Metrologia 9, p. 107 (1973).

11. D. Kleppner, H. M. Goldenoerg, and N. F. Ramsey

"Theory of tne Hydrogen Maser" Phys . Rev. 126,

p. 603 (1962).

10
-15 3 12. Stabilities of 2 x 10 for around 10 s sample time

were reported independently by R. F. C. Vessot, Smithsonian Astrophysical Observatory, Cambridge, MA; H. E.

Peters, NASA-GSFC, Greenbelt, MD; and C. Audoin,

Laboratoire de l'Horloge Atomique, Orsay > France

.

13. H. Hellwig, R. F. C. Vessot, M. W. Levine, P. W.

Zitzewitz, D. W. Allan, and D. J. Glaze "Measurement of the Unperturbed Hydrogen Hyperfine Transition Frequency" IEEE Trans, on Instr. and Meas

.

IM-19 , p. 200 (1970)

.

14. D. Morris "Hydrogen Maser Wall Shift Experiments at

the National Research Council of Canada" Metrologia 7,

p. 162 (1971)

.

15. L. Essen, R. W. Donaldson, E. G. Hope, and M. J.

Bangham "Hydrogen Maser Work at the National Physical

Laboratory" Metrologia 9, p. 128 (1973).

16. R. F. Lacey "Thallium Beam Frequency Standards"

Metrologia 3, p. 70 (1967).

17. R. E. Beehler, and D. J. Glaze "Evaluation of a Thallium Atomic Beam Frequency Standard at the

National Bureau of Standards" IEEE Trans, on Instr.

and Meas. IM-15 , p. 55 (1966).

18. H. Hellwig, R. McKnight, E. Pannaci, and G. Wilson

"Barium Oxide Beam Tube Frequency Standard" Proc

.

22nd Ann. Freq. Contr. Symp. , Ft. Monmouth, NJ,

p. 529 (1968).

19. F. Strumia "A Proposal for a New Absolute Frequency

Standard, Using a Mg or Ca Atomic Beam" Metrologia 8,

p. 85 (1972).

20. H. Hellwig, and H. E. Bell "Some Experimental Results

with an Atomic Hydrogen Storage Beam Frequency Standard" Metrologia 8, p. 96 (1972).

21. H. Hellwig, and H. E. Bell "Experimental Results with

Atomic Hydrogen Storage Beam Systems" Proc. 2 6th Ann.

Freq. Contr. Symp,, Ft. Monmouth, NJ, p. 242 (1972).

22. H. Peters "Hydrogen as an Atomic Standard" Proc. 26th

Ann. Freq. Contr. Symp., Ft. Monmouth, NJ, p. 230 (1972)

11
23. E. N. Fortson, F. G. Major, and H. G. Dehmelt "Ultrahigh Resolution AF = 0, ±1 (He ) HFS Spectra by an Ion-Storage Collision Technique" Phys. Rev. Letters

16, p. 221 (1966)

.

24. H. A. Schiissler "The Ion Storage Tube, A Proposal

for a New Atomic Frequency Standard" Metrologia 7,

p. 103 (1971).

25. C. Audoin, Laboratoire de l'Horloge Atomique,

Orsay France; private communication.

26. A. J. Wallard "Characteristics of the 633 run He-Ne

Laser Stabilized by Saturated Absorption in Iodine

Vapour" Proc. 27th Ann. Freq. Contr. Symp. , Ft.

Monmouth, NJ, p. 376 (1973).

27. J. L. Hall, Atomic Physics 3, Plenum Press, NY,

p. 615 (1973).

28. J. L. Hall and C. Borde "Measurement of Methane

Hyperfine Structure Using Laser Saturated Absorbtion" Phys. Rev. Letters 30, p. 1101 (1973).

29. E. E. Uzgiris and N. F. Ramsey "Large Storage Box

Hydrogen Maser" IEEE J. of Quant. Electr. QE-4

,

p. 563 (1968).

30. V. Reinhardt, J. Lavanceau "A Comparison of the Cs and

H Hyperfine Frequencies by Means of Loran-C and Portable

Clocks," Proc. 28th Ann. Symp. on Freq. Contr., Ft.

Monmouth, NJ, (1974) , to be published.

31. R. F. C. Vessot and M. W. Levine "A Method for Eliminating

the Wall Shift in the Atomic Hydrogen Maser" Metrologia

6, p. 116 (1970)

.

32. P. Debely "Hydrogen Maser with Deformable Storage Bulb"

Proc. 24th Ann. Symp. on Freq. Contr., Ft. Monmouth,

NJ, p. 259 (1970)

.

33. K. M. Evenson, J. S. Wells, F. R. Petersen, B. L.

Danielson, and G. W. Day "Accurate Frequencies of

Molecular Transitions Used in Laser Stabilization:

The 3.39ym Transitions in CH. and the 9.33-and 10.18-

um Transitions in C0 9

" Appl. Phys. Letters 22, p.

192 (1973). *

12
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-646

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE Status Report on Primary Frequency Standards

5. Publication Date September 1973

6. Performing Organization Code 273. 00

7. AUTHOR(S)

Helmut Hellwig

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Labs

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2730144

11. Contract/Grant No.

2. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as #9

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) This report surveys the state-of-the-art in atomic frequency

standards with exclusive regard to their use as primary frequency

reference; i. e. , only accuracy is discussed. The report covers

operational standards as well as devices which are still in the research

or exploratory development phase. It is predicted that accuracies of

better than 1x10 will be achieved within a few years, and that, as a consequence of new techniques, accuracy may be treated statistically

in the not too distant future. Also, clocks may become available which

state accuracy continually.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper- name; separated by semicolons) Atomic frequency standards; Accuracy; Cesium beam tubes;

Clocks; Hydrogen masers; Lasers; Primary frequency standards.

18. AVAILABILITY [X] Unlimited

^j For Official Distribution. Do Not Release to NTIS

| x] Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13, 46:646

~[2 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

USCOMM.DC 29042-P74

NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics, mathematics, and chemistry. Comprehensive scientific papers five complete details of the work, including laboratory data, experimental procedures, and theoretical and mathematical analyses. Illusti'ated with photo- graphs, drawings, and charts. Includes listings of other NBS papers as issued. Published in two sections, available separately:

• Physics and Chemistry (Section. A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a

year. Annual subscription: Domestic, $17.00; Foreign,

$21.25. • Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemistry, logical design and programming of computers and computer

systems. Short numerical tables. Issued quarterly. Annual subscription: Domestic, $9.00; Foreign, $11.25. DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. DIMENSIONS/NBS highlights and reviews such

issues as energy research, fire protection, building technology, metric conversion, pollution abatement,

health and safety, and consumer product performance.

In addition, DIMENSIONS/NBS reports the results of Bureau programs in measurement standards and tech- niques, properties of matter and materials, engineering standards and services, instrumentation, and automatic

data processing. Annual subscription: Domestic, $6.50; Foreign, $8.25.

N0NPERI0DICALS Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of high-level national and international conferences sponsored by NBS, precision measurement and calibration volumes, NBS annual reports, and other special publications appropriate to this grouping such as wall charts and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396). See also Section 1.2.3.

Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies. Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose of the standards is to establish nationally recognized requirements for products, and to provide

all concerned interests with a basis for common under- standing of the characteristics of the products. The

National Bureau of Standards administers the Voluntary Product Standards program as a supplement to the activities of the private sector standardizing

organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The purpose of the Register is to serve

as the official source of information in the Federal Gov- ernment regarding standards issued by NBS pursuant

to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations). FIPS PUBS

will include approved Federal information processing standards information of general interest, and a complete index of relevant standards publications. Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological marketplace. NBS Interagency Reports—A special series of interim

or final reports on work performed by NBS for outside sponsors (both government and non-government). In

general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Infor- mation Service (Springfield, Va. 22151) in paper copy

or microfiche form. Order NBS publications (except Bibliographic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington, D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service

(Publications and Reports of Interest in Cryogenics). A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00. Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the preceding bibliographic services to the U.S. Department

of Commerce, National Technical Information Service, Springfield, Va. 22151. Electromagnetic Metrology Current Awareness Service (Abstracts of Selected Articles on Measurement

Techniques and Standards of Electromagnetic Quantities from D-C to Millimeter-Wave Frequencies).

Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and remittance to the Electromagnetic Metrol- ogy Information Center, Electromagnetics Division, National Bureau of Standards, Boulder, Colo. 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC. 20234

OFFICIAL BUSINESS

Penalty for Private Use. $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

'^e-i^6

* U. S. GOVERNMENT PRINTING OFFICE 1974- 67 7-231/1208 REG. 8