Hydrogen cell and ion chamber for obtaining photoelectric data on resonance scattering at lyman alpha lin

Blamont, J. E.

Delache, P.

Stober, A. K.

English

NASA Technical Reports Server

RESONANCE SCATTERING 
A N  ATOMIC HYDROGEN CELL 
AT LYMAN-ALPHA BY 
bY 
J. E. Blamont and P. Delache 
Centre National de la Recherche ScientiJiqzce 
France 
and 
A. K. Stober 
Goddard Space Flight Center 
Greenbelt, M aryland I 
i 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 0 WASHINGTON, D. C. 0 JUNE 1964 
https://ntrs.nasa.gov/search.jsp?R=19640012104 2020-03-24T06:25:19+00:00Z
TECH LIBRARY KAFB, NM 
I llllll1111111111 Ill11 llll 111111 Il1 Il1 
RESONANCE SCATTERING AT LYMAN-ALPHA 
BY AN ATOMIC HYDROGEN CELL 
By J. E. Blamont and P. Delache 
Centre National de la Recherche Scientifique 
France 
and 
A. K.  Stober 
Goddard Space Flight Center 
Greenbelt, Maryland 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
~~ 
For sale by the Office of Technical Services, Department of Commerce, 
Washington, D.C. 20230 -- Pr ice  $0.50 
RESONANCE SCATTERING AT LYMAN-ALPHA 
BY AN ATOMIC HYDROGEN CELL 
by 
J. E. Blamont and P. Delache 
Centre National de la Recherche Scientifique 
France 
and 
A. K. Stober 
Goddard Space Flight Center 
SUMMARY 
This paper describes the experimental conditions for 
direct photoelectric observation of the optical resonance and 
polarization of the hydrogen Lyman-alpha line (1216A). 
i 

CONTENTS 
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  i 
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
Hydrogen Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
DESCRIPTION OF THE EXPERIMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
The Light Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
Experiments with the First Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
Modification of the Light Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
The Second Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
Results Observed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4 
Evaluation of the Number of Hydrogen Atoms in the Cell . . . . . . . . . . . . . . . . . .  5 
Best Performance Obtained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5 
A Third Cell for Observing the Polarization of the Lyman-Alpha Light . . . . . . . . . . .  5 
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6 
iii 
RESONANCE SCATTERING AT LYMAN-ALPHA 
BY AN ATOMIC HYDROGEN CELL 
J. E. Blamont and P. Delache 
Centre National de la Recherche Scientifique 
France 
and 
A. K. Stober 
Goddard Space Flight Center 
INTRODUCTION 
Rocket and satellite ,studies of the Lyman-alpha (1216A) line emitted in the upper atmosphere 
have been made using detectors which have large bandwidths, such as ion chambers with various 
window materials and filling gases (References 1 and 2), or  photomultipliers with cesium iodide or  
copper iodide photocathodes (Reference 3). High-resolution photographic measurements have been 
made with a grating spectrograph (Reference 4) and photographic plates; the system requires long 
exposures and a recovery system to obtain data. 
This paper is  concerned chiefly with the experimental results of a hydrogen (filter) cell and 
ion chamber for obtaining photoelectric data. 
Hydrogen Cells 
Lyman-alpha is the resonance line of the hydrogen atom. If a cell containing a large number 
of hydrogen atoms is placed in front of the detector and is excited by an incident beam of light, the 
hydrogen atoms will absorb selectively the central frequency (core) of the Lyman-alpha emission 
from the existing beam with a width that is a function of the temperature of the atoms and their 
number density (optical thickness). This experiment has been performed by the Naval Research 
Laboratory (Reference 5). The number density of the hydrogen atoms is not very critical when the 
exciting spectrum is a line spectrum containing a narrow Lyman-alpha emission line. In such a 
cell there is a lower limit for the density of hydrogen atoms above which the cell will absorb all the 
Lyman-alpha contained in the exciting beam. Such a system, a broad bandpass filter with a narrow 
absorption feature, can be used when the energy of the spectrum of interest is defined by the detec- 
tor  (1050 to 1300A for an ion chamber) and contains two parts of the same order of magnitude: 
1 
the absorbable part, centered at the Lyman-alpha line with a width of 10- * A; and the remainder of 
the spectrum from 1050-1300A, nonabsorbable. This system cannot be used to observe the Lyman- 
alpha radiation coming from the sun, the absorbable part being too small compared with the non- 
absorbable part (not more than 1 part in 100). 
On the contrary, a scattering cell, where the resonance light, scattered by hydrogen atoms, is 
observed at 90 degrees of the incident beam, is equivalent to a very narrow band filter (about lo-' 
A). Of course, the "transmission" in the center of the line will be small because the geometry of 
the cell forbids the detection of all the scattered photons, but the transmission will be exactly zero 
(except from the effects of s t ray light) for all the other wavelengths. 
DESCRIPTION OF THE EXPERIMENT 
The first cell was a rather complicated cell, Figure l(a), in which the hydrogen atoms were 
produced by dissociation of hydrogen molecules on a hot tungsten wire. The cell contained two fila- 
ments mounted in light horns located out of direct view of the detector. Four LiF  (lithium fluoride) 
windows were epoxy-sealed to the cell. One was the "entrance" window facing the monochromator 
light source, and one the "exit" window on the opposite side of the cell for observing the direct 
(absorbed) light. Two other windows were mounted at right angles to the light beam: one near the 
entrance window, and one near the exit window. 
Hydrogen was allowed to enter the cell through a palladium leak. The cell was isolated from 
the vacuum system by a stopcock, thus permitting evacuation and pressure regulation. 
The Light Source 
The light source system was a 3-meter high- 
dispersion vacuum monochromator illuminated 
by a hydrogen-discharge continuum lamp. The 
resolution of the monochromator was on the 
order of 1/20A at Lyman-alpha emission. 
Experiments with the First Cell 
The absorption was first observed. The 
first problem was the outgassing due to the 
large diameter of the filaments, high filament 
current needed, and envelope glass tempera- 
ture. A good test for the "true" absorption was 
obtained by observing a band of the hydrogen 
molecule. When the heating of the filament 
would not create any absorption at 1234A, an 
absorption measurement would be made at 
Lyman- alpha. 
(a) First cell 
FILAMENT 
LIGHT 
LiF 
ENTRANCE 
WINDOW OBSERVATION 
WINDOW 
LiF EXIT 
WINDOW 
(bl Second cell 
Figure 1-Atomic hydrogen cells. 
2 
The Lyman-alpha profile of the light coming out of the system was observed by scanning the 
line with the monochromator, with and without the filament heated. This profile was already 
strongly self-reversed because of the hydrogen atoms at the exit port of the light source. However, 
a careful study of the profiles showed that this reversal  was higher in the presence of hydrogen 
atoms in the cell. Under these conditions, even with very large slits, it was impossible to see any 
resonance signal at either of the two windows. 
Modification of the l i g h t  Source 
To have a higher intensity in the center of the Lyman-alpha line, the density of hydrogen inside 
the lamp was lowered by running the discharge with a mixture of helium and hydrogen. The pro- 
cedure was the following: 
A discharge was started in pure helium. The Lyman-alpha profile was observed with the slits 
of the monochromator compatible with the detector sensitivity. This may not be the Lyman-alpha 
line of the hydrogen atoms left in the gas of the helium tank but the H-alpha line of singly ionized 
helium; its intensity was very small. The hydrogen pressure was slowly increased by using a needle 
valve. As the intensity of Lyman-alpha increased, the line profile did not self-reverse. It then 
appeared that conditions changed very suddenly in the discharge lamp; the profile became strongly 
self-reversed, and the total intensity increased greatly. At the same time, the color of the dis- 
charge in the light source changed from orange to red. It was thus possible to determine the best 
conditions (maximum intensity, minimum self-reversal) from the color of the discharge. 
The Second Cell 
The design of the second cell, Figure l(b), took into account the facts mentioned above. 
The small rhenium wire heater filament was mounted on an aluminum oxide ring and required 
5 watts for white heat. The detectors were attached directly to the windowless cell by O-rings or 
Apiezon wax seals. 
The outgassing problem was solved by eliminating the taper joint system of the first cell and by 
using a secondary pump. 
The stray light was removed by the use of a light horn; however, the detector at the bottom of 
the horn “saw” part of the direct (absorbed) light reflected on the curved part  of the horn. 
The palladiumleak system was the same as that used onthe first cell. Pressure was measured 
with a thermocouple gage. Foreign gas could be introduced into the cell through a needle valve. 
(The experimental apparatus is shown in Figure 2). 
3 
Figure 2-The experimental apparatus. 
Results Observed 
With the above-mentioned conditions, both absorbed and scattered light signals were observed. 
A typical recording is shown in Figure 3. On the left channel, the scattered light signal is seen 
and, on the right channel, the absorbed light signal. 
Reading from below, we (1) start with Lyman-alpha excitation and atomic hydrogen (filament- 
heated); (2) close the shutter - that is, suppress the excitation - and both signals fall to zero (the 
L a  
-H 
L a  
f H  
Scattered Light 
I-H 
1 Lo+H 
Absorbed Light 
0 = Zero 
L a  = Lyman-alpha 
H = Hydrogen 
L a +  H = Lyman-alpha i Hydrogen 
Figure 3-Recording scattered light and 
absorbed light. 
zero of the left recorder is not the zero of the 
chart paper); (3)  return to the former situation, 
Lyman-alpha+ hydrogen; (4) stop heating the fila- 
ment (no more atomic hydrogen). The "absorbedtq 
signal rises back to the value marked Lyman- 
alpha, and the resonance signal falls to zero at 
the same time, indicating no stray light. Then 
we (5) close the shutter, and the absorbed signal 
falls again to zero while the resonant signal does 
not show any variation. This indicates that the 
filament is not seen by the detector. It can be 
noted at this point that another proof of the fact 
that there are hydrogen atoms which absorb and 
4 
I 
scatter the light, and not any outgassing, is the very short time constant of variation of the signals. 
We can say that it is less than the electronic time constant (0.2 second). 
A study of the resonance signal as a function of the hydrogen pressure showed that a maximum 
of scattered light was obtained at the total pressure of 30 to 40 microns. Introduction of helium 
makes the signal drop by a factor of 2 or more, whatever the total pressure is. 
Evaluation of the Number of Hydrogen Atoms in the Cell 
This is a rather difficult point, since we do not know whether the density is uniform throughout 
the cell. However, the density can be evaluated from two experiments: 
1. By constantly increasing the filament heat, the absorption reaches a plateau while the res- 
onance signal is still increasing. This seems to indicate that the optical thickness of the whole 
system between the entrance window and the detector is rather high (2 or more). 
2. The resonance signal increases but does not reach a maximum. However, at higher filament 
current the light we observe is roughly 1/1000 of the "absorbable light"; the absorbable light is that 
part  of the light effectively absorbed as observedby the bottom detector (30 percent of the increas- 
ing light). If an optical depth of 1 is assumed, the scattered light would be 1/500 of the incident 
absorbable light. Since an optical depth of 1 is 10" atoms/cm2, it is believed that the density in 
hydrogen atoms is on the order of 10" cmm3. The dissociation of hydrogen is then on the order of 
Best Performance Obtained 
Power dissipated in the filament for maximum resonance signal: 6 watts 
Hydrogen pressure for maximum resonance signal: 30 microns 
Signal-to-noise ratio: 20 
A Third Cell for Observing the Polarization of the Lyman-Alpha Light 
Figure 2 shows the three cells: the first cell at the left, the horn-shaped cell at the right,and 
a third one in the center. This cell was made to observe the polarization of the light at Lyman- 
alpha by reflection from a LiF  crystal at the Brewster angle. A Malus-type design was adopted. 
We observed a reflected light strongly polarized (at least 90 percent) with a reflection coefficient 
of the order of 15 percent. This system will be used for measuring the polarization of the Lyman- 
alpha airglow. 
(Manuscript Received September 18, 1963) 
5 
FEFEFENCES 
1. Chubb, T. A.; and Friedman, H., "Photon Counters for the Far Ultraviolet," Rev.  Sci. Inst. 
26(5):493-498, May 1955. 
2. Stober, A. K., "Ceramic Vacuum Ultraviolet Ion Chambers," NASA Technical Note D-1180, 
March 1962. 
3. Dunkelman, L., Fowler, W. B., and Hennes, J. P., "Spectrally Selective Photodetectors for the 
Middle and Vacuum Ultraviolet," AppZ. Optics 1(6):695-700, November 1962. 
4. Purcell, J. D., and Tousey, R., "The Profile of Solar Lyman-a," Memoirs SOC. R .  Sci. Liege 
4:283-294, 1961. 
5. Morton, D., and Purcell, J. D., "Observations of the Extreme Ultraviolet Radiation in the Night 
Sky Using an Atomic Hydrogen Filter," Planet. Space Sci. 9:455-458, August 1962. 
6 NASA-Langley, 1964 G-514 
“The aeronautical and space activities of the United States shall be 
conducted so as io  contribute . . . t o  the expansion o f  human knowl- 
edge of phenomena in  the atmosphere and space. T h e  Admitzistration 
shall provide fo r  the widest practicable and appropriate dissemination 
of jnf ormatiotr concerning its artisities and the resirlts thereof .” 
-NATIONAL AERONAUTICS AND SPACE ACT OF 1958 
NASA SCIENTIFIC AND TECHNICAL PUBLICATIONS 
TECHNICAL REPORTS: 
important, complete, and a lasting contribution to existing knowledge. 
TECHNICAL NOTES: 
of importance as a contribution to existing knowledge. 
TECHNICAL MEMORANDUMS: Information receiving limited distri- 
bution because of preliminary data, security classification, or other reasons. 
CONTRACTOR REPORTS: Technical information generated in con- 
nection with a NASA contract or grant and released under NASA auspices. 
TECHNICAL TRANSLATIONS: Information published in a foreign 
language considered to merit NASA distribution in English. 
TECHNICAL REPRINTS: Information derived from NASA activities 
and initially published in the form of journal articles. 
SPECIAL PUBLICATIONS: Information derived from or of value to 
NASA activities but not necessarily reporting the results .of individual 
NASA-programmed scientific efforts. Publications include conference 
proceedings, monographs, data compilations, handbooks, sourcebooks, 
and special bibliographies. 
Scientific and technical information considered 
Information less broad in scope but nevertheless 
Details on the availability o f  these publications may be obtained from: 
SCIENTIFIC AND TECHNICAL INFORMATION DIVISION 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
Washington, D.C. PO546 


Resonance scattering at lyman-alpha by an atomic hydrogen cell

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server