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1051
Tlib AVx.U;L>i- iiii .UliJuD FOxv TLL FOK&ATIOH
OF .AN ION—PAIR BY TLx, TOTAL AB-OIIFTIOK
OF lOOO VOLT i.L-CT'AOITL IN .IR
By
Frederick Ail11am Kuether
_iN ABoTRa OT
ubmi fctca to the uciiool of dradufl.te 0t idies of Lid >
S t ? <.1 e Co H e r e of Agriculture ana Arno lie a Sc 1 once
in partial f\ilflll:ru nt of the r. - ao i rcrr.n t s
for the decree of
rOG lOu OF FAILO ,..'01' AY
ijep- rl;.i'.nt of Physics
1951
-roved
x’xiL, iWjLiiu.Gii j_, •-.iuK-.fY Uh 1 l'-Oti TILL FORk-VI'ION
OF ,-.i; IOk-FAlu BY i'ii TOTAL . u3;_ ORFTIOF
OF lOOO VOl_T ^LuCTnOfb IF .iIR
Frcderick '.Villiam Kuebher
nn instrument has been construe ted -for the measurement
of the ionization properties of slow electrons in n:as.
Preliain&ry measurements indicate that 1031 ev electrons
totally absorbed in air produce 27*5 * 3,j ion-pairs on the
average. This allows the determination of '.V, the average
enei’ijy required for the formation of an ion-pair, to be
38.5 ev * 4.5(. for the particular case of lOOO ev electrons
absorbed in air.
The method of measurement, which appears --uite simple,
consists of a compsri£ on of the current due to positive ions
removeu from the ionization chamber by suitable electric
fields with the current uue to the electrons entering the
ionization chamber. The ratio of these currents, which of
course cannot be measureu simultaneously, yields K, the
aver-.ye number of ion-pairs formeu per initial low cnerjy
electron. If the low eneryy electron with energy L ev
prouuces K ion-pairs per* initial electron, ;V, the avers, ye
enerjy r&^uircu for the formation of an ion-pair is yivon by
th e ra t. io h/K •
The present instrument is capable of measuring R, the
r- n_c of electrons, as well as .7 for electrons of oneryy be
tween 200 ev anti. ROOO ev. '.Vith this information and the
experimentally known specific ionization ion—pair formation
per unit track lenyth, the experimentally difficult to
a
me a sure s, the total track lenyth, may be determined as
function of Initial electron cnoru.
TABLE OP CONTENTS
Section Pa^e
Ionization Processes 1
Ionization Measurements 0
The Importance of delta-ruy Ionization 9
Experimental Observations on electrons In Air 14
Design Considerations 19
Vacuum Requirements of the Ionization
Chamber 91
Vacuum Reruiremcats of the dun Chamber 25
Electrical and j^lectronic Components for
the Electron Gun 97
Other electronic and Electrical Components 40
pressure Measuring ampul :.mv nt 51
Vacuum ^e-ls 51
Vapor Traps 52
Clcariin0 of Vacuum surfaces 52
Operational details 54
Evacuation of Ionization aim Gun Chamber's 54
Care of the Pore pumps anu diffusion Pumps 55
Positioning and Focusinu of Beam on dmall
Orifice 56
determinetion of the m t r ^ y distribution
of electrons Facsinp through the dmall
Orifice into the Ionization Chamber 58
The .Tuition of the ptop; in0 Gas 67
Aemoval of tn«, Ionization Products 68
determine tion of the baturntion Condition 77
diffusion and ueconibimtion effects 80
Ice suits 81
3u20^£ited Improvements 86
Conclusion an^ acknowledgments 90
Bibliography 91
LIST OF FIGUiUiS
I^ure I'a^e
1 16
6 A Oeneral View of tlie Lqaixraent 20
3 Interior Surface of the 20'1 by 30:t Door 22
4 Section of Loor Seal 23
5 Electron Gun I.iounted on And pi ate 28
6 electron Gun and Voltajc SuppGlea 31
7 300 v Iie^ula ted Tower Supply 32
S The Control iunel 38
o P] e u\' ■ c • t : «nd Oe r’no n 0j2 f'lce 41
— x - - — ■*• \j - —
10 retail of Vacuum Interface 43
11 An Ion Collector 44
12 Current Lea curins network 47
13 jj. c. Curre.it ^qlil'icr 49
14 j_ncn-i_y Liotribetion Curve 63
15 An Ion Collection Carve 69
16 An Ion Collection Curve 72
17 An Ion Collection Curve 74
18 78
LIST OF TABLES
Table Pa^e
I Lei ta— ray s rjtcocdi;\; lOOev Lnerjy Prouu.ced
by electron a, Protons, anti Alp!ia-partlclos 11
II Frequency of Ion-Clusters containing;
Various Numbers of Ionizations 12
III Felc-itive Ion Yield of Clusters 13
IV Lxnerimental Values in tbe Tietcnniiv: tion
of K • 84
THIi AVLRAGL jA^RGTY RL JJlRLd FOR Tlid NORkATION
CF aK ION-P^IR BY ILL TOTAL ABSORPTION
OF lOOO VOLT LLjiOTRONS IN AIR
IONIZATION PAOCLSSrL
Various pax*ticles, such as alpha-particlos, neutrons,
protons, electrons, gamma-rays, x-rays, etc. are often
thought of as possessing different over-all patterns of
Ionization as the particles fly through matter. The differ
ences in these patterns are more frequently emphasized than
their common features. As will be indicated in the follow
ing sections, a very significant fraction of the total
ionization caused bp the initial particle Is due to delta-
rays. .delta-rays are the low energy (a few hundred volts
or less) electrons ejected by the initial particle from the
molecules with which It collides.
When the initial particle traverses a medium with
energy equal to the Ionization potential of the medium,
there is a finite probability that the particle will yield
all its energy by an inelastic collision and an electron
will be ejected from the target atom or molecule with
negligible velocity. It Is vastly more probable that the
atom or molecule is merely excited and later returns to Its
grouna state by radiation. The atom or molecule may also
have Its kinetic energy increased by the collision.
2
As the energy of the particle Is increased., the prob
ability of ionization (electron ejection, Ion-pair formation)
Increases greatly. When the energy is increased to twice
the ionization potential it becomes possible for two ion-pairs
to be formeu by the initial particle. The initial particle
is itself not necessarily Involved In forming the second
ion-pair, for the positive Ion or more probably the electron
of the first ion-pair may be ejected with sufficient energy
to cause ionization by collision, hence It Is not only the
behavior In collision of the Initial particle but also those
of the positive and particularly of the slow electron or
delta-ray which contributes to the observed collision phe
nomena, As the energy of the incident ray or particle becomes
large enough to form many Ion-pairs, the possibilities for
the sequence in Ionization become very numerous. However
the total ionization can only be due to the ionization caused
by the components.
For an alpha-particle traversing air the Ionization
phenomena must be due to the alpha-particle, ejected electrons,
positive ions, and negative Ions. The negative ion is formed
by attachment of an electron to a neutral molecule or atom.
Since oxygen is electro-negative,25 the negative Ions In
air would be Og, etc. For some target atoms and molecules
there are further complicating effects such as the creation
of multiply charged Ions of mercury, Hg^, Hg"*"*, Hg’4'4'*, Kg’4"*"4”4,
etc. which may be formed by relatively low energy collisions.
