NUCLEONICS DATA SHEET No. 3 
Shielding Constants 
Gamma Rays from Thermal-Neutron Capture 
By PHILLIP S. MITTELMAN 
and ROBERT_A. LIEDTKE 
Nuclear Development Associates, Inc. 
White Plains, New York 
Gamma RAYS from absorption of ther- 
mal neutrons in a reactor and its shield 
are often of major importance to the 
shield designer. Table on p. 51 lists all 
available data on these gamma rays in 
useful form. Not all elements are 
listed since not all have been measured. 
In all cases, the target material is 
listed. Usually this is the natural ele- 
ment, but in some instances single 
isotopes are tabulated as indicated by 
the mass number following the element 
name. In these instances, the capture 
cross section applies only to the isotope 
indicated. Tabulated (n, y) cross sec- 
tions were usually taken from applica- 
ble papers of the Kinsey-Bartholomew- 
Walker group (1-14). If no such 
reference existed, the cross section was 
obtained from AECU-2040 (35). 
Gamma rays emitted by an element 
after thermal-neutron absorption fall 
into two classes: ‘‘capture” and ‘“‘de- 
cay” gamma rays. Capture gamma 
rays originate when a nucleus, having 
absorbed a thermal neutron, releases 
the binding energy of the neutron (5-10 
Mev). Usually this energy is given 
off promptly (~10-" sec) in the form 
of one or more gammarays. “Decay” 
gamma rays are produced if the de- 
excited nucleus is still unstable, and 
decays by beta and gamma emission to 
a stable nucleus; pertinent information 
is tabulated in the last column. 
Capture-gamma-ray spectra are tab- 
ulated by four major types. Three are 
illustrated by Figs. 1-3 and described 
in accompanying captions. A fourth 
“type,” that for which the gamma rays 
are weak or nonexistent because parti- 
cle emission is the favored mode of de- 
excitation, is necessarily not illustrated. 
Boron-10 and lithium-6 are the only 
representatives of that class included 
here (because of importance to shield- 
ing design). 
In addition to spectral classification, 
the number of photons emitted in 
202 
n 
o 
T 
é 
8 
3S 
= 
§ 
8 
wu 
z 
ZS ey ia 
Gomma- Ray Energy (Mev) 
FIG. 1. Type 1 capture-gumma-ray spec- 
trum shows few gamma rays. Ground- 
state-transition line dominates. Most of 
de-excitation energy is carried by single 
6-8-Mev gamma rays 
3 
nYPE;2 
(Vanadium) 
NIE) (photons/Mev/copture) 
OF OO (610) 
o @ 
Oo NN ff 
Ta een 
Gamma-Ray Energy (Mev) 
ah 
FIG. 2. Type 2 capture-gamma-ray spec- 
trum shows many gamma rays. Distinct 
line structure is evident. , This is typical of 
light and medium-weight elements with 
fairly large level spacing between levels 
and transitions between levels as likely 
as transitions to ground 
N (E) (photons/Meyeapture) 
Pt 
4 5 6 TZ. 8 
Gamma-Ray Energy (Mev) 
FIG. 3. Type 3 capture-gamma-ray spec- 
trum shows many gamma rays, no line struc- 
ture below 5 Mev. This is typical of heavier 
elements; they have high level density and 
overlapping highest levels. Distribution 
peaks below half maximum energy 
various energy intervals per 100 cap- 
tures is tabulated. In some cases suffi- 
cient data were available to determine 
the total number of photons in each 
energy interval. In other cases, only 
the intensity of the resolved lines could 
be determined. For spectral types 1 
and 2 the difference is small, but for 
type-3 spectra the difference is signifi- 
cant. Fortunately, sufficient data were 
available to determine the number of 
unresolved photons for most type-3 
spectra. For those elements for which 
the number of unresolved photons has 
been determined, the spectral type is 
marked with an asterisk (*). 
* * * 
This article is based largely on NDA 10-99 
prepared under contract with Pratt and 
Whitney Aircraft Division of United Aircraft 
Corporation. Table has been revised and 
brought up to date by R. A. Liedtke under 
AEC contract AT(30-1)862C. 
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86. Neutron Cross Section, AECU 2040 
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