A Mossbauer spectrometer with high efficiencies in both transmission and backscattering techniques is described. The device contains a sodium iodide crystal for detecting radiation caused by the Mossbauer effect, and two photomultipliers to collect the radiation detected by the crystal. When used in the transmission technique, the sample or scatterer is placed between the incident radiation source and the detector. When used in a backscattering technique, the detector is placed between the incident radiation source and the sample of scatterer such that the incident radiation will pass through a hole in the crystal and strike the sample. Diagrams of the instrument are provided

Singh, J. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19740006978 2020-03-17T14:35:45+00:00Z
Unked States Patent
Singh
[in 3,781,562
[45] Dec. 25, 1973
[54] MOSSBAUER SPECTROMETER
RADIATION DETECTOR
[75] Inventor: Jag J. Singh, Yorktown, Va.
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D.C.
[22] Filed: Dec. 8, 1972
121] Appl. No.: 313,381
[52] U.S. Cl 250/360, 250/361, 250/369,
250/492
[51 ] Int. Cl. GOlj 39/18
[58] Field of Search 250/276, 361, 367,
250/458, 485, 486
[56] References Cited
UNITED STATES PATENTS
2,858,452 10/1958 Wouters 250/367
2,911,534 11/1959 Brannon et al 250/486.
2,666,145 1/1954 Eversole et al 250/367
3,404,277 10/1968 Jerko : 250/359
3,257,558 6/1966 Cook 250/272
3,612,875 11/1971 Ord. 250/360
Primary Examiner—Harold A. Dixon
Attorney—Howard J. Osborn et al.
[57] ABSTRACT
The invention is a versatile Mossbauer spectrometer
with high efficiencies in both transmission and back-
scattering techniques. The heart of the invention is a
detector which includes an NaI(Tl) crystal for detect-
ing radiation caused by the Mossbauer effect. The
crystal has a hole in its center to allow the collimated
incident radiation to pass through and it is coupled
through two quartz optical windows to two photomul-
tiplters such that all of the scintillation radiation pro-
duced by the radiation detected by the crystal will
flow into the two photomultipliers. When used in a
transmission technique the sample or scatterer is
placed between the incident radiation source and the
detector; and when used in a backscattering technique
the detector is placed between the incident radiation
source and the sample or scatterer such that the inci-
dent radiation will pass through the hole in the crystal
and strike the sample.
6 Claims, 2 Drawing Figures
T
21
ELECTRO MECHANICAL
SOURCE TRANSDUCER
\
MOSSBAUER SPECTROMETER
34
PATENTEDKC25I373 3,781,562
19
FIG. 1
T
ELECTRO MECHANICAL
SOURCE TRANSDUCER
(CONTROL UNIT)
FIG. 2
PHOTOMULTIPLIER PHOTOMULTIPLIER
33 30
ATTENUATOR INVERTER
32^
SINGLE
CHANNEL
ANALYZER
v_
MOSSBAUER SPECTROMETER
31
T 1
AMPLIFIER
^34
• . • • " • 3,781,562
1 2
MOSSBAUER SPECTROMETER RADIATION It is therefore the primary object of this invention to
DETECTOR provide a Mossbauer spectrometer that utilizes a scat-
ORIGIN OF THE INVENTION teri"g technTe which Provides better results **" Pre"UKIUIM ui- i Ht IN vtN 1 ION
 yious ^ 5^^ and transmission techniques.
The invention described herein was maide by an em- 5 Another object of this invention is to provide a radia-
ployee of the United States Government and may be tion counter for use in a Mossbauer spectrometer that
manufactured and used by or for the Government for has the advantages of subtending a very large solid
governmental purposes without the payment of any angle at the scatterer and of 'seeing* only the scattered
royalties thereon or therefor. radiation.
oAr-vror>u\sr\ r»c -rue IKIWCWTU™ 10 Other objects and advantages of this invention willBACKGROUND OF THE INVENTION
 become apparent hereinafter fn the speciflcation ^ in
The invention relates generally to spectrometry and the claims.
more specifically concerns a versatile Mossbauer spec- „ ,„ ^ ^ ,^,,,^,^,^
trometer with high efficiencies in both transmission and SUMMARY OF THE INVENTION
backscattering techniques. l5 The invention is a versatile Mossbauer spectrometer
Mossbauer spectrometry has not found many appli- that is efficient both in transmission and reflection ge-
cations in such areas as surface physics and structural ometries. It includes a radiation detector consisting of
mechanics of bulk matter. This is mainly the result of a 0.6 inch by 2 inches by 2 inches NaI(T£ ) crystal with
thin sample requirement for the commonly used trans- a 0.6 diameter hole in its center. The crystal encapsula-
mission geometry. It is, however, possible to eliminate tion incorporates 1.5 inch diameter, 0.005 inch thick
this limitation by using a scattering technique in which front and back beryllium windows that permit easy
no special sample preparation is required. At present penetration by the conversion X-rays and the reemitted
this technique is not used since the results are not very 14.4 keV gamma rays into the sensitive region of the
satisfactory, certainly not as good as in a transmission Nal(Tl) crystal. The crystal is coupled, at its two ends,
geometry. There are several reasons for it: (1) low effi- via quartz optical windows, to two matched photomul-
ciency due to a very low solid angle subtended by previ- tipliers. The electronic gains of the two photomulti-
ous detector configurations; (2) relatively pronounced pliers are equalized and the signals at the respective
interference (in the case of CO57) by the 122 keV collectors are added and amplified before passing
gamma ray-induced fluorescence as a result of low
 30 through a single channel analyzer (SCA). The limits of
Mossbauer counting rates and (3) imprecise source- the SCA are set to accept all pulses in the range of from
velocity definition in previous counting arrangements. 5 to 15 keV. These accepted pulses are applied through
In a transmission geometry, the incident radiation an inverter to an attenuator to a Mossbauer spectrome-
passes through the Mossbauer absorber before falling ter.
* —' DESCRn-nON OF THE DRAW,NOS
the intensity of the transmitted radiation. The reemit- FIG. 1 is a cutaway view of the detector portion of
ted radiation, because of its isotropicity, does not en- this invention; and
tirely compensate for loss, in the forward direction, due FIG. 2 is a block diagram showing the use of the de-
to absorption out of the incident beam. The transmitted 40 lector in FIG. ! in a backscattering geometry.
radiation spectrum, therefore, shows a net reduction in
 nFTAn Fn nPSPRIPTION OF THF INVENTIONthe counting rate at resonance velocity. Since only a DETAILED DESCRIPTION OF THE INVENTION
certain fraction, Mossbauer fraction, of the incident Turning now to the embodiment of the invention se-
beam exhibits resonance fluorescence, one is necessar- lected for illustration in the drawings the number 1 £ in
ily faced with a large nonresonant component at the de- 45 FIG. £ designates an NaI(T£) crystal. The dimensions
tector. This nonresonant component has the net effect of crystal 11 are 2 inches by 2 inches by 0.6 inch with
of reducing the S/N ratio, and consequently the resolu- a 0.6 inch diameter hole 12 at its center. Crystal 11 is
tion, in the transmission spectra. The situation could be coupled through one-fourth inch thick quartz optical
considerably improved if the detector did not 'see' non- windows 113 and £4 to photomultipliers IS and &$, re-
resonant component of the incident radiation. One way SO spectively. The crystal £1 is completely encapsulated
of achieving this is to use the scattering geometry in with 1.5 inch diameter and 0.005 inch thick beryllium
which the detector "sees" only the resonantly scattered windows 17 and 18, plus any other suitable material,
radiation. Scattering technique in transmission geome- These beryllium windows 17 and 18 permit easy pene-
try has not been used because the absorber must be tration by the conversion X-rays and the reemitted 14.4
thin, just as in the conventional absorption technique, 55 keV gamma rays. Each of the photomultipliers 15 and
to let the incident, as well as the reemitted, radiation £6 includes an input terminal 19 for applying a voltage
pass through it. and an output terminal 20 for removing the produced
Scattering technique in backscattering geometry has signals and provision for adjusting the focusing voltage
previously been used with considerable advantage. In to equalize the multiplier gains,
this use the scatterer or sample is placed at an angle Even though crystal 11 has been disclosed as having
with the incident radiation. In such an arrangement, a rectangular shape, it can be many different shapes,
only the backscattered radiation can reach the detector The only requirement is that crystal £ £ be shaped on its
and one should, therefore, except considerable im- two ends to provide for direct coupling of optical win-
provement in the resolution in the spectra. However, dows (light guides) to transmit all the scintillation radi-
even this technique has .not yet been successfully ex- ation to the windows of the two photomultipliers. Also
picked because of the extremely long counting periods crystal 11 can have dimensions different from those
needed for good statistical accuracy in the spectra. disclosed without departing from the invention.
3,781,562
The detector disclosed in FIG.-1 can be used in both
transmission and reflection geometries. FIG. 2 is a
block diagram of the use of the detectors in FIG. 1 in
a reflection or backscattering geometry. In this use, an
electro-mechanical source transducer 21 controlled by 5
control units (not shown) produces motion of the CO57
source 21. The radiation emitted by this source is colli-
mated by a collimator assembly 23. The collimated ra-
diation beam 22 from the source 21 passes through the
hole 12 of the detector and strikes the specimen 24 10
being studied. Specimen 24 is shown stationary; how-
ever, it could have motion as well as source transducer
21. The resulting backscattering radiation 25 is de-
tected by the NaI(Tl) crystal and then passed through
the quartz optical windows to photomultipliers 15 and is
16 which have a high voltage Hv applied to them. The
solid angle subtended by the detector, at the point of
beam 22 incidence on scatterer 24, can be maximized
by adjusting the distance between the scatterer and the
detector. The outputs of photomultipliers 15 and 16 20
are applied to a summing network consisting of resis-
tors 26, 27 and a potentiometer 28. The purpose of po-
tentiometer 28 is to equalize the sensitivities of the two
photomultiplier channels. The sum of the outputs from
photomultipliers appears at the slider of potentiometer 25
28. This sum voltage is amplified by preamplifier 29
and amplifier 30 and then applied to a single channel
analyzer 31. The SCA 31 limits are adjusted to accept
all pulses in the range of from 5 to 15 keV. The output
of the SCA is applied, through an inverter 32 and an at- 30
tenuator 33, to a Mossbauer spectrometer 34.
The advantages of this invention are numerous. It is
highly efficient and equally suitable for transmission
and backscattering geometries. When used under opti-
mal conditions, the detector subtends a solid angle of 35
3.15 steradians at the specimen location. This high
solid angle, coupled with freedom from non-M6ssbauer
radiation components at the detector, permits the use
of more intense radiation sources thereby effecting an
order of magnitude savings in time. Additionally, 40
shorter run times eliminate the drawbacks associated
with long time electronic gain drifts and thus lead to
improved sensitivity of the Mossbauer spectrometers.
Also, it can be used under environmental conditions
without any special preparation of the sample or scat-
terer.
It is to be understood that the form of the invention
herewith shown and described is to be taken as a pre-
ferred embodiment. Various changes can be made
without departing from the spirit or scope of the inven-
tion as defined in the subjoined claims. For example,
the shape, dimensions and composition of crystal 11,
the source of incident radiation, and the balancing cir-
cuit for the two photomultipliers can be different from
that disclosed without departing from the invention.
What is claimed as new and desired to be secured by
Letters Patent of the United States is:
1. A detector for use in a Mossbauer spectrometer
wherein a beam source of gamma rays irradiates a sam-
ple for producing secondary gamma rays characteristic
of the Mdssbauer effect comprising:
a crystal having a composition suitable for detecting
radiation caused by the Mossbauer effect and hav-
ing a hole in it for passing the substantially unatten-
uated primary radiation of said beam source of
gamma rays, and
two photomultipliers connected to two sides of said
crystal such that substantially all of the scintillation
radiation produced by the radiation detected by
said crystal flows out of said crystal into said two
photomultipliers.
2. A detector according to claim 1 wherein the com-
position of said crystal is NaI(Tl).
3. A detector according to claim 1 wherein said crys-
tal is coupled to said two photomultipliers through two
quartz optical windows.
4. A detector according to claim 3 including a beryl-
lium window on each side of said hole.
5. A detector according to claim 3 including means
'for summing the outputs of said two photomultipliers.
6. A detector according to claim 5 including means
for equalizing the sensitivities of the two photomulti-
pliers.
45
50
55
60
65


Mossbauer spectrometer radiation detector

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