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Ç N A E M 33
T. A. E. C.
ÇEKMECE NUCLEAR RESEARCH CENTER
ISTANBUL - TURKEY
A H O M E - M A D E N E U T R O N C R Y S T A L S P E C T R O M E T E R F O R
R E S E A R C H A N D T R A I N I N G
R. ö . Akyüz, Ç. Cansoy and F. Domaniç
A HQME-NRDE NEUTRON CRYSTAL SPECTROMETER FOR
RESEARCH AND TRAINING
R. ö. Akyüz, Ç. Cansoy and F. Domaniç
S U M M A R Y
In order to make experimental research in the field of low energy nuclear physics, a crystal neutron spectrometer is constructed in ÇNAEM. The energy range is between 0,025 - 6 eV. The available monochromator single crystals are Be, NaCl,(CaCC^), Pb. The minimum precise rotation of spectrometer is 1 minute of arc in Bragg angle. Two Soller type collimators, one from reactor to crystal and the other from sample to counter, are used. The net angular divergence is approximately 5 minutes of arc. Counting system consists of two channels, one of which is used for monitoring the reactor. Counter tubes are B ^ en riched BF3 proportional counters. The main problem in construc tion was, to design and give precise movement to the heavy crystal shielding.
-I. INTtODUCTION
After the foundation of TR-1 Reactorx in ÇNAEM, the possibility of constructing a neutron crystal spectrometer which will be used in low energy nuclear research, was taken into consideration. The necessary help and information were provided by BNL (USA) members who have a great experience in this field?*
II. DETAILS OF THE INSTRUMENT
A) C o l l i m a t i o n :
The Soller type collimator which is located in the beam tube No.4 of reactor consists of narrow steel tubes in a stainless steel cover. There are 64 rectangular sectioned tubes with dimensions of 0,4 x 0,7 x 194 cm. The beam is 5.25 cm high and 2.7 cm wide. Additional beam defining are obtained by means of boron carbide bricks that are placed where needed. Those bricks are made of paraffin and 320-mesh B4C mixture. The amount of the paraffin is held to a minimum
x
TR-1 Reactor Which is a 1 MW Power swinming pool reactor sited in ÇNAEM (Çekmece Nuclear Research and Training Center) is the first Turkish research reactor,
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Dr. Vance L. Sailor and Mr. Robert Schmidt of Nuclear Cryogenics Group of BNL(USA).
The Collimators were fabricated in BNL under the auspices of •BNL-ÇNAEM cooperative program.
-2-so the bricks have rea-2-sonably aood mechanical strength and can keep their shape indefinitely.
B) S p e c t r o m e t e r T a b l e :
The main structural features of the ÇNAEM instrument are modeled after the BNL spectrometer.^ It is built on a large scale, so that with a beam wide enough to give
sufficient intensity and good resolution. The table and the arm are built as to support heavy loads of shielding and at the same time to maintain good precision in their motion. The arm which is made of a U beam is, 2.95 m long and can support a load more than 80 kg. at the end without any appreciable deflection. The arm and the crystal can be rotated independently or can be coupled together to move in a 2:1 ratio.
The Bragg angle can be read from a steel millimetric tape which is set on a circular frame, at a distance of 171.974 cm. from the axis of rotation.(Fig. 1) At this dis tance, an angle of 6017 minutes of arc corresponds to 301 cm on the tape. The errors in absolute Bragg angle are less than *1 min. of arc. The backlash of gear train is measured by means of an auto-collimator and it is found to be less than *1 min. of arc.
(1)
C) D e t e c t i n g S y s t e . n a n d M o n i t o r :
The neutron detector is a BF^ proportional counter, 5.1 cm in diameter ard 50 cm. in length. It is filled to a pressure of 40.6 cm. Hg with BF3 enriched in B10 to 96%. The counter operates in-2000 Volts and has a 600 Volt plateau. It is shielded by 30 cm. of borax added paraffin to minimize back ground. The efficiency of the detector is almost 1 in the energy region where the instrument works.
The beam is monitored by a snail BF3 counter mounted close to the exit of the 1st collimator. It is positioned to give
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3.10 counts per minute. All the data are normalized to the monitor, so it is important that the monitor remains stable, especially for the duration of any particular run. It was indicated by the tests that the stability of the monitor is good to about 1% over a period of more than a month of con tinuous operation.
D) S h i e l d i n g :
The instrument is not designed for automatic operation. Therefore, in order to make possible the nanual rotation of the system there was needed a local shielding around the crystal to get rid of the biological and the back-ground effects of radiation caning from the reactor fast neutrons and incoherent scattering from the crystal. This was the main problem that was considered. It took quite a long
-4-time to solve this problem and three different types of crystal shieldings were tested with the help of the health physicists. The last form is shown in Fig. 2.
The whole shielding consists mainly of two parts. One of them is close to the reactor wall, it has a plane-concave shape and is fixed. The other shielding surrounding the crystal is a cylinder having an i.d. of 40 cm. and an o.d. of 120 cm. All of the shielding is made of borax added paraffin containing 5 cm. thick lead layer at all internal sides to prevent y radiation. Additional pieces of shielding with convenient shapes are used
in places where leakings are seen. And a piece of shielding involving 10 cm. thick lead and 20 cm. thick paraffin is hanged on the cylinder where the direct beam hits.
The moving part of the shielding is cylindirical and weighs about 2000 kg. It is inpossible to give the mechanical parts a better precision and rotational steps less than 1 min. of arc because of that weight.
III. SPECTROMETER ALIGNMENT
The alignment of the spectrometer must satisfy several
requirements common to this type of instrument to avoid geometrical errors in measuring 'angles. The precision with which each of the various adjustments were made is estimated as follows:
1) The well collimator and the second collimator are parallel to within ± 1 0 seconds of arc;
2) The axis of rotation is centered in the neutron beam to ±0.005 cm ;
3) The crystal is oriented with the reflecting planes parallel to the axis of rotation to ±1 minutes of arc ;
4) The crystal slab is centered over the axis of rotation (when crystal is used in transmission) to within ±0.02 cm. ;
5) The zero angle is determined within ±0.2 minutes of arc.
Step (1) in the alignment is acccnplished by Optical (theodolite) and mechanical methods. Steps (2) and (5) are achieved by means of the neutron beam itself. The greatest uncertainty is in step (4) because the geometrical center of the crystal slab does not necessarily coincide with the "center" of the Bragg reflection. The crystal position which gives the maximum reflection is selected as being the optimum position.
In general, errors in the alignment cause errors in the measured angles of about 30 seconds of arc in the whole range of the scale.
IV. MONOCHROMATORS
An incident neutron beam having a de Broglie wave length satisfying Bragg's law for any set of crystal planes may be
-6-reflected. That is, a Bragg reflection can occur if
ni = 2dhkj. Sin 6hki (1)
where n is an integer; A=h/mV is the de Broglie wave length of a neutron having momentum mV; is the spacing corresponding to the crystal plane with Miller indices h, k, and 0^$, is the angle of the incidence of the neutron on these planes. The energy of the neutron diffracted in the crystal is
E = Const.
dwa si*20hk
l(2)
It is obvious that with planes of small d and/or in small Bragg angle positions, higher neutron energies can be obtained. Because of the excessive background increase due to the direct beam , the minimum useful angle is 4° of arc. So to higher the energy, one has to use crystals which give planes with smaller d. But the
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thermal neutron flux distribution of the TR-1 Reactor and the lower value of the total thermal flux (10^n/(sec.cm.)) prevents the
spectrometer to work at the energies higher than 2 eV. On the other hand, the constructional form of the cylindrical shield does not allow the Bragg angle to be more than 20° of arc, so the mininum energy is about 0,02 eV. The available monochromator crystals and
x ” ~
In the region close to the direct beam back-ground increases by contributions from randomly scattered neutrons and y rays of the direct beam,
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V. CHARACTERISTICS
A ) . R e s o l u t i o n :
As given with its argument in reference (2) , the full width at half maximum of the rocking curve of the crystal is expressed as
"where 6 is the mosaic spread of the crystal and “ is the angular divergence of each of the collimators. “ has the value of 7 minutes of arc.
The resolution width at energy E is determined by the formula
, 3/2 AE - kE
where k = dhk£ *ûe /0.0715is a constant which depends on the crystal plane only. The resolution widths at various energies and for different crystal planes are listed in Table 2. A0 can be determined as follows :
In the case of very snail mosaic spread A<f> = A0 = a / /2 1 5
This is true only when the effect of total reflection of the wall material of the collimators is negligible. So, in general,
Sailor, Foote, Landon and Wbod: Rev. Sci. Instr., 27, 26(1956)
-8-the exact value of A0 is determined from -8-the half width of a rocking curve of calcite (CaCO^) crystal which has almost zero mosaic spread. AG depends on the critical angle of the wall material and hence is a function of the neutron energy. For example, A0 has the value of 9.5 minutes of arc at 0.028 eV, 6 minutes of arc at 0.085 eV, 5.5 minutes of arc at 0.18 eV and 5 minutes of arc at 0.26 eV.
B) P r e c i s i o n o f t h e E n e r g y S c a l e :
Sources of error in determination of energy is discussed in Ref. (1). Then the energy scale of the spectrometer is checked by measuring the transmissions of materials which
have well defined resonances in the energy region of the instru ment. The measurements with Rh and In give sufficient results for the precision of the energy scale. The differences are not much than reading errors of the Bragg angle which is less than * 0,25 minute of arc.
VI. EXPERIMENTATION
The first experiments are planned to be the measurements
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of the total neutron cross-sections of Cd and Sm . Of course the determination of cross sections is made by the transmission method. The ratio of counts taken with and without sample before the counter will give the transmission. A pneumatic lever which can be controlled remotely, changes easily the sample with the dumny hole and vice versa. Each time the position of sample
changes not much than ±0.1 nm. and since the sample is wider than the incident neutron beam 4 nm. at each side, although the accumulations would occur during the sample changings, no consider able error can take place.
VII. ACKNOWmX^MENTS
The authors are indebted to Dr. Vance L. Sailor and Mr. Robert Schmidt of BNL for their very kind helps both from BNL and during their visits to ÇNAEM in planning and construction of the instrument. We wish to express our appreciation to the people of the work shops of ÇNAEM, Mr. Necdet Birinci who helped in technical designing, Mr. İbrahim Büyüksayar and Mr. Samet Yavrutiirk who gave their great technical experience to construct the spectrometer. And we also gratefully acknowledge of the assistance of the Health Physicists for their great help in the testing of the shielding.
TABLE I
THE AVAILABLE CRYSTALS AND PLANES
Crystal Plane Spacing (d), A°
200 2.82003 NaC 220 1.99406 1011 1.73260 Be 1231 1.73230 Ca003 112 3.02946 (Calcite) TABLE 2
RESOLUTION WIDTHS AT VARIOUS ENERGIES
Energy (eV) Planes E(eV)
CaC03 (112) 0.0022 NaC (200) 0.0017 0.0973 NaC (220) 0.0012 Be (1011) 0.0011 Be (1231) 0.0004 CaC03 (112) 0.0057 NaC (200) 0.0043 0.181 NaC (220) 0.0030 Be (1011) 0.0029 Be (1231) 0.0011 CaCO (112) 0.060 NaC J (200) 0.045 0.872 NaC (220) 0.032 Be (1011) 0.031 Be (1231) 0.012
g en er al v ie w o f th e S p ec
