Misenta, R.

Olivi, L.

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Survey Measurements on the Optimalisation of the Scatterer Shape for a Cold Neutron Source. EUR 2953.

EUR 2953.e EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM SURVEY MEASUREMENTS ON THE OPTIMALISATION OF THE SCATTERER SHAPE FOR A COLD NEUTRON SOURCE by L. OLIVI and R. MISENTA 1966 Joint Nuclear Research Center Ispra Establishment - Italy Reactor Physics Department Experimental Neutron Physics LEGAL NOTICE This document was prepared under the sponsorship of the Commission of the European Atomic Energy Community (EURATOM). Neither the EURATOM Commission, its contractors nor any person acting on their behalf : Make any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe privately owned rights ; or Assume any liability with respect to the use of, or for damages resulting from the use of any information, apparatus, method or process disclosed in this document. This report is on sale at the addresses listed on cover page 4 at the price of FF 4.- FB 40.— DM 3.20 Lit. 500 Fl. 3.— When ordering, please quote the EUR number and the title, which are indicated on the cover of each report. Printed by L. Vanmelle, S.A. - Gent Brussels, May 1966 This document was reproduced on the basis of the best available copy. EUR. 2953.e SURVEY MEASUREMENTS ON THE OPTIMALISATION OF THE SCATTERER SHAPE FOR A COLD NEUTRON SOURCE by L. OLIVI and R. MISENTA European Atomic Energy Community — EURATOM Joint Nuclear Research Center — Ispra Establishment (Italy) Reactor Physics Department — Experimental Neutron Physics Brussels, May 1966 — 14 Pages — 5 Figures — FB 40 The output of subthermal neutrons was measured for differently shaped lucite samples in a graphite stack which simulated a tangential beam hole of a reactor. The samples were cooled to liquid nitrogen temperature. The obtained values were normalized to the output of a circular slab by the introduction of a shape factor. The measurements show that an elliptical slab in the reflection position and a black body shaped scatterer have gain factors by 20 - 50 % higher than a circular scatterer of the same thickness. The usual gain factor, which is defined as the neutron output a t high temperature to that at low temperature is for all shapes between 2.5 and 3. EUR. 2953.e SURVEY MEASUREMENTS ON THE OPTIMALISATION OF THE SCATTERER SHAPE FOR A COLD NEUTRON SOURCE by L. OLIVI and R. MISENTA European Atomic Energy Community — EURATOM Joint Nuclear Research Center — Ispra Establishment (Italy) Reactor Physics Department — Experimental Neutron Physics Brussels, May 1966 — 14 Pages — 5 Figures — FB 40 The output of subthermal neutrons was measured for differently shaped lucite samples in a graphite stack which simulated a tangential beam hole of a reactor. The samples were cooled to liquid nitrogen temperature. The obtained values were normalized to the output of a circular slab by the introduction of a shape factor. The measurements show that an elliptical slab in the reflection position and a black body shaped scatterer have gain factors by 20 - 50 % higher than a circular scatterer of the same thickness. The usual gain factor, which is defined as the neutron output at high temperature to that at low temperature is for all shapes between 2.5 and 3. EUR 2953.e EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM SURVEY MEASUREMENTS ON THE OPTIMALISATION OF THE SCATTERER SHAPE FOR A COLD NEUTRON SOURCE by L. OLIVI and R. MISENTA 1966 Joint Nuclear Research Center Ispra Establishment - Italy Reactor Physics Department Experimental Neutron Physics TABLE OF CONTENTS I. Introduction 1 II. Experimental set-up and flux distribution 2 III. Measurements on different scatterer shapes 3 1. Method 3 2. Circular slabs 3 3. Elliptical slabs 4 4. Black body scatterer 5 IV. Discussion of the results 5 LIST OF FIGKUES Fig.1 Horizontal section of the experimental set-up la ι So - Circular slabs ïb « RSo - Elliptical slabs in reflection 1o ι TSo - Elliptical slabs in transmission Fig. 2 The background corrected counting rates for circular lucite slabs as function of the thickness diameter of the slabs : 175 mm Fig. 3 The background corrected oounting rates for circular luoite slabs measured in reflection and in transmission as function of the thickness Fig. 4 The background oorreoted counting rates for a block body scatterer as function of the shell thickness d. Fig. 5 The gain and shape factors for circular and elliptical slabs as funtion of the thickness SUMMARY The output of subthermal neutrons was measured for differently shaped lucite samples in a graphite stack which simulated a tangential beam hole of a reactor. The samples were cooled to liquid nitrogen temperature. The obtained values were normalized to the output of a circular slab by the introduction of a shape factor. The measurements show that an elliptical slab in the reflection position and a black body shaped scatterer have gain factors by 20 - 50 % higher than a circular scatterer of the same thickness. The usual gain factor, which is defined as the neutron output at high temperature to that at low temperature is for all shapes between 2.5 and 3. - 1 -I. Introduction The dependance of the cold neutron output on the moderator shape has been mentioned "by Webb in his survey article on cold neutron sources and Webb proposed some scatterer geometries which can lead to a higher output of cold neutrons / 1 / from a cold mode-rator in the reactor. For the planned installation of a cold neutron source in a tan-gential beam hole of the Ispra-I reactor the question has been raised wether a substantial increase in the output of cold neutrons can be obtained by appropriately choosing the shape of the scatterer cell. In order to answer this question survey measurements have been performed on differently shaped lucite samples . The spatial flux distribution in a tangential beam hole of a reactor was mocked up by a graphite stack with a channel of large cross section and with americium-beryIlium neutron sources. The counting rate of a BF counter with a beryllium filter in front of it was taken as measure for the cold neutron output. With this mock-up the cold neutron output of cylindrical and elliptical scatterers and a scatterer with a black body shape was measured at room temperature and at about 80°K. ï The experimental investigation of this question was suggested by W.KLEY. Manuscript received on March 15, I966. - 2 -II· Experimental set-up and flux distribution The neutron flux in a tangential beam hole is roughly mocked up in a graphite stack with a transversal channel (Fig.1). Three Am-Be neutron sources with a total activity of 11 C and a total γ neutron output of 2.6x10 n/sec are placed in a distance of 28 cm from the channel. The neutrons are moderated by diffusing through the graphite and scattered by the scattering sample Sc into the collimator. The collimator has been chosen that only neutrons coming from the scatterer reach the counter. After having passed a beryllium filter with a length of 20 cm the neutrons are counted by a BF counter. In the transversal channel is a vacuum insulated cryostat in which the samples are mounted and cooled. The heat from the samples is transferred to a copper box filled with liquid nitrogen by natural convection of helium gas. Some measurements on the longitudinal, transversal and angular distributions of the thermal flux inside the channel have been made with a Cd-covered BF counter with a length of 16 cm. The longitudinal flux distribution shows the expected cosine-like behaviour. The transversal flux decreases from the side of the neutron sources to the other side with a slope of approximately 0.85%/cm up to a distance of 16 cm and then with a slope of about 0.335Vcm . The angular dependance of the flux has been measured in the central position of the transversal channel by rotating a BF counter which had a Cd cover with a slit. By placing the slit in the direction of the source the counting rate was higher by 30fo than the oounting rate observed in the opposite sense. The Cd ratio is approximately 45 in the whole channel. The distance is measured from the channel wall which is close to the neutron source. - 3 -I I I . Measurements 1. Method For the measurements a scatterer sample is mounted in a cryostat. Then the counting rate at room temperature is taken. The sample is cooled by natural convection of gaseous helium between a copper box which is filled with liquid nitrogen and the sample. As soon as the cooling starts the counting rates inorèases. After 8-10 hours the counting rate reaches a constant value, indicating that the scatterer has reached a steady state temperature. This value and the value of the counting rate taken at room temperature are the quantities of physical interest. For the evaluation of the results the background is subtracted from the measured counting rates. As background the counting rate measured without a scattering sample has been taken. All oounting rates have been normalized to 10 min. From the corrected counting rates the gain factor is calculated. This factor is defined as the ratio of the corrected counting rate at low temperature to that at room temperature. 2. Circular slabs The counting rates of circular luoite slabs with a diameter of 17.5 om have been measured at room and at low temperature for thickness up to 6 cm. Fig. 2 shows the background corrected counting rates normalized to 10 min at the two temperatures and Fig. 5 the gain factors as funotion of the thickness. From the figures it is seen that the optimum thickness is between 3 and 4 cm and that there is no substantial increase in counting rate and gain factor for samples thicker than 2.5 cm. 3E All scatterers have been made from lucite, which has the formula (CcH8°2) a density of 1.2 g/cnr5 and 5.7x10 h atoms/ sum cm . - 4 -The arrows in Fig.2 give the statistical error calculated from the total number of collected counts and by taking into account the statistical error of the background. In general the statistical error of the "background corrected counting rates is between +2 and +7fo* The gain factors have been calculated by taking the counting rates from the curves from the Fig. 2 and calculating the ratios. For this reason no experimental points are indicated in Fig.5· Based on the statistical errors of the counting rates at room and at low temperature an error of +10?& or less is estimated for the gain factors shown in Fig.5. 3. Elliptical slabs The oounting rates of elliptical slabs with axis of about 42.5 cm and 17·5 cm and which were inclined by 26° against the channel wall have been measured in the two positions, reflection (Fig.1b) and transmission (Fig. 1c) at room and at low temperature for thickness up to 5.2 cm. Fig.3 shows the background corrected counting rates at the two temperatures and Fig. 5 the gain factors for elliptical lucite slabs in reflection and in transmission as function of the thickness. From the figures it is seen that the optimum thickness for elliptical slabs is with 3 to 4 cm the same as for circular slabs. An important difference is seen by comparing the counting rates and the gain factors measured in reflection and transmission. For a thickness of 3.5 cm the counting rate measured in reflection is 375 cts/10' and for transmission 246 cts/10· whereas the respective gain factors are 2.5 and 3.0. - 5 -4. Black-body scatterer A black body shape has been approximated by a cylindrical shell with an outer diameter of 17·5 cm and a height of 23 cm which is closed at one end by a circular slab of lucite with a thickness of 3.5 cm and at the other end by a circular beryllium piece with a thickness of 5 cm and a diameter of 10 cm (see insert of Fig.4). The counting rates for the samples at room and at low temperature have been measured for three thickness of the shell. Fig.4 shows the background corrected counting rates at room and at low temperature for three different thickness of the shell. From the figure it is seen that the counting rate reaches a maximum with a shell thickness of about 12 mm. IV. Discussion of the results For the discussion of the results a gain factor is used. Further­more a shape factor is introduced which is defined by the ratio back-ground corrected counting rate for a given sample at a temperature Τ to background corrected counting rate for a cylin­drical sample with the same thickness at the same temperature T. In Fig.5 the gain factors and the shape factors for the circular and the elliptical slabs in reflection and in transmission are plotted as function of the thickness. For a slab thickness of 3.5 cm the background corrected counting rates at room and low temperature, the gain factors and the shape factors for the different scatterer shapes have been compiled in Table 1. The defined shape factor includes not only the shape of the scatterer but also the position of the scatterer to the direction of the neutron current and the counting tube, e.g. for the ellip­tical slabs measured in reflection and transmission the shape is the same only the orientation against the neutron current and the counting tube is different. circular slab elliptical slab in reflection in transmission blackbody scatterer dp ■ 12 mm, 1 = 23 cm background corrected counting rate (cts/101) at room low temperature 122 148 82 185 322 374 246 478 I gain factor 2.64 2.54 3.00 2.58 shape factor at room , low temperature 1.0 1.21 0.67 1.51 1.0 1.15 0.76 1.48 I ON I Table 1 : The background corrected counting rates in cts/10' at room and low temperature, the gain factors and the shape factors at room and low temperature for the different scatterer shapes with a thickness of 3.5 cm - 7 -The obtained shape factors of 1,2 for an elliptical slab in reflection and of 1,5 for a black body shaped scatterer show that the output of cold neutrons from a scatterer placed in a neutron flux with a gradient can be increased by 20^ in comparison with a circular slab, by choosing an elliptical slab of the same thickness and working in reflection or by 50$ by choosing a black body shaped scatterer. This increase in the output is already obtained with the scatterers at room temperature. For an elliptical slab the shape factor is about 0,7 in transmission and 1,2 in reflection. The difference in the shape factor for an elliptical slab in reflection and transmission can be qualitatively understood. Due to the flux gradient the number of neutrons scattered by the scatterer surface which is seen by the counter is larger in the reflection position than that number in the transmission position. In order to consider the shape factor of 1,5 for the black body shaped scatterer it is important to take into account that the shape factor comprises two different effects : - the difference in the number of neutrons which are scattered per sec into the collimator by the circular slab and by the black body shaped scatterer ; - and the difference in the probability that a neutron is mode-rated in the circular slab and in the black body shaped scatterer. Since these two effects have not been separated by the measure-ments it is difficult to say which effect causes the inorase in the cold neutron output of the black body scatterer . Since the shape factor for each of the three scatterers is the same at room and at low temperature survey measurements on the optimalisation of the scatterer shape can be made with hydrogenous samples at room temperature. _ The two effects could be separated by counting in addition the number of neutrons above the Cd cut off which are scattered into the collimator by a circular slab and by a black body shaped scatterer. - 8 -A difference between the gain factors for a circular slab, an elliptioal slab in reflection and a black body shaped soatterer has not been found. The gain factors for all these geometries is 2.5 to 2.6. For an elliptical slab in transmission a gain faotor of 3 has been measured which is higher by about 209& than the gain factors of the three other scatterers. However the neutron output at low temperature is still lower by about 30$ than the output of a circular slab. Bibliography 1) F.J. Webb Reactor Science and Technology, 17, I87 (1963) Fifl.1 t Horizontal section of the experimental set-up 1a ι So - Circular slab 1b ï ESo- Elliptical slabs in reflection 1c : TSo- Elliptical slabs in transmission C : Counter Bo t Boron carbide Be : Berylliura filter Ρ ι Paraffine S t Neutron souroe G : Graphite K Ï Cryostat 300 200 100 η τ ï Fig.2 : The background corrected counting rates for oiroular lucite slabs as function of the thickness Diameter of the slabs ι 175 HUD δ counting rates at room temperature i oounting rates obtained with the cooled samples 300 200 100 ζ / / / li ¡ι y IL·/-TÆ γ r / / r X τ y s " ■ * - * . I - \ _ X \ V î I i À cm. Fig.3 t Thö background corrected counting rates for circular luoite slabs measured in reflection and in transmission as function of the thickness S ó counting rates at room temperature O · counting rates obtained with the cooled samples ■ measured in reflection --_- measured in transmission 800 M l ih 600 400 200 -4 10 15 d. mm. Fig.4 : The background corrected counting rates for a black body scatterer as function of the shell thickness d. Be ι ciroular beryllium piece with a diameter of 10 om and a thickness of 5 cm d ι thickness of the shell i •4-! ? o c s / i / 7 V > / / v ^ „ ———«. ^ ^*^ -^ . - * ■ " * -í r* ■» s Fig.5 ι The gain and shape factors for oircular and elliptical slabs as function of the thickness circular slab elliptioal slab in reflection elliptical slab in transmission NOTICE TO THE READER AU Euratom reports are announced, as and when they are issued, in the monthly periodical EURATOM INFORMATION, edited by the Centre for Information and Documentation (CID). 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Survey Measurements on the Optimalisation of the Scatterer Shape for a Cold Neutron Source. EUR 2953.

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