An Improved Slow Neutron Spectrometer at the Research Reactor-R1

EG9601747

AN IMPROVED SLOW NEUTRON SPECTROMETER AT THE RESEARCH REACTOR-R1

M.A. ABU EL-ELA

Reactor and Neutron Physics Department, Nuclear Research Center. AEA. Cairo. Egypt.

ABSTRACT

An improved slow neutron selector has been aligned at channel number 6 of the nuclear research Reactor-Rl-lnshas. The flight path is 3.75 meter. The collimator-rotor-collimator system has the dimensions 0.3 x 2.5 x 70 cm3 with the rotor diameter 16 cm and 3 slits of 0.3 x 2.5 cm2 cross section. The rotor rotation rate varies between 600 r.p.m and 3600 r.p.m. The counting system has one of the best modern high electronic advanced technology time analyzer with minimum Dwell time 2 jisec, 8192 channels and a Double detector inputs of TTL and NEG NIM standard pulses. The analyzer external Triggering signals are of TTL standard type. A special design 3He detector for Time of flight spectrometry has been used in the SNS. The reactor bare thermal neutron spectrum has been successfully measured.

INTRODUCTION

Neutron spectrometers can be arranged into 4 generations, since the discovery of the neutron in 1932. The first 2 generations are a twin characterized by the dual property of the neutron.

12 k The first generation of the twin consider the neutron as a particle, in this case, mechanical treatment could be done, through a mechanical chopper.

These mechanical choppers^ " ' can be mainly classified into, fast 3 5 4 6 17 I9 2 neutron choppers^ ' ) and slow neutron choppers^ ' " « ' °)$ Both are jointly arranged by 3 rotor parameters; 1 -neutron scattering rotors; 2-neutron absorbing rotors; 3- shape of slit inside the rotor, (straight, curved, cigar, barrel, helical). These choppers started by one slit rotor, followed by multi- slits rotor ^ followed by multi-rotors spectrometers^ >5'2°).

The second generation of the twin consider the neutron as a wave, in this case, wave reflection and diffraction treatment could be used, through a single crystal. This generation started by a (straight) single crystal*18^, followed by 2 single crystals, the first crystal as neutron monochromator and the second crystal as neutron analyzed \ (Neutron Triple axes spectrometer).

The third generation is a combination between the first generation and the second generation,

The fourth generation considers the neutron spin. The high advanced technology helped much more in solving many problems, to provide the neutron spin spectrometers and the associated neutron guide tubes.

The present spectrometer is one of the simplest slow neutron mechanical choppers, and high advanced technology electronics. The mechanical part is the same as was done in 1974' \ The electronic system is completely different from the old one, between the use of valve tubes and the use of integrated circuits ICs, besides, avoiding the use of mechanical tachogenerator (amplidyne) to the use of electronic tachogenerator, that prevents any additional mechanical load on the motor of the rotor. The use of NEG N1M logic pulses instead of TTL pulses introduces a better resolution for the counting system. Special time of flight (TOF) He detector with diameter half of the BF3 old one , and better resolution is used. The time multichannel analyzer provides a dwell time of 2|isec, compared with 16 u.sec in the old one with the ratio 1:8 and 10 jisec in the most recent

125 neutron spectrometer in the reactor and neutron physics department of the necular research center^, with the ratio 1:5. This facility is very important, specially in the case of neutron elastic and inelastic scattering, where the need to distinguish between closed peaks is required. The use of IC's in building the system had helped in decreasing the ratio between the noise level and the effective values.

The spectrometer can be used in the neutron wavelength range from 0.5 A to 6 A. This range can be extended to 10 A by increasing the power of the reactor, and using a higher vacuum flight path tube.

THE EXPERIMENTAL PROCEDURES

1- An inpile collimator had been designed from Pb with dimensions 25x2.5x0.4 cm3, to decrease the spectrometer background and keep it safe.

2- The neutron beam center had been determined, by using dual scanning, in the horizontal position and the vertical one, at flight path lengths 3.75, 5, 6, 6.5 meters.

3- The spectrometer has a collimator-rotor-collimator system. In order to keep very carefully the intensity at maximum, the following scheme 123 (Rotor-lst collimator-2nd collimator) had been used: v —~ ••* •••-•i- '

Neutron 1 ? | y 1 i | a beam Istcollimator Rotor 2nd collimator

3.1- The rotor (1) had been adjusted to give the maximum counting rate, by using the orientation screws at the base of mechanical part. 3.2- The collimator (2) had been adjusted using the same scheme and the orientation screws at the base of the collimator. 3.3- The collimator (3) had been adjusted using the same scheme as in (2).

126 4- Start triggering signals:

4.1 Two magnetic cells had been designed and built in aluminum disk of the same diameter of the rotor. The carrier aluminum disk (of magnetic cells) had been fixed at the same shaft of the motor and the rotor, with the magnetic cells perpendicular to the slits in the rotor. 4.2- 10 magnetic pick up heads had been tested, to select the best, the selected magnetic pick-up head had been fixed in the proper position, as close as possible to the magnetic cells during its rotation point by point with the rotor, to produce the start signals, that trigger the multichannel time analyzer. 4.3- An adjustable system had been designed to absorb the motor vibration, and keep it stable at the same center line with rotor and the carrier disk of the magnetic cells.

5- The safety of the spectrometer:

5.1- The collimator-rotor-collimator mechanical part of the spectrometer (the hot area) had been surrounded by 1 mm cadmium, slow neutron high absorbing material; about 80 cm high boron paraffin mixture blocks, 5 cm of high boron condensed polyethylene blocks, and 5 mm of lead. 5.2- The path of the neutron beam from the reactor to the hot area of the collimator-rotor-collimator is rounded by 20cm boric acid, 2cm iron, and 5mm lead. 5.3- The flight path had been rounded by 10cm boric acid, 10cm paraffin, 10cm boric acid, and 5mm lead. 5.4- The detector area had been completely rounded by 10cm boric acid, 10cm paraffin, 10cm boric acid and 5mm lead. 5.5- At the end of the spectrometer a beam cature had been used to stop any leaked y-rays and fast neutrons.

6- Supply and counting system :

Since the control system should be far from the reactor hall, the needed supply cables to the spectrometer, and the data cables had been

127 extended for 70 meters from the reactor hall to the control room, through special tunnels.

7- Special technique had been used to get the start triggering pulses before the rotor opens the slits, to create the neutron bursts, with a suitable lagging time. This technique is useful to determine the following: (1) The zero time channel of the measured neutron spectra. (2) The background of the measured neutron spectra, neutron burst by neutron burst. This means that there is no need to get the associated background in a separate measurement, and save the reactor operation time.

RESULTS AND DISCUSSION

1- The spectrometer layout can be seen in Fig. (1).

2- The used shielding materials in building the spectrometer had provided a safe spectrometer. This result has been assured by the official radiation survey around the spectrometer of the Protection Department. The radiation level is lower than the international base level. It means that the spectrometer is completely protected from the other experiments in the reactor hall and vice versa.

3- The reactor neutron beam and the spectrometer: 3.1- The reactor neutron beam spectrum, as a neutron source, has a very wide range, starting from fast neutrons (fission process in the core) and ending by cold neutrons. 3.2- The present spectrometer is a slow neutron spectrometer type. So, there is a problem, how can you use the thermal and cold neutrons as a neutron source to the spectrometer, in the presence of associated y-rays and fast neutrons? 3.3- The spectrometer is connected with the reactor neutron beam in two points. The first point is the rotor, and the second is the detector. The rotor material is a high scatterer to the thermal and cold neutrons, so it prevents the passage of these neutrons, and controls the passage of these

128 3456 789 10 11 12 13

| ro

14 15 16 17 18 19 Fig.(l) Spectrometer Layout

. Inpile shield, 2- Inpile collimator, 3- Lead. 4- Outpile collimator, 5- Lead, 6- Iron , 7- Boric acid, 8- Cadmium, 9- Borated paraffin, 10- Lead, 11- Shielding wall, 12- Detector shield, 13- Detector, 14- fan, 15- Motor, 16-Rotor, 17- Magnetic phono preamplifier, 18- Borated polyethylene blocks, 19- Beam cature neutrons only through its slits. So, the created neutron bursts through the slits of the rotor contain all the reactor spectrum contents, y-rays and fast neutrons, slow and cold neutrons, while the passing spectrum, when the slits are closed, contains only y-rays and fast neutrons, it will be treated as background as follows later.

The second point is the detector, that should be selected very carefully as follows :

1.1- The detector material should have a very high absorption cross section to the thermal and cold neutrons and give interaction products to be registered.

1.2- At the same time it should has a very low absorption cross section to the associated y-rays and fast neutrons and consequently give no interaction products to be detected.

1.3- The detector size must be small as much as possible, not to affect the flight path length of the spectrometer. All three previous conditions have been verefied in the used detector. It is an american type time of flight (TOF) He detector. Now, the measured spectrum represents thermal and cold neutrons, while the y- rays and fast neutrons are not registered by the detector. Fig.( 2 ) represents the measured slow neutron spectrum of the reactor. Fig. ( 3 ) gives the same spectrum with another scale to show the associated background.

4- The transmission function had been treated theoretically and experimentally^ 17\ the results can be shown in Fig.(4).

5- The spectrometer calibration :

The spectrometer had been calibrated by using the accurate value of the beryllium cut-off X = 3.952 A, that corresponds to time of flight value = 3744 fisec.

130 Tenne1ec/Huc1eus Inc. HCS-II Idl Jo Reactor slow neutron spectrum Hax.170498 cts / 4:13:18 pw Juf 29, 1995 Nov 14, 1995 6:53:41 pw HHHHHHHHII ;j|j|||jijjj))lfflj|)^ Acquire: Off Display: 256 ^BliiWpBli^llJ^ Overlap: Off Scale: 256K Group: El Roi No: Noiree Roi: Off Raw: 268K Port: 528 Dae: Say Caic: OB 00 Syinch: Int Zero time channel usec 7368 I ./0-5 ]A -2% Cts: 7548 Passes: 188888 Elapsed: 8 Regaining: 188888 Dwell: 16 usec

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Fig.(2) Reactor slow Eeiitroo spectrum, Angular velocity 1562 r.p.m Tennelec/HucIeus Inc : Io Reactor slow neutron spectrum flax. cts / 4:13 29, 1995

Nov 14, 1995 '. '. • - 6:50:25 PH j Acquire: Off Display: 1824 Overlap: Off Scale: IK broup« m Roi No: NoYie Roi: Off Port: 528 Dae: Saw Calc: On

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Fig.(3) The value opposite to cursor corresponds to the background associated with the spectrum ie Fig.(2) * ©

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133 The results of measuring the transmitted neutron spectrum through 12 cm Be, using TTL and NEG NIM standard pulse can be seen in Fig.(5.1), Fig.(5.2) and Fig.(6.1), Fig.(6.2), respectively, for 240000 neutron burst registration, the results give good agreement between the calculated time of flight value 3744 [isec and the measured time of flight value 3776 jisec that corresponds to the Be cut-off X = 3.952 A.

The first peaks represent the zero time channel, while the second peaks gives the Be cut-off channel. It may be noticed that the two peaks are fixed and do not fluctuate, that represents a good stability for this number of neutrons bursts 240000.

The counts opposite to the cursor position in Fig.(5.3) and Fig.(6.3) represent the background values. The time between the channel number zero and the cursor position in Fig.(5.4) and Fig.(6.4) represent the half duration time of the chopper. The fixed position of the cursor in both figures for 240000 neutron burst represents a good stability for the spectrometer. Using NEG NIM pulses save 1/2 the measuring time with TTL pulses, because in the case of NEG NIM, the minumum pulse-pair resolving time is 5 nsec, while in the case of TTL, the minimum pulse-pair resplving time is 20 nsec.

6- Tliis spectrometer can be used in the following multipositions :

1- In combination with the neutron diffractometer.

2- Using the available curved slit rotor, as Small Angle Scattering (SAS) spectrometer.

3- Using the available special technique as Fixed Scattering Angle (FSA) spectrometer.

4- Using a very good goniometer as a texture spectrometer.

5- Fast neutron themialization research activity.

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Fig.5.1 Spectrometer calibration using TTL pulses (zero time channel)

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135 Id: Be 12cn IRANS . AH25 DL.3 J_JJL 84:51:23 an Sep 85, 1995 Nov 13, 1995 111 3:32:18 pn «•iM»Blil Acquire: off jllMWBM Illii Display: 512 jijl Overlap. Off Scale: 4K Group: El Roi No: None Roi: Off Ran: 268K Port: 528 Dae: Sau 1/N :K .-•• Calc: On '••-M*^ V ' .:-V V*^v.v./-r/--- Synch: Int usec 4688 Cts: 1387 Passes: 248800 Elapsed: 8 Rewaining: 248088 Dwell: 64 usec

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Iennelec/fhicleus Inc. HCS-1I Id: Be 12 cn IRANS. AH25 DL.3 L 94:51:23 an Sep 05, 1995 Nov 13, L99J 5 3:39:24 PR mmmsmMmmmmliiiiiBMBiii^BiiBIII •••• w||||!|!||M mm Acquire: o* * I^^^^^^^^^^^^^^IPPIH Display: 512 •HI nUHliil Overlap. Off Scale: 4K Group: O Roi No: None Roi: Off Ran: 268K Port: 528 Dae: Sau . - \ Calc: On r Synch: Int usec 18944 Cls: 8 Passes: 248688 Elapsed: 8 Reitaining: 240606 Dwell: 64 usec

Filenane: b:lar22fl.spfii F1g.5.4 Spectrometer calibration using TTL pulses (half rotor duration time channel)

136 tennelec/Nucleus Inc. WCS-U Id: Be 12 en Trans. H£C NIH AH25 DL.3 82:35:39 an Sep 96, 1995 Nov 13 6:41:84 pM Acquire: Off Display: 512 Overlap: Off Scale: 8K Croup: El Roi No-' None Zero time channel Roi: Off Ran: 268X Port: 528 Dae: Saw Calc: On Synch: Inl usec 7744 Cts: 3796 Passes: 249608 Elapsed: 0 Rewainlng: 240808 Dwell: 64 usec

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Fig.6.1.Spectrometer calibration using NEG NEW pulses(zero time channel)

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137 Ttnntlffo/Nuoltuf Inc. HCS-H Id: Be 12 CM Trans. NEC HIM AM25 DL.3 12:39:39 AM Sep OS, 1999 Nov 13, 1995 'iBBlllllliM*™™*!!""™"™^ '" 6.'38:87 Acquire: Display: "z BBMBMBM . Overlap: Off Scale: 8K Group.' El Roi No: None Roi: Off Ran: 268K Port: 528 Dae' Saw L '•-• : ''•• Calc: On Synch: hit usec 4608 Cts: 2600 Passes: 240000 Elapsed: 0 Regaining: 240000 Dwell: 64 usec

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Filename: b:lar239.SPfl Fig.6.4.Spectrometer calibration using NEG NIM pulses(half rotor duration time channel)

138 Having finished the set-up and adjustment of the spectrometer, we are starting to do some measurements of neutron cross sections of metals with different crystal grain size and metal hydrides.

ACKNOWLEDGEMENT

I am deeply grateful to Professor Dr.M.K.Fayek, Deputy of the Atomic Reactors Division for Technological Affairs, for sponsoring and encouragement. Without his support, this work could not have been achieved.

139 REFERENCES

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