Characteristics of poly- and mono-crystalline BeO and SiO2 as thermal and cold neutron filters

Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Characteristics of poly- and mono-crystalline BeO and SiO2 as thermal and cold neutron filters M. Adib a, N. Habib a, I.I. Bashter b, H.N. Morcos a, M.S. El-Mesiry a, M.S. Mansy a,⇑ a b

Reactor Physics Department, NRC, Atomic Energy Authority, Cairo, Egypt Physics Department, Faculty of Science, Zagazig University, Egypt

a r t i c l e

i n f o

Article history: Received 4 May 2015 Received in revised form 21 May 2015 Accepted 1 June 2015

Keywords: Neutron cross-sections of SiO2 and BeO Thermal and cold neutron filters Mono and poly crystals

a b s t r a c t A simple model along with a computer code ‘‘HEXA-FILTERS’’ is used to carry out the calculation of the total cross-sections of BeO and SiO2 having poly or mono-crystalline form as a function of neutron wavelength at room (R.T.) and liquid nitrogen (L.N.) temperatures. An overall agreement is indicated between the calculated neutron cross-sections and experimental data. Calculation shows that 25 cm thick of polycrystalline BeO cooled at liquid nitrogen temperature was found to be a good filter for neutron wavelengths longer than 0.46 nm. While, 50 cm of SiO2, with much less transmission, for neutrons with wavelengths longer than 0.85 nm. It was also found that 10 cm of BeO and 15 cm SiO2 thick mono-crystals cut along their (0 0 2) plane, with 0.5° FWHM on mosaic spread and cooled at L.N., are a good thermal neutron filter, with high effect-to-noise ratio. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction The use of long-wavelength neutrons at reactor and pulsed sources often requires a filter for the removal of unwanted epithermal neutrons. For instruments that only require neutrons having wavelength k P 0.41 nm, the polycrystalline beryllium filter provides the best performance and is commonly used. For shorter wavelengths in the thermal neutron range, strong attenuation from Bragg scattering occurs in polycrystalline materials since the Bragg condition may be satisfied over a continuous range of short wavelengths, whereas perfect mono crystals eliminate the coherent Bragg scattering for all but a few narrow wavelength ranges that satisfy the Bragg condition for the particular crystal orientation [1]. Other materials that may be produced economically as low-mosaic-spread (imperfect) mono crystals are routinely used as filters for these thermal neutron applications [2–4]. Mono-crystal filters of sapphire (Al2O3) at room temperature (R.T.) [5,6] or silicon [7,8] and magnesium oxide [9,10] at cryogenic temperatures, are frequently used. Freund [4] has reviewed a variety of mono-crystal filter materials and calculated the total neutron cross-sections from transmission measurements. He uses a simple model to determine the ⇑ Corresponding author. Tel.: +20 1142173078; fax: +202 44620787. E-mail addresses: [email protected], mohamedmansy_np@ yahoo.com (M.S. Mansy). http://dx.doi.org/10.1016/j.nimb.2015.06.001 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

single-phonon, multiple-phonon and absorption cross-sections as functions of the neutron wavelength and fits the data to a general formula with two adjustable parameters. There is a good agreement between the calculated neutron cross-sections and the experimentally determined ones for several materials. Adib et al. [11] have been study the use of beryllium oxide mono crystal as a thermal neutron monochromator. Also Rustad et al. [12] have measured the neutron cross-section of quartz (SiO2) mono crystal with 100 mm thick cut randomly at R.T. and L.N. temperature as a function of wavelength, using the Columbia crystal spectrometer at the Brookhaven graphite reactor. However, in their work they did not study the effect of crystal mosaic spread value upon the disturbing parasitic Bragg dips in the filtered thermal neutron spectrum transmitted through SiO2 mono-crystal due to Bragg reflections from different (h k l) planes. Furthermore, they do not estimate the optimal crystal thickness and mosaic spread of SiO2 mono-crystal to be used as high efficient neutron filter. Therefore, the present work concerns a detailed study of the attenuation of thermal neutron beams through poly and mono-crystalline beryllium and silicon oxides. Moreover, the optimal thickness, orientation, transmission direction and mosaic spread for the efficient transmission of thermal reactor neutrons are also given. A Computer program ‘‘HEXA-FILTERS’’ has been developed in order to provide the required calculations in the neutron wavelength band from 0.001 nm to 1.2 nm.

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104 Table 1 Physical parameters of oxide crystals. Physical property

BeO

SiO2

Structure Lattice constants (nm)

Hexagonal a0 = b0 = 0.2698 c0 = 0.4378 25.011 Be: (1/3, 2/3, 0) (2/3, 1/3, 1/2) O: (1/3, 2/3, z) (2/3, 1/3, 1/2 + z), z = 0.378

Hexagonal a0 = b0 = 0.4913 c0 = 0.5405 60.080 ; u  ; 1=3) Si: (u, 0, 0) (0, u, 2/3) (u O: (x, y, z) (y, x, 2/3-z) , x  y, z + 2/3) ( (y x, y  x, 1/3-z) , z) (y  x,  x, z + 1/3) (x  y, y u = 0.4698, x = 0.4145, y = 0.2663, z = 0.1189 3 0.8903E + 28 Si = 0.4149E14 O = 0.5805E14 10.627 0.171 560 0.0270 5.92

Molecular weight (a.m.u.) Atomic positions

No. of molecules/unit cell No. of unit cells/m3 Coherent scattering lengths (m)

2 2.2257E+28 Be = 0.7790E14 O = 0.5805E14 11.78 0.0076 1470 0.0012 5.24

Total scattering cross-section rbat (barn) Absorption cross-section rabs (barn) Debye temperature HD (K) C1 C2 nm2 eV1

The total cross-section determining the attenuation of neutrons by crystalline material is given by:

different (h k l) planes. In case of polycrystalline material the reflections are from all planes having spacing dhkl P k=2, while in case of mono crystal, reflections are from the (h k l) planes satisfying the Bragg equation:

rtotal ¼ rabs þ rtds þ rBragg

nk ¼ 2dhkl sin hhkl

2. Methodology

where

ð1Þ

rabs is the absorption cross-section due to nuclear capture

processes where rabs ¼ C 1 E1=2 with C 1 a constant which can be calculated from values provided by Sears [13]. rtds is the thermal diffuse or inelastic scattering (TDS), and rBragg corresponds to elastic or Bragg scattering. As shown by Freund [4] the second contribution rtds can be split in two parts, rmph (multi phonon) and rsph (single phonon), depending on the neutron energy. The single phonon scattering cross-section, concerns the energy range E  kBHD, where kB is Boltzmann’s constant and HD is the Debye temperature characteristic of the material. It is determined by the phonon annihilation processes. While rmph is the predominant in the energy range E P kBT having a parameter C 2 which is dependent on the scattering material and given by equation C 2 ¼ 4:27 expðA=61Þ [4]. The contribution of Bragg scattering rBragg to the total attenuation arises from coherent elastic scattering due to reflections from

ð2Þ

where n is the order of reflection, hhkl is the glancing angle to the ðh k lÞ plane. It was shown by Bacon [14] that for a polycrystalline material with grain size less than 104 mm, the total coherent Bragg scattering cross-section can be given as:

N c k2 X 2 F dhkl :e2w 2 d Pk=2 hkl

rcoh ¼

ð3Þ

hkl

where N c is the number of unit cells per cubic centimeter and F 2hkl is the structure factor of the unit cell. In the case of mono-crystal, the Bragg scattering cross-section is given by Naguib and Adib [15]:

0

rBragg ¼

1

B C 1 1 C ln BY Nt0 @ ð1  Phhkl ÞA

ð4Þ

hkl

where N is the atom number density, t 0 is the thickness of the crystal in the beam direction, Phhkl is the reflecting power of the (h k l) plane inclined by the angle hhkl to the incident beam direction and is given by Bacon [14] as:

30

Total neutron cross-section σ (barn)

20

Present w ork 293 K Exp. Fermi et al. [17]

E xp. Z hezherun et al. [18]

101

10 9 8 7 6 5 4

110

Phhkl dh ¼

102

ð5Þ

For the reflection method and

100

Phhkl dh

002

¼ v sinhðAaÞeAð1þaÞ dh

ð6Þ

and

3

A ¼ lt 0 =c0

2

For 1 1E -3

avdh 1 þ a þ ð1 þ 2aÞ1=2 coth½Að1 þ 2aÞ1=2 

0.01

0.1

Neu tron Energy (eV) Fig. 1. Total neutron cross-section of polycrystalline beryllium oxide at 293 K.

transmission

ð7Þ method,

where

Q hkl

a ¼ l WðDÞ

and

dh ¼ dk=2dhkl cos h in which l is the linear absorption coefficient. WðDÞ has a Gaussian distribution with standard deviation g on mosaic blocks of the mono crystal. As shown by Bacon [14], the integrated reflectivity Rh from imperfect crystal of infinite absorption is given by:

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104

1.0 0.9

1.0

(a) BeO at 3 00 K

100

10 cm 20 cm

0.8

0.7

0.7

0.6

0.6

0.5 002

0.4

110

0.3

102

101

110 102

0.4

0.2

0.1

0.1

0.3

101

103

0.3

103

0.2

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

5 cm 15 cm 25 cm

0.0 0.1

1.2

0.2

0.3

0.4

Neutron W avelength (nm)

10 cm 30 cm 50 cm

20 cm 40 cm

0.7

0.8

0.9

1.0

1.1

1.2

(d) SiO 2 at 77 K

0.9

100

0.8

0.7

0.7 1 0 cm 3 0 cm 5 0 cm

100 0.6

Transmission

Transmission

0.6

1.0

(c) SiO 2 at 300 K

0.8

101

0.5 0.4 0.3

0.6

20 cm 40 cm

101

0.5 0.4 0.3

111

0.2

111

0.2

102 110

102

110

0.1

0.1 0.0 0.1

0.5

10 cm 20 cm

Neutron W avelength (nm)

1.0 0.9

(b) BeO at 77 K

002

0.5

0.2

0.0 0.1

100

0.9

Transmission

Transmission

0.8

5 cm 15 cm 25 cm

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Neutr on W avelength (nm)

Neutron Wavelength (nm)

Fig. 2. Neutron transmission through polycrystalline BeO and SiO2.

10 18000

9

Incident Cold Neutron Flux Transmission through 25 cm BeO Transmission through 50 cm SiO2

Neutron Flux (n.cm-2 .s-1)

100

12000

9000

6000

7 6 5 4 3 2

3000 100 002

0 0.1

Present Work at 86 K Exp. at 8 6 K

8

Total cross-section σ (barn)

15000

Present Wo rk at 293 K Exp. at 2 93 K

0.2

0.3

0.4

1

101

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Neutron W avelength (nm)

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Neutron Wavelength (nm) Fig. 3. Cold neutron transmission through 25 and 50 cm of polycrystalline BeO and SiO2 respectively at moderating temperature (L.N. = 77 K).

Fig. 4. Total neutron cross-section of SiO2 mono-crystal at (R.T.) and (L.N.).

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104 1 .0

chkl ¼ dc dhkl

0 .9

þ pffiffi32la c

0 0

0 .8

nh

h

i

4 3a20

fhhc þ kkc þ ðhkc þ khc Þ=2g þ llc =c20 cos w i o M c hhc þ kkc þ ðkhc þ hkc Þ=2  M1c sin w 

ð10Þ

Transmission

0 .7

where

0 .6

1 1 Mc ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; dc ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 l2c 4 hc þ kc þ hc kc 2 ðhc þ kc þ hc kc Þ þ 2

0 .5 0 .4

3a0

c0

0 .3

While the inclination angle ahkl can be given by:

0 .2

P r e se n t w o rk T r an s m i ssio n th r o u g h 1 0 cm S iO 2 at 8 6 K E x p . T r a n sm iss io n t h r o u g h 1 0 c m Si O 2 a t 8 6 K

0 .1 0 .0 0 .0

0.1

0.2

0 .3

0 .4

0.5

0.6

0.7

0 .8

0 .9

1.0

cos ahkl ¼ dhkl dc 1.1

1.2

N e utr o n W a ve le n gth (n m ) Fig. 5. Neutron transmission through 10 cm perfect SiO2 mono-crystal at (L.N.).

Rh ¼

Z

þ1

1

Phhkl dh

ð8Þ

The integrated reflectivity from imperfect crystal of infinite absorption reaches saturation for bulk crystal thickness of t0 . Such behavior is due to extinction effects inside the thickness t 0 of the crystal [14]. Therefore, when mono crystal is used as a neutron filter in transmission geometry, to take into account the extinction effects within the crystal thickness t 0 Eq. (8) is calculated for a thickness ts , where ts  to and Eq. (7) is multiplied by a factor v ¼ t 0 =t s . As shown by Naguib and Adib [16], the reflecting power P hhkl for any imperfect mono crystal depends upon the direction cosine of the diffracted beam chkl and the inclination of the (h k l) plane to the crystal surface ahkl . Let a large mono crystal having a Hexagonal Close Packed (H.C.P) structure with lattice constants a0 , c0 and cut along the plane with Miller indices ðhc kc lc Þ is fixed on a goniometry table such that its surface is parallel to XY plane. By simple plane transformation, the equation describing the cutting plane ðhc kc lc Þ can be given as:

Z=dc ¼ 1

ð9Þ

Assuming also, the angle between the neutron beam direction and the direction ½hc kc lc  is w, then c0 ¼ cos w, and the direction cosine of the diffracted beam chkl from any (h k l) plane can be expressed as: 1.0 012 110

0.9 201 0.8

103

S iO 2

203

0.7

Transmission

111 0.6

011

0.5 0.4

BeO 0.3 103 203 112

0.2 0.1 0.0 1E-3

0.01

0.1

1

Neutron Energy (eV) Fig. 6. Neutron transmission through BeO and SiO2 (0 0 2) plane.

10



    hhc 4 h hc llc þ þ þ k þ k c 2 a20 c20 3a20 2

ð11Þ

For cutting plane is ð0 0 lc Þ, chkl is given by

chkl ¼ dhkl



 

 1 2 h sin w cos w þ pffiffiffi þk c0 3a0 2

ð12Þ

A software computer code ‘‘HEXA-FILTERS’’, was developed in order to calculate the required calculations. The code is an adapted version of the ISCANF [15] and ISCANF-I [16] codes. 3. Results and discussions In order to check the applicability of the computer code and the used model, the calculations were carried out for poly and mono-crystalline BeO and SiO2 crystals and compared with the experimental ones. The main physical parameters used for calculation are listed in Table 1. 3.1. Polycrystalline BeO and SiO2 Experimental total neutron cross-section values for polycrystalline beryllium oxide reported by Fermi et al. [17] and Zhezherun et al. [18] are displayed in Fig. 1. For comparison the calculated total neutron cross-section of polycrystalline beryllium oxide with fitting parameter C2 = 5.24 (nm2 eV1) was calculated in the energy range from 0.1 meV up to 0.1 eV. The result of calculation was also displayed in Fig. 1 as dashed line. Fig. 1 shows an overall agreement between the calculated and experimental data. Such agreement justifies the applicability of the used model along with the adapted computer code. Since SiO2 have the same structure as BeO therefore the same computer code with the parameters given in Table 1 is used for its cross-section calculation. Neutron transmission of the polycrystalline beryllium and silicon oxides were calculated as a function of thickness and at both room (R.T. = 300 K) and liquid nitrogen (L.N. = 77 K) temperatures in the energy range from 1 meV up to 10 eV. The result of calculation is displayed in Fig. 2. It seems that 25 cm thick polycrystalline BeO cooled at (L.N.) temperature has a better effect-to-background ratio for neutrons with wavelengths longer than 0.46 nm. While a 50 cm thick for SiO2 for wavelengths longer than 0.85 nm. The calculated cold neutron flux having Maxwellian distribution with neutron gas temperature close to liquid hydrogen (T  20 K) incident on a 25 cm thick polycrystalline BeO and 50 cm SiO2 cooled at (L.N.) before and after their transmission is displayed in Fig. 3. Fig. 3 shows that 25 cm of BeO transmits only about 3.4% of neutrons of wavelengths within the range of (0.43–0.47) nm where, the reflections from plane (0 0 2) is forbidden while (1 0 0) is allowed. However, it transmits about 81.2% for neutron with wavelengths longer 0.46 nm since the reflection from (1 0 0) plane as the longest inter-planar spacing is also forbidden. The figure also demonstrates that 50 cm of SiO2 transmits about 8.1% for neutron

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104

1.0 0.9

1.0

η=0.05°

(a)

0.9

0.8

0.8 0.7

0.6

0.6

Transmission

Transmission

SiO2 0.7

0.5 0.4

BeO

SiO2

0.5 BeO

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

η=0.1°

(b)

0.0

1E-3

0.01

0.1

1

10

1E-3

Neutron Energy (eV)

0.9

η=1.0°

(d) SiO2

0.7

SiO2

Transmission

Transmission

10

0.8

0.6 0.5 0.4 0.3

1

1.0

η=0.5°

(c)

0.8 0.7

0.1

Neutron Energy (eV)

1.0 0.9

0.01

BeO

0.6 0.5 0.4 0.3

0.2

0.2

0.1

0.1

BeO

0.0

0.0 1E-3

0.01

0.1

1

10

Neutron Energy (eV)

1E-3

0.01

0.1

1

10

Neutron Energy (eV)

Fig. 7. Neutron transmission through BeO and SiO2 at different mosaic spread values.

with wavelengths range from (0.66 nm to 0.85) nm where, while transmits about 30.4% of the incident neutrons wavelengths with longer than 0.85 nm. 3.2. Mono-crystal BeO and SiO2 The neutron cross-section and transmission data reported by Rustad et al. [12] were measured using the Columbia crystal spectrometer at the Brookhaven graphite reactor. The SiO2 samples were selected at random and cut to the shape of rectangular blocks measuring (5  6.6  1.9) cm without regard to crystal orientation. The total neutron cross-section and transmission values reported by Rustad et al. [12] at (R.T.) and (L.N.) temperatures are displayed as circles and triangles in Figs. 4 and 5 respectively. The calculated values are also displayed in as solid and dashed lines assuming that SiO2 mono-crystal is nearly perfect and the neutrons incident perpendicular to the (1 1 1) plane. One can notice that the calculated values are in good agreement with the experimental ones. The relative error was calculated and displayed in the figure for most values the error bars do not exceed the symbol size. As shown by Adib et al. [19] when mono crystal is used as thermal neutron filter the favorable cutting plane is that having the longest inter-planer distances (dhkl ). For beryllium and silicon oxides (0 0 2) plane satisfy these requirements. To show the contribution of parasitic Bragg reflections on neutron transmitted through a

10 cm of beryllium and silicon oxide mono crystals cut along (0 0 2) planes assuming the same FWHM on mosaic spread (g = 0.5°) at room temperature at take-off angle (w = 0°). The calculated neutron transmission is displayed in Fig. 6. Fig. 6. give indication that beryllium oxide with thickness 10 cm, mosaic spread g = 0.5° and cut along its (0 0 2) plane is better than silicon oxide mono crystal as a thermal neutron filter since no Bragg reflections occur for En < 19 meV but for SiO2 En < 5 meV. To overcome the Bragg reflections occurring at higher energies an optimum choice of crystal mosaic spread is essential. Neutron transmission through 10 cm beryllium and silicon oxide mono crystals cut along (0 0 2) planes for different FWHM on mosaic spread at take-off angle (w = 0°) were calculated and displayed in Fig. 7. As observed from Fig. 7 for FWHM in mosaic spread >0.5° parasitic Bragg reflections could limit the use of beryllium and silicon oxide mono crystals as a thermal neutron filters. To find the optimum mono crystal thickness, the neutron transmission through different crystal thickness, were calculated at (R.T.) and (L.N.) temperatures. Fig. 8a and b shows the result of calculation through beryllium oxide (0 0 2) mono crystal having FWHM on mosaic spread of 0.5° at (R.T.) and (L.N.) temperatures respectively. While Fig. 8c and d for silicon oxide at the same conditions. From Fig. 8a and b it would appear that a 7.5 cm thick beryllium oxide cooled at (L.N.) is

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104

1.0

1.0

(a) BeO at 300 K

(b) BeO at 77 K

0.9

0.8

0.8

0.7

0.7

0.6

0.6

Transmission

Transmission

0.9

0.5 0.4

5 cm 7.5 cm 10 cm

0.3

0.5 0.4 5 cm 7.5 cm 10 cm

0.3

0.2

0.2

0.1

0.1

0.0

0.0

1E-3

0.01

0.1

1

10

1E-3

0.01

Neutron Energy (eV) 1.0

1

10

1.0

(c) SiO 2 at 300 K

0.9 0.8

0.8

0.7

0.7

0.6

5 cm 7.5 cm 10 cm 15 cm

0.5 0.4

(d) SiO2 at 77 K

0.9

Transmission

Transmission

0.1

Neutron Energy (eV)

0.6 0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

5 cm 7.5 cm 10 cm 15 cm

0.0

1E-3

0.01

0.1

1

10

1E-3

Neutron Energy (eV)

0.01

0.1

1

10

Neutron Energy (eV)

Fig. 8. Neutron transmission through BeO and SiO2 at different thickness and moderator temperature.

sufficient for removing neutrons with energies >1 eV Tn < 4%) while providing high transmission (Tn > 93%) for neutrons with energies <0.025 eV. Fig. 8c and d demonstrates that silicon oxide mono crystal with thickness 15 cm and cooled to (L.N.) is capable of removing neutron with energies >0.2 eV (Tn < 4%) but transmits (Tn > 81%) for neutrons with energies <0.025 eV. To show the filtering features of beryllium and silicon oxide mono crystals, the incident neutron flux having Maxwellian distribution, with neutron gas temperature close to 300 K along with transmission through beryllium and silicon oxide mono crystals at moderating temperatures of 300 K and 77 K, were calculated and displayed in Fig. 9a and b respectively. From Fig. 9a one can observe that, a 7.5 cm thick for BeO (0 0 2) having FWHM on mosaic spread of >0.1° cooled to 77 K, can be successfully used to transmit a thermal neutron flux, while significantly rejecting the accompanying slowing down flux (dE/E), with neutron energies En > 1 eV. While Fig. 9b shows that a 15 cm silicon oxide cut along (0 0 2) plane having FWHM on mosaic spread of >0.5° cooled to 77 K, more effective in attenuating of epithermal neutrons and the accompanying c-rays with average energies (Ec = 2.0 MeV) but with less transmission of thermal neutrons.

4. Conclusion Optimum filtering characteristics have been determined for poly and mono-crystalline beryllium and silicon oxides. A simple model along with a computer code ‘‘HEXA-FILTERS’’ which is an adapted version of the ‘‘ISCANF’’ code was used to carry out the calculations. A reasonable agreement was obtained between calculated neutron cross-sections and measured ones for both poly and mono-crystalline materials which supports the applicability of the adapted code. Calculation shows that a 25 cm polycrystalline beryllium oxide cooled at liquid nitrogen is sufficient of almost removing epithermal neutron and transmits <1% of fast ones and <5% of c-rays with average energy (Ec = 2.0 MeV) accompanying the neutron flux, while providing reasonable intensity of thermal neutrons (81.7%). The same features were seen for 50 cm thick of polycrystalline SiO2, however with less intensity. Such transmission behavior supports the application of BeO and SiO2 polycrystalline as a cold neutron filter. It was also shown that, 7.5 cm thick beryllium oxide mono crystal cut along its (0 0 2) plane having mosaic spread of 0.5° and cooled to 77 K, can be successfully used as thermal neutron filter

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M. Adib et al. / Nuclear Instruments and Methods in Physics Research B 358 (2015) 98–104 106

η= 0 .05 °

( a ) B e O ( 0 0 2)

-2

-1

Neutron Flux (n.cm .s )

105

10

4

103

102

101

100

In ciden t Th erm al Neu tro n F l ux T ran sm i ss io n t hro ug h 7 .5 cm B eO at 30 0 K T ran sm i ss io n t hro ug h 7 .5 cm B eO at 77 K

1 0 -3

1 0 -2

1 0 -1

100

101

N e u tr on E n e rg y ( e V )

10

6

10

5

η = 0 .0 5 °

-2 -1

Neutron Flux (n.cm s )

(b ) S iO 2 ( 0 0 2 )

104

10

3

102

10

1

Incid ent Th erm al N eut ron F lu x Trans m is si on th rou gh 15 c m S iO 2 a t 3 00 K Trans m is si on th rou gh 15 c m S iO 2 a t 7 7 K

100 10

-3

10

-2

10

-1

10

0

10

1

N e u tr on E n e rg y ( e V )

Fig. 9. Thermal neutron flux per unit wave-length interval transmitted through BeO and SiO2 cut along (0 0 2) at different moderating temperatures.

with low background from the accompanying epithermal neutrons. Almost the same features were seen with SiO2 but with less transmission of thermal neutrons. References [1] J. Barker, D. Mildner, J. Rodriguez, P. Thiyagarajan, Neutron transmission of single-crystal magnesium fluoride, J. Appl. Crystallogr. 41 (6) (2008) 1003– 1008, http://dx.doi.org/10.1107/S0021889808032858.

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