EURADOS Working Group 6 Exercise on Unfolding ...

WG6: Computational Dosimetry European Radiation Dosimetry Group e.V.

Exercise on neutron spectrum unfolding in Bonner sphere spectrometry J.M. Gómez-Ros1, R. Bedogni2, C. Domingo3, J.S. Eakins4, N. Roberts5 and R.J. Tanner4 1

CIEMAT, E36.P2.02, Av. Complutense, E-28040, Madrid, Spain

2

INFN – LNF Laboratori Nazionali di Frascati, Via E. Fermi n. 40 – 00044 Frascati (Rome) Italy 3

UAB, Grup de Recerca en Radiacions Ionitzants, Departament de Física, Edifici C, Campus UAB, E-08193 Bellaterra (Spain)

4

Public Health England, CRCE, Chilton, Didcot, Oxon OX11 0RQ, United Kingdom

5

NPL, Neutron Metrology Group, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom

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Content: Exercise on neutron spectrum unfolding in Bonner sphere spectrometry............................. i Content: ...............................................................................................................................iii 1.

INTRODUCTION ............................................................................................................ 1

2.

THE BONNER SPHERE SPECTROMETER .................................................................. 2

3.

PROPOSED PROBLEMS .............................................................................................. 3 PROBLEM 1: Medical accelerator ..................................................................................... 3 PROBLEM 2: A simulated workplace field ......................................................................... 5 PROBLEM 3: An irradiation room with a radionuclide source ............................................ 7 PROBLEM 4: Skyshine scenario ....................................................................................... 8

4.

INSTRUCTIONS FOR PRESENTING THE RESULTS................................................... 9

5.

Miscellaneous information .........................................................................................10 WORKSHOP ............................................................................................................................. 10 PUBLICATIONS ........................................................................................................................ 10 EXERCISE PROPOSED BY: ....................................................................................................... 10 CORRESPONDENCE: ............................................................................................................... 10 RESULTS SHOULD BE RETURNED TO:.................................................................................... 10

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Exercise on neutron spectra unfolding in Bonner sphere spectrometry

1. INTRODUCTION The Bonner sphere spectrometer (BSS) has been well established in neutron spectrometry since the 1960s, mainly because of its nearly isotropic response and wide energy range (from thermal up to 1 several hundred MeV) . A BSS consists of a neutron detector sensitive to thermal neutrons placed in the centre of moderating spheres of different sizes made of polyethylene. The combination of the thermal sensor and the moderating spheres has sensitivity to neutrons over a broad energy range so the neutron spectrum can be inferred from the readings of a set of spheres, by means of an unfolding procedure. The main purpose of this exercise is to survey the usage of unfolding methods by individual participants by comparing the results obtained from realistic unfolding exercises, since the user plays a crucial role in the quality of the results. Four different workplaces with neutron fields with energies below 20 MeV have been considered: an irradiation room with an iron moderated radionuclide source a medical accelerator within the treatment room a skyshine scenario an irradiation room with a water moderated radionuclide source The exercise is open to all the participants working on Bonner sphere spectrometry that would be interested in gaining knowledge and experience on the topic of unfolding. It is also open to those with an interest in unfolding methods but who may not have applied them to Bonner sphere systems. Participants may attempt as many of the problems as they wish and are welcome to submit solutions using different unfolding methods.

It is planned to publish the exercise results in a peer-reviewed journal, including all the participants as co-authors. One paper per problem is envisaged. In addition, a EURADOS report with detailed summaries of the results is foreseen.

RESULTS SHOULD BE RETURNED by e-mail to Carles Domingo ([email protected]) before 30th April 2017

1 D.J. Thomas and A.V. Alevra. Bonner sphere spectrometers – a critical review. Nucl. Instrum. Meth. A. 476 (2002) 12-20.

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J.-M. Gomez-Ros, R. Bedogni, C. Domingo, J. S. Eakins, N. Roberts and R.J. Tanner

2. THE BONNER SPHERE SPECTROMETER The Bonner spheres considered in this exercise are an idealized set based on perfect spherical geometry: The detector is pure 3He with a 0.5 mm thick steel case. The external diameter of the detector is 33 mm and the internal diameter 32 mm. A set of 13 spheres has been used: bare, 2˝, 2˝ + 1 mm Cd, 3˝, 3.5˝, 4˝, 4.5˝, 5˝, 6˝, 7˝, 8˝, 10˝ and 12˝ (diameter given in inches where 1˝ = 2.54 cm). The moderator is pure CH2, with a density of 0.947 g cm-3. Thermal scattering for polyethylene at room temperature has been applied in all calculations. The response calculations used plane parallel beams with the same radius as the moderator/detector: the code MCNPX 2.6 was used. The organizers are aware that active detectors are not appropriate for use in pulsed fields, such as that in the medical linac problem, but do not consider that this impacts on the nature of the unfolding problem. The fluence response matrix is shown in Figure 1.

Figure 1: The response matrix of idealized Bonner sphere spectrometer

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3. PROPOSED PROBLEMS PROBLEM 1: Medical accelerator A medical accelerator GE Saturne 43, 25 MV, is situated in the centre of a 7×7×3 m3 room, concrete walls. The accelerator head is mounted vertically with the top of the target located 2 m above the floor. The field size is 10×10 cm2. A 40×40×40 cm3 water phantom with PMMA walls, 1.5 cm thick, is located in the beam with the upper surface at 90 cm from the top of the target.

Figure 2. Sketch of the LINAC room, indicating the position of the accelerator and the two measurement points.

Two measurement points have been considered (see Figure 2): one at the entrance of the maze (point 1) and the other at 1 m from the isocentre (point 2). The uncertainty for the measurements made in both points is 2%.

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Table 1. The BSS counts normalized to absorbed dose to water of 1 Gy at the isocentre, are given below. Point 1

Point 2

Counts (x103Gy-1)

Counts ( x103Gy-1)

Bare

8734

16716

2˝

7280

14534

2˝ + 1 mm Cd

1481

3771

3˝

9224

23471

3.5˝

10099

30408

4˝

10822

33755

4.5˝

11186

39586

5˝

10533

41044

6˝

9240

43241

7˝

7702

38637

8˝

5888

32222

10˝

3397

19908

12˝

1795

11822

Sphere label

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PROBLEM 2: A simulated workplace field The problem uses a metrology laboratory room with a moderated radionuclide source to simulate a workplace field. The source consists of an ISO 241Am-Be source suspended in a stainless steel tube, which is clad by a lead shield. The steel tube extends from the floor to the ceiling of a 2.5 × 5.0 × 7.75 m3 laboratory, which has wooden panels covering all surfaces. Neutron absorbing material is placed behind the panels. The measurement point is fixed at the same height as the source (1.25 m), at 1 m distance from it. Adjacent to the steel tube, and in-line with the measurement point, is a water-filled container of ~50 cm depth that moderates the neutrons. The container is sufficiently wide to cover the solid-angle subtended at the source by the largest Bonner sphere, and is supported by a 2 cm thick wooden table at a height that gives equal coverage in the vertical direction. The uncertainty of the measurements is 4%.

Figure 3. Schematic of the water moderated simulated workplace field.

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Table 2. The activity of the source is 10 MBq. The count rates measured with the BSS are given below. Sphere label

Count rate (s-1)

Bare

28.4

2˝

26.3

2˝ + 1 mm Cd

4.92

3˝

29.1

3.5˝

32.0

4˝

34.1

4.5˝

34.3

5˝

31.3

6˝

28.2

7˝

22.7

8˝

19.8

10˝

13.1

12˝

6.87

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PROBLEM 3: An irradiation room with a radionuclide source The source consists of an ISO 241Am-Be source in the centre of an iron sphere (r = 10 cm) and is located in the centre of a 8x8x8 m3 irradiation room with 50 cm thick concrete walls, floor and ceiling. The measurement point is fixed at the same height as the source, at 4 m distance from the source along one diagonal of the room’s horizontal plane. The uncertainty of the measurements is 2%. Table 3. The activity of the source is 3.7×1010 Bq. The count rates measured with the BSS are given below. Sphere label

Count rate (s-1)

Bare

7.13

2˝

6.28

2˝ + 1 mm Cd

1.51

3˝

8.21

3.5˝

9.38

4˝

10.18

4.5˝

10.78

5˝

11.13

6˝

10.11

7˝

9.12

8˝

7.75

10˝

4.82

12˝

2.77

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PROBLEM 4: Skyshine scenario This exercise attempts to simulate the situation of an environmental measurement at 100 m from a nuclear plant. The plant is a cylindrical room of height 10 m and radius 10m. The walls of the plant are made from concrete and the roof is made from thin concrete tiles. A source of (α, n) neutrons is located in the middle of the building; the total activity of the source is unknown. The walls provide strong attenuation of the direct field so the main component of the field at 100 m is from air scattered neutrons. The Bonner spheres were placed with their centres 1.5 m above the ground. The uncertainty of the measurements is 5%.

Table 4. The count rates measured with the BSS are given below. Sphere label

Count rate (h-1)

Bare

418

2˝

605

2˝ + 1 mm Cd

228

3˝

794

3.5˝

949

4˝

1006

4.5˝

1186

5˝

1084

6˝

839

7˝

950

8˝

761

10˝

472

12˝

270

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4. INSTRUCTIONS FOR PRESENTING THE RESULTS A participant can present results for one or more problems. Should the participant decide to present solutions obtained with multiple codes, one separate submission per code is required. The following quantities and information should be provided by the participant: Total neutron fluence normalized as as appropriate for each problem with its associated uncertainty (one sigma). Ambient dose equivalent normalized as as appropriate for each problem with its associated uncertainty (one sigma). The ICRP74/ICRU57 conversion coefficients should be used. Percentages of neutron fluence in the following energy intervals: • E