What can we do to get better
neutron dose equivalent
measurements?
David Thomas
NPL Workshop 26th October 2006
What are we trying to do?
Determine effective dose
Effective dose, E, is the
weighted sum of various
organ doses, HT
E = ∑wT ⋅ HT
T
Oesophagus
Thyroid
Lung
Breast
Bone marrow
Colon
Liver
Stomach
Bladder
Gonads
Skin
Bone surface
Remainder
2
0.05
0.05
0.12
0.05
0.12
0.12
0.05
0.12
0.05
0.20
0.01
0.01
0.05
NPL Workshop 26th October 2006
2
Conversion coefficient (pSv cm )
Effective dose
conversion coefficients
E(A-P)
E(P-A)
E(ROT)
E(RLAT)
E(ISO)
100
10
1
10
-9
ICRP74
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
Neutron energy (MeV)
3
NPL Workshop 26th October 2006
Survey instruments Ambient
dose equivalent
The ambient dose equivalent, H*(d), is the dose equivalent that would be
produced by the corresponding expanded and aligned field, in the ICRU
sphere at a depth of d mm on the radius opposing the direction of the
aligned field.
Expanded and
aligned field
•
4
ICRU
sphere
NPL Workshop 26th October 2006
Comparison of E and H*(10)
2
Conversion coefficient (pSv cm )
Conversion factors from ICRP Publication 74 or ICRU Report 57
for effective dose and ambient dose equivalent
E (A-P)
E (ROT)
H*(10)
100
10
1
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
Neutron energy (MeV)
File ICRP74.opj
5
NPL Workshop 26th October 2006
Typical area survey
instrument response
functions
H*(10) response
Under- and over-read for samples of area survey meters
10
1
10
0
Thermal
Harwell 0949 (Leake detector)
Bertold LB6411
10
-1
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Neutron energy (eV)
6
4
10
5
10
6
10
7
Dosemeters.opj
NPL Workshop 26th October 2006
Implications of area survey
instrument response
functions 1
10
Harwell 0949 (Leake detector)
Berthold LB6411
3.5
3.0
2.5
1
2.0
Dose equivalent spectrum
H*(10) near a transport flask
Clab D
1.5
1.0
Dose equivalent response
Ambient dose equivalent H*(10)
4.0
0.5
0.0
0.1
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Neutron energy (eV)
7
4
10
5
10
6
10
7
10
NPL Workshop 26th October 2006
Implications of area survey
instrument response
functions 2
Leake counter
2.0
Field types
Gas cooled reactor
PWR
Simulated workplace
Fuel cycle
Transport flasks
MOx fabrication
Source fabrication
Calibration
1.8
Reading/H*(10)
1.6
1.4
1.2
1.0
0.8
0.6
10
100
2
H*(10)/Φ (pSv cm )
8
NPL Workshop 26th October 2006
Personal dose equivalent
The personal dose equivalent, Hp(d), is the dose equivalent in soft tissue, at a
depth of d mm, below a specified point on the body.
•
Body or
slab
phantom
Note : in ICRU Report 47, the ICRU has considered the definition of the personal dose
equivalent to include the dose equivalent at a depth, d, in a phantom having the composition of
ICRU tissue. Then, Hp(10), for the calibration of personal dosemeters, is the dose equivalent at
a depth of 10 mm in a phantom composed of ICRU tissue, but of the size and shape of the
phantom used for calibration (a 30cm x 30cm x 15cm parallelepiped), and the conversion
coefficients, hp,slab(10), are calculated for this configuration.
9
NPL Workshop 26th October 2006
Personal dose equivalent
conversion coefficients
Conversion coefficients, from ICRP Publication 74
or ICRU Report 57, for personal dose equivalent
2
Conversion coefficient (pSv cm )
1000
o
hp(10,0 )
o
100
hp(10,15 )
o
hp(10,45 )
o
hp(10,75 )
10
1
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
Neutron energy (MeV)
10
-1
10
0
10
1
10
File ICRP74.opj
NPL Workshop 26th October 2006
Personal dose equivalent
conversion coefficients and E
Conversion coefficients, from ICRP Publication 74
or ICRU Report 57, for personal dose equivalent
o
hp(10,0 ) and for E(A-P)
2
Conversion coefficient (pSv cm )
1000
100
E(A-P)
o
hp(10,0 )
10
1
-9
10
ICRP74
-8
10
-7
10
-6
10
-5
-4
10
10
-3
10
-2
10
-1
10
0
10
1
10
Neutron energy (MeV)
11
NPL Workshop 26th October 2006
Neutron personal dose equivalent response
100
Hp,m(10)/Hp,c(10)
10
1
0,1
0,01
-8
10
0° ALOKA PDM-313
60° ALOKA PDM-313
10
-3
10
-1
10
1
En / MeV
12
NPL Workshop 26th October 2006
Neutron personal dose equivalent response
100
0° Saphydose-n
Hp,m(10)/Hp,c(10)
10
1
0,1
0,01
-8
10
10
-3
10
-1
En / MeV
13
10
1
NPL Workshop 26th October 2006
Neutron personal dose equivalent response
14
NPL Workshop 26th October 2006
Requirements
• Several approaches to improving neutron dosimetry, e.g.
¾ Improved survey instruments and dosemeters
¾ Improved knowledge of workplace spectra
¾ Improved knowledge of device response functions
¾ Education of the practitioners – explaining the problems
• What is the most important?
15
NPL Workshop 26th October 2006
End
16
NPL Workshop 26th October 2006
0.8
10
Ambient dose equivalent H*(10)
0.7
0.6
0.5
0.4
1
0.3
Dose equivalent spectrum
H*(10) near a transport flask
Clab D
241
Am-Be
0.2
Dose equivalent response
Harwell 0949 (Leake detector)
Berthold LB6411
0.1
0.0
0.1
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Neutron energy (eV)
17
4
10
5
10
6
10
7
10
NPL Workshop 26th October 2006
Neutron spectra
0.55
Neutron fluence and dose equivalent spectra outside containment
for a typical gas-cooled reactor
252
compared to a Cf dose equivalent spectrum
E.Φ(E) or H*(10) normalised to unity
0.50
0.45
0.40
252
Cf
0.35
0.30
0.25
0.20
Fluence
H*(10)
0.15
0.10
0.05
0.00
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Neutron energy (eV)
18
4
10
5
10
6
10
7
10
NPL Workshop 26th October 2006
Personal dosemeters for
neutrons
•
In general there are two mechanisms commonly used
to detect neutrons in personal dosemeters – both
active and passive:
¾
Detection of recoil protons from a hydrogenous
layer,
¾
Detection of albedo neutrons from the body
(albedo dosemeters).
19
NPL Workshop 26th October 2006
Proton recoil devices
Proton recoil approach - used in etch track
dosemeters, electronic dosemeters, etc.
n
•
Converter
layer
p
Detector
By simply counting the number of recoil protons from a
hydrogenous layer get a reasonable estimate of personal
dose equivalent for fast (> 50 keV) neutrons
20
NPL Workshop 26th October 2006
Albedo devices
Li(n,T) capture per source neutron
-4
6
Albedo devices consist of 2 or more thermal
neutron detectors with thermal neutron
absorbing material shielding the detectors
from the direct neutron field or from the
backscattered neutrons.
A careful choice of the combination of the
signals from the detectors gives a dose
equivalent response up to 1 to 10 keV.
1.6x10
6
Element 1 TLD + Li - shielded from body
6
Element 3 TLD + Li - shielded from direct
-4
1.2x10
-5
8.0x10
-5
4.0x10
0.0
Combined response for:
1 x TLD element 1 and 2 X TLD element 3
-5
Body
Hp(10) response
Thermal
shield
1.6x10
albedo
neutrons
Direct neutrons
-5
1.2x10
-6
8.0x10
-6
4.0x10
0.0
-8
10
Thermal
neutron
detector
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
Neutron energy (MeV)
21
NPL Workshop 26th October 2006