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The angular dependence of an Si energy deposition spectrometer response at several radiation sources
Radiation Measurements 39 (2005) 323 – 327 www.elsevier.com/locate/radmeas
The angular dependence of an Si energy deposition spectrometer response at several radiation sources夡 František Spurnýa,∗ , François Trompierb , Jean-François Bottollier-Depoisb a Department of Radiation Dosimetry, Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Na Truhláˇrce 39/64, Prague
18086 Praha 8, Czech Republic b External Dosimetry Department, Institute for Radiation Protection and Nuclear Safety (IRSN), Fontenay-aux-Roses, France
Received 10 February 2004; received in revised form 18 June 2004; accepted 20 June 2004
Abstract An MDU–Liulin spectrometer based on an Si-diode was mainly used during the last few years with the goal to use them for measurements onboard aircraft. To investigate its ability to obtain such measurements, the detector was tested in some radiation reference fields, like 60 Co and other photon beams, neutrons of an AmBe and 252 Cf sources and in high-energy radiation fields at CERN. Due to the high geometrical asymmetry of the Si-diode semiconductor, an angular dependence of the response would be expected. This work presents analyses and discusses the results of angular dependence studies obtained at the different radiation sources mentioned. It was found that these angular dependences vary with the type and energy of radiation. The influence of these variations on the use as a dosimeter onboard aircraft is also studied and discussed. © 2004 Elsevier Ltd. All rights reserved. Keywords: Si-semiconductor spectrometer; Angular response; Aircrew dosimetry
1. Introduction ICRP 60 recommendations (ICRP, 1991) suggest to consider aircrew members as occupationally exposed persons. The radiation fields on aircraft board are complex; they contain particles with energies up to few hundred MeV (EURADOS, 1996). Obviously, one distinguishes the component with low linear energy transfer (LET) composed mostly of electrons, high-energy protons, mesons and 夡 The studies were partially supported through EC Project FIGMCT00-0068 and the Project of the Grant Agency of Czech Republic No. 202/01/0710. ∗ Corresponding author. Tel.: +420-283-841-772; fax: +420-283842-788. E-mail address: [email protected] (F. Spurný).
1350-4487/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.06.009
photons, and the high LET component represented mostly by neutrons; few high-energy heavier charged particles are still present. There are few types of equipment available to characterize such a field. The spectrometer MDU–Liulin with an Si-diode, originally developed for radiation measurements onboard cosmic vehicles was also proposed for measurements onboard aircraft (Dachev and Spurny, 2002; Spurny and Dachev, 2002, 2003). To appreciate its possibilities for that purpose, four Liulin spectrometers were characterized in photon, neutron and high-energy radiation reference fields (Spurny et al., 2003; Spurny and Datchev, 2002; Spurný, 2004). Due to geometrical asymmetry of the Si-diode, an angular dependence of the response would be expected. This work presents, analyzes and discusses the results of angular dependence studies obtained at the different radiation sources mentioned. It was found that these
324
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327
angular dependences vary with the type and energy of radiation. The influence of these variations on the use as a dosimeter onboard aircraft is discussed. 2. Experimental 2.1. MDU–Liulin energy deposition spectrometer The main purpose of MDU–Liulin energy deposition spectrometer (Dachev and Spurny, 2002; Spurný, 2004) is to monitor simultaneously the doses and numbers of energy deposition events in an Si-diode. It consists of the detector itself, a charge-sensitive preamplifier, two microcontrollers, a flash memory and Li-ion cells. Pulse analysis technique is used to measure the deposited energy in the detector. The amplitude of the pulses is proportional by a factor of 240 mV/MeV to the energy loss in the detector. Adjustment of the energy scale is made through the 60 keV photons of 241Am. The amplitudes are digitized and organized in a 256-channel spectrum. The dose D (Gy) is calculated from the spectrum as D = K · (Ei Ai )/MD,
(1)
where MD is the mass of the detector in kg; Ei is the energy loss in the channel i; Ai is the number of events in it; and K is a coefficient. MDU–Liulin Si-diode has the dimensions 10 × 20 × 0.003 mm3 ; it is covered from the front side by 0.3 mm of epoxy and 0.4 mm of Al. A polyethylene layer, 1 mm thick (see Fig. 1a), is positioned between Al and epoxy layer in one of units, DRD 01. Four units of Liulin were tested at radiation sources described further, two of them from Prague’s laboratory (DRD 01, 02) and two others from the IRSN’s laboratory (FAR 01 and FAR 02). Units DRD 01, 02 and FAR 02 are a little smaller (60 × 25 × 95 mm3 —see Fig. 1b) and powered by accumulators; the unit FAR 01 is powered by batteries, and its dimensions are 100 × 45 × 100 mm3 . 2.2. Radiation fields 2.2.1. Isotopic sources Reference isotopic radiation sources available at IRSN and at NPI were utilized for angular dependence studies. At IRSN, the irradiation was performed in a panoramic hall used regularly for calibration of personal dosimeters by 60 Co gamma radiation. The Liulin units were exposed at roughly 2 m from the center of 60 Co source, where the kerma rate in air was 1 mGy per hour. Exposure to neutrons of an AmBe source (emission rate 1.07 × 107 s−1 ) was realized in the same hall, with the distance between the center of source and the Si-detectors of MDU equal to 100 cm. Ambient dose equivalent rate due to neutrons would be, at that distance, about 0.127 mSv per hour when scattering is not considered. A lead cup covered
Fig. 1. (a) Synoptic scheme of the detector, and (b) photo of a Liulin unit with accumulators; head with Si-diode—small area above number 1.
the AmBe source to reduce the contribution of 60 keV photons of 241Am. At NPI, photon angular response studies were performed at the 60 Co source of the previous primary etalon of the exposure of Czech Republic; kerma-rate in air at the irradiation point was 2.85 mGy/h. Neutron irradiation there was performed at the 252 Cf source used for the regular calibration of Czech routine neutron personal dosimeters. The Hp (10) rate at the point of exposure was about 5 mSv/h during our tests. 2.2.2. CERN high-energy stray radiation fields High-energy stray radiation fields have been realized at the beam H6 of the SPS facility at CERN to simulate neutron component of the onboard aircraft radiation field. A secondary beam of positively charged particles (protons: pions; 2:1; 120 GeV/c) is allowed to enter into collision with a thick copper target (∅70 mm × 500 mm). The target is located in one of the two shielding configurations built for the purpose of the experiment with a concrete (800 mm) or an iron (400 mm) top as already described (Mitaroff and Silari, 2002). Different measurement positions are available on the top and on the side of both shielding configurations. High-energy stray radiation fields are monitored with a beam monitor—precise ionization chamber (PIC). Angular response studies were realized at the beam intensity of about 4000 PICs per burst. The information on neutron spectra is available from the calculation by means of FLUKA
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327 Table 1 Comparison of D(Si) rates measured by means of Liulin units for 60 Co photons at FAR (E dep below 1 MeV), AmBe neutrons and neutrons at CERN behind concrete shield (Edep above 1 MeV for all)
DRD 01 DRD 02 FAR 01 FAR 02 Average
D(Si), Gy/h as measured at 60 Co
1237 1147 1077 1096 1139±41
AmBe
CERN-SCa
CERN-TCb
1.76 1.60 1.55 1.59 1.62±0.05
34.7 31.0 25.8 29.7 30.2±2.1
29.5 25.0 26.3 29.5 27.6±1.3
Relative event distributions - calculation 1.E+00 200 - 0
Relative event number
Liulin unit
3. Results and discussion
Cs - 0 Co - 0 200 - 30
1.E+02
Cs - 30 Co - 30 200 - 60
1.E+03
Cs - 60 Co - 60
1.E+04
0
300
600 900 Edep, keV
1200
1500
Fig. 2. MC (EGS 4) calculated angular dependence of Liulin response to photons (0, 30, 60—angles of incidence; 0 ⇒ perpendicular incidence to the thickness 0.3 mm).
Angular dependence - spectra, 60Co, DRD 01 1000 0, 0.3 mm
100 Dose distribution
2.2.3. Onboard measurements We tried to verify angular response of Liulin units also onboard aircraft, during two long-haul flights of the Czech Airlines realized during November (Prague–Montreal–Prague) and December (Prague–Dubai–Prague) 2003. The unit DRD 01 was exposed in one flight direction with the front turned to sky, in other to the bottom of aircraft, and the unit DRD 02 was exposed inversely.
1.E+01
1.E+05
a SC—side concrete; b TC—top concrete.
Monte Carlo code; reference data were recently published (Mitaroff and Silari, 2002).
325
45, 0.3 mm 90, 10 mm 90, 20 mm
10 1 0.1
3.1. Data interpretation; comparison of MDU units 0.01
It was observed that there is a difference in energy deposition spectra when Liulin is exposed to photons (low LET radiation) and/or neutrons (Spurny and Datchev, 2002; Spurny et al., 2003; Spurný, 2004). In the spectra of deposited energy due to photons impulsions with energy deposited in the region above 1 MeV are practically absent. Different is the situation in the case of irradiation by fast and/or high-energy neutrons. The energy deposited is going up to the maximal value of about 21 MeV, particularly in the case of neutrons of CERN high-energy fields. We will present the results obtained also separately for the values of energy deposition below (low Edep ) and above (high Edep ) 1 MeV. The comparison of MDU units used for angular dependence studies can be estimated from the data given in Table 1, where the results obtained in the case of perpendicular radiation incidence are presented. One can see there that the responses of all units are similar; typical relative variation of the mean value is about 5%, maximum difference does not exceed 25%. The results of all angular dependence studies are therefore presented relatively to the perpendicular incidence; variation obtained for a unit can be judged as typical for all others (see also further).
0
0.5
1 Edep, MeV
Fig. 3. Experimentally registered MDU-response to photons.
angular
1.5
dependence
2
of
3.2. Angular dependence of the response to photons Independently of experimental studies described in this report, we already acquired an indication that the angular response of MDU to photons is not pronounced. This indication followed from MC calculation by EGS 4 code; results obtained are presented in Fig. 2 (Spurny et al., 2003). One can see that for 200 keV and 137 Cs photons the angular dependence is practically absent; for 60 Co there is a little change in spectrum for incidence angle 60◦ . Such an effect was also observed experimentally as can be seen in Fig. 3 up to the angle of incidence 90◦ . As far as the integral dose values obtained by means of Eq. (1), are concerned, the value at 90◦ is not less than 85% of the value at perpendicular incidence (0◦ ).
326
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327 Table 2 The influence of the detector’s orientation on the DRD 02 readings at side concrete position (beam intensity—about 4000 PIC counts per burst)
Liulin angular dependence at AmBe source Relative response for different Edep
1.2 1 0.8 D01high D01tot
0.6
D02high
D(Si), Gy/h, in region of Edep
oriented to
Low
High
Total
Side shield Beam direction Opposite to beam Roof of hall Floor at side shield Average
40.39 32.58 29.07 25.62 25.87 30.71 ± 3.06a
31.37 35.91 31.94 39.96 41.39 36.11 ± 2.27
71.76 68.49 61.01 65.58 67.26 66.82 ± 1.98
D02tot F01high
0.4
F01tot F02high
0.2
F02tot
0 20
0
60
40
80
Angle of incidence,
100
o
Fig. 4. Angular dependence of all four Liulin units at AmBe neutron source; dependences for total D(Si) as well as that above 1 MeV deposited energy are given.
1.2 1 high Edep
0.8
a 1 standard deviation of the average value.
more flat, due to low angular dependence of the response to photons, also emitted by this source. One can also observe that even when the MDU unit is exposed from the back of the equipment, the response is still about 40% for the total, and 30% for the high Edep region in both cases when compared to the perpendicular incidence.
Angular dependence - Cf 252
Relative response
Head of unit
total
3.4. MDU angular response in CERN high-energy radiation field
0.6 0.4 0.2 0 0
30
60
90
120
150
180
Angle of incidence,o
Fig. 5. Angular dependence of Liulin DRD 01 unit at 252 Cf neutron source; dependences for total D(Si) as well as that above 1 MeV deposited energy are given.
3.3. MDU angular responses at fast neutron sources All four available Liulin units were compared at IRSN laboratory exposed 1 m from Pb-covered AmBe neutron source. The results obtained are presented in Fig. 4. One can see first that the angular dependencies are actually very similar for all four units studied, both for high Edep region as well as for total dose. The response in high Edep region decreases roughly twice when comparing parallel (90◦ ) to perpendicular (0◦ ) incidence, by about 25% for the total. Studies at 252 Cf neutron source were performed with the unit DRD 01, for angles of incidence up to the irradiation from the back of the unit. The results obtained are presented in Fig. 5. One can see that the response in high Edep region is constant to about 30◦ , afterwards decreases to about 0.4 for parallel incidence, a little more than for AmBe neutrons. As far as the total response at 252 Cf source is concerned, it is
All results presented until now were obtained with more or less parallel radiation beams. The irradiation geometry at CERN is different. The radiation field is more isotropic, with the preferential direction from a shield. Besides, angular dependence studies at CERN reference field at top shield positions are complicated due to the presence of a muon background (Mitaroff and Silari, 2002). It seemed preferable to us to study the angular dependence at side shield positions, where muon background is absent. A complete study was performed for the angular and position dependence of Liulin response using DRD 02 module, including self-shielding positions. The results obtained are presented in Table 2. One can see there that: • The variations of the readings are actually registered. • They are the highest for the low Edep region (relative 1 of mean about 10%), less important for the high Edep region, and the least for the total signal (relative 1 of mean about 3%). Similarly, less extensive tests were also performed for FAR 02 unit, with similar results; relative standard deviation was 11% for Elow , 9% for Ehigh . 3.5. Liulin angular response onboard aircraft The values of H ∗ (10) calculated from the readings of DRD 01 and DRD 02 units during two long-haul returned
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327
327
Table 3 Influence of the orientation of MDU units onboard aircraft, head of DRD 01 unit oriented from Prague to sky, to Prague to bottom, for DRD 02 inversely Flight
D(Si)a in Gy estimated with DRD 01 Low Edep
Prague–Montreal 10.0 Montreal–Prague 8.5 Prague–Dubai 5.7 Dubai–Prague 6.9 a Relative uncertainty (2) 10% for low E dep ,
DRD 02 High Edep
Total
2.7 12.7 2.1 10.6 1.0 6.7 1.3 8.2 20% for high Edep and total.
flights mentioned are presented in total and for both Edep regions in Table 3. One can see that the differences observed for both Edep regions and total value of H ∗ (10) are less important than the estimated uncertainty.
Low Edep
High Edep
Total
9.9 8.2 5.4 6.4
2.2 1.9 1.1 1.2
12.1 10.1 6.5 8.6
to estimate still more quantitatively the importance of this effect.
References 4. Conclusions The main conclusions based on the results presented can be summarized as: 1. The influence of the detector’s orientation is observed for all radiations tested. 2. For isotopic sources, this influence is relatively less important in the case of photons. It can be calculated from the data presented that the average response at 2isotropic impact is about 0.93 of that for perpendicular incidence for 60 Co photons, and still higher for lower photon energies. 3. Angular dependence is a little more important for fast neutrons. It can be calculated from the data presented, that the average response at 2-isotropic impact would be about 0.78 of that for perpendicular incidence for 252 Cf neutrons, and higher for AmBe neutrons. 4. The influence of detector orientation is also observed at CERF high-energy reference fields. The influence is, however, limited in extent, as can be seen in Table 2. 5. Onboard radiation field is still more isotropic than CERF field; the influence of angular dependence registered during two long-haul flights mentioned is actually practically absent. Nevertheless, more complete studies are needed
Dachev, Ts., Tomov, B., Matviichuk, Yu., Dimitrov, Pl., Lemaire, J., Gregoire, Gh., et al., 2002. Calibration results obtained with LIULIN-4 Type dosimeters. Invited paper. Adv. Space Res. 30, 917–925. EURADOS, 1996. Exposure of air crew to cosmic radiation. McAulay, I., Bartlett, T.D., Dietze, G., Menzel, H.G., Schnuer, K., Schrewe, U.J. (Eds.), EURADOS Report 1996-01, Luxembourg, 1–77. ICRP, 1991. Recommendations of the International Commission on Radiological Protection. ICRP Publication, vol. 60. Pergamon Press, Oxford, 1991. Mitaroff, A., Silari, M., 2002. The CERN–EU high-energy reference field (CERF) facility for dosimetry at commercial flight altitudes and in space. Radiat. Prot. Dosim. 102, 7–22. Spurný, F., 2004. Response of a semiconductor spectrometer to neutrons. Radiat. Meas., submitted for publication, January 2004. doi:10.1016/j.radmeas.2004.05.006 Spurny, F., Dachev, Ts., 2002. Aircrew onboard dosimetry with a semiconductor spectrometer. Radiat. Prot. Dosim. 100, 525– 528. Spurný, F., Dachev, Ts., 2003. Long-term monitoring of the onboard aircraft exposure level with a Si-diode based spectrometer. Invited paper. Adv. Space Res. 32, 53–58. Spurny, F., Gschwindt, R., Makoviˇcka, L., Dachev, Ts., 2003. Response of a semiconductor spectrometer to photons—comparison of experiment with MC calculation. Nucl. Energy Safety 11 (49), 396–400.
The angular dependence of an Si energy deposition spectrometer response at several radiation sources夡 František Spurnýa,∗ , François Trompierb , Jean-François Bottollier-Depoisb a Department of Radiation Dosimetry, Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Na Truhláˇrce 39/64, Prague
18086 Praha 8, Czech Republic b External Dosimetry Department, Institute for Radiation Protection and Nuclear Safety (IRSN), Fontenay-aux-Roses, France
Received 10 February 2004; received in revised form 18 June 2004; accepted 20 June 2004
Abstract An MDU–Liulin spectrometer based on an Si-diode was mainly used during the last few years with the goal to use them for measurements onboard aircraft. To investigate its ability to obtain such measurements, the detector was tested in some radiation reference fields, like 60 Co and other photon beams, neutrons of an AmBe and 252 Cf sources and in high-energy radiation fields at CERN. Due to the high geometrical asymmetry of the Si-diode semiconductor, an angular dependence of the response would be expected. This work presents analyses and discusses the results of angular dependence studies obtained at the different radiation sources mentioned. It was found that these angular dependences vary with the type and energy of radiation. The influence of these variations on the use as a dosimeter onboard aircraft is also studied and discussed. © 2004 Elsevier Ltd. All rights reserved. Keywords: Si-semiconductor spectrometer; Angular response; Aircrew dosimetry
1. Introduction ICRP 60 recommendations (ICRP, 1991) suggest to consider aircrew members as occupationally exposed persons. The radiation fields on aircraft board are complex; they contain particles with energies up to few hundred MeV (EURADOS, 1996). Obviously, one distinguishes the component with low linear energy transfer (LET) composed mostly of electrons, high-energy protons, mesons and 夡 The studies were partially supported through EC Project FIGMCT00-0068 and the Project of the Grant Agency of Czech Republic No. 202/01/0710. ∗ Corresponding author. Tel.: +420-283-841-772; fax: +420-283842-788. E-mail address: [email protected] (F. Spurný).
1350-4487/$ - see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.06.009
photons, and the high LET component represented mostly by neutrons; few high-energy heavier charged particles are still present. There are few types of equipment available to characterize such a field. The spectrometer MDU–Liulin with an Si-diode, originally developed for radiation measurements onboard cosmic vehicles was also proposed for measurements onboard aircraft (Dachev and Spurny, 2002; Spurny and Dachev, 2002, 2003). To appreciate its possibilities for that purpose, four Liulin spectrometers were characterized in photon, neutron and high-energy radiation reference fields (Spurny et al., 2003; Spurny and Datchev, 2002; Spurný, 2004). Due to geometrical asymmetry of the Si-diode, an angular dependence of the response would be expected. This work presents, analyzes and discusses the results of angular dependence studies obtained at the different radiation sources mentioned. It was found that these
324
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327
angular dependences vary with the type and energy of radiation. The influence of these variations on the use as a dosimeter onboard aircraft is discussed. 2. Experimental 2.1. MDU–Liulin energy deposition spectrometer The main purpose of MDU–Liulin energy deposition spectrometer (Dachev and Spurny, 2002; Spurný, 2004) is to monitor simultaneously the doses and numbers of energy deposition events in an Si-diode. It consists of the detector itself, a charge-sensitive preamplifier, two microcontrollers, a flash memory and Li-ion cells. Pulse analysis technique is used to measure the deposited energy in the detector. The amplitude of the pulses is proportional by a factor of 240 mV/MeV to the energy loss in the detector. Adjustment of the energy scale is made through the 60 keV photons of 241Am. The amplitudes are digitized and organized in a 256-channel spectrum. The dose D (Gy) is calculated from the spectrum as D = K · (Ei Ai )/MD,
(1)
where MD is the mass of the detector in kg; Ei is the energy loss in the channel i; Ai is the number of events in it; and K is a coefficient. MDU–Liulin Si-diode has the dimensions 10 × 20 × 0.003 mm3 ; it is covered from the front side by 0.3 mm of epoxy and 0.4 mm of Al. A polyethylene layer, 1 mm thick (see Fig. 1a), is positioned between Al and epoxy layer in one of units, DRD 01. Four units of Liulin were tested at radiation sources described further, two of them from Prague’s laboratory (DRD 01, 02) and two others from the IRSN’s laboratory (FAR 01 and FAR 02). Units DRD 01, 02 and FAR 02 are a little smaller (60 × 25 × 95 mm3 —see Fig. 1b) and powered by accumulators; the unit FAR 01 is powered by batteries, and its dimensions are 100 × 45 × 100 mm3 . 2.2. Radiation fields 2.2.1. Isotopic sources Reference isotopic radiation sources available at IRSN and at NPI were utilized for angular dependence studies. At IRSN, the irradiation was performed in a panoramic hall used regularly for calibration of personal dosimeters by 60 Co gamma radiation. The Liulin units were exposed at roughly 2 m from the center of 60 Co source, where the kerma rate in air was 1 mGy per hour. Exposure to neutrons of an AmBe source (emission rate 1.07 × 107 s−1 ) was realized in the same hall, with the distance between the center of source and the Si-detectors of MDU equal to 100 cm. Ambient dose equivalent rate due to neutrons would be, at that distance, about 0.127 mSv per hour when scattering is not considered. A lead cup covered
Fig. 1. (a) Synoptic scheme of the detector, and (b) photo of a Liulin unit with accumulators; head with Si-diode—small area above number 1.
the AmBe source to reduce the contribution of 60 keV photons of 241Am. At NPI, photon angular response studies were performed at the 60 Co source of the previous primary etalon of the exposure of Czech Republic; kerma-rate in air at the irradiation point was 2.85 mGy/h. Neutron irradiation there was performed at the 252 Cf source used for the regular calibration of Czech routine neutron personal dosimeters. The Hp (10) rate at the point of exposure was about 5 mSv/h during our tests. 2.2.2. CERN high-energy stray radiation fields High-energy stray radiation fields have been realized at the beam H6 of the SPS facility at CERN to simulate neutron component of the onboard aircraft radiation field. A secondary beam of positively charged particles (protons: pions; 2:1; 120 GeV/c) is allowed to enter into collision with a thick copper target (∅70 mm × 500 mm). The target is located in one of the two shielding configurations built for the purpose of the experiment with a concrete (800 mm) or an iron (400 mm) top as already described (Mitaroff and Silari, 2002). Different measurement positions are available on the top and on the side of both shielding configurations. High-energy stray radiation fields are monitored with a beam monitor—precise ionization chamber (PIC). Angular response studies were realized at the beam intensity of about 4000 PICs per burst. The information on neutron spectra is available from the calculation by means of FLUKA
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327 Table 1 Comparison of D(Si) rates measured by means of Liulin units for 60 Co photons at FAR (E dep below 1 MeV), AmBe neutrons and neutrons at CERN behind concrete shield (Edep above 1 MeV for all)
DRD 01 DRD 02 FAR 01 FAR 02 Average
D(Si), Gy/h as measured at 60 Co
1237 1147 1077 1096 1139±41
AmBe
CERN-SCa
CERN-TCb
1.76 1.60 1.55 1.59 1.62±0.05
34.7 31.0 25.8 29.7 30.2±2.1
29.5 25.0 26.3 29.5 27.6±1.3
Relative event distributions - calculation 1.E+00 200 - 0
Relative event number
Liulin unit
3. Results and discussion
Cs - 0 Co - 0 200 - 30
1.E+02
Cs - 30 Co - 30 200 - 60
1.E+03
Cs - 60 Co - 60
1.E+04
0
300
600 900 Edep, keV
1200
1500
Fig. 2. MC (EGS 4) calculated angular dependence of Liulin response to photons (0, 30, 60—angles of incidence; 0 ⇒ perpendicular incidence to the thickness 0.3 mm).
Angular dependence - spectra, 60Co, DRD 01 1000 0, 0.3 mm
100 Dose distribution
2.2.3. Onboard measurements We tried to verify angular response of Liulin units also onboard aircraft, during two long-haul flights of the Czech Airlines realized during November (Prague–Montreal–Prague) and December (Prague–Dubai–Prague) 2003. The unit DRD 01 was exposed in one flight direction with the front turned to sky, in other to the bottom of aircraft, and the unit DRD 02 was exposed inversely.
1.E+01
1.E+05
a SC—side concrete; b TC—top concrete.
Monte Carlo code; reference data were recently published (Mitaroff and Silari, 2002).
325
45, 0.3 mm 90, 10 mm 90, 20 mm
10 1 0.1
3.1. Data interpretation; comparison of MDU units 0.01
It was observed that there is a difference in energy deposition spectra when Liulin is exposed to photons (low LET radiation) and/or neutrons (Spurny and Datchev, 2002; Spurny et al., 2003; Spurný, 2004). In the spectra of deposited energy due to photons impulsions with energy deposited in the region above 1 MeV are practically absent. Different is the situation in the case of irradiation by fast and/or high-energy neutrons. The energy deposited is going up to the maximal value of about 21 MeV, particularly in the case of neutrons of CERN high-energy fields. We will present the results obtained also separately for the values of energy deposition below (low Edep ) and above (high Edep ) 1 MeV. The comparison of MDU units used for angular dependence studies can be estimated from the data given in Table 1, where the results obtained in the case of perpendicular radiation incidence are presented. One can see there that the responses of all units are similar; typical relative variation of the mean value is about 5%, maximum difference does not exceed 25%. The results of all angular dependence studies are therefore presented relatively to the perpendicular incidence; variation obtained for a unit can be judged as typical for all others (see also further).
0
0.5
1 Edep, MeV
Fig. 3. Experimentally registered MDU-response to photons.
angular
1.5
dependence
2
of
3.2. Angular dependence of the response to photons Independently of experimental studies described in this report, we already acquired an indication that the angular response of MDU to photons is not pronounced. This indication followed from MC calculation by EGS 4 code; results obtained are presented in Fig. 2 (Spurny et al., 2003). One can see that for 200 keV and 137 Cs photons the angular dependence is practically absent; for 60 Co there is a little change in spectrum for incidence angle 60◦ . Such an effect was also observed experimentally as can be seen in Fig. 3 up to the angle of incidence 90◦ . As far as the integral dose values obtained by means of Eq. (1), are concerned, the value at 90◦ is not less than 85% of the value at perpendicular incidence (0◦ ).
326
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327 Table 2 The influence of the detector’s orientation on the DRD 02 readings at side concrete position (beam intensity—about 4000 PIC counts per burst)
Liulin angular dependence at AmBe source Relative response for different Edep
1.2 1 0.8 D01high D01tot
0.6
D02high
D(Si), Gy/h, in region of Edep
oriented to
Low
High
Total
Side shield Beam direction Opposite to beam Roof of hall Floor at side shield Average
40.39 32.58 29.07 25.62 25.87 30.71 ± 3.06a
31.37 35.91 31.94 39.96 41.39 36.11 ± 2.27
71.76 68.49 61.01 65.58 67.26 66.82 ± 1.98
D02tot F01high
0.4
F01tot F02high
0.2
F02tot
0 20
0
60
40
80
Angle of incidence,
100
o
Fig. 4. Angular dependence of all four Liulin units at AmBe neutron source; dependences for total D(Si) as well as that above 1 MeV deposited energy are given.
1.2 1 high Edep
0.8
a 1 standard deviation of the average value.
more flat, due to low angular dependence of the response to photons, also emitted by this source. One can also observe that even when the MDU unit is exposed from the back of the equipment, the response is still about 40% for the total, and 30% for the high Edep region in both cases when compared to the perpendicular incidence.
Angular dependence - Cf 252
Relative response
Head of unit
total
3.4. MDU angular response in CERN high-energy radiation field
0.6 0.4 0.2 0 0
30
60
90
120
150
180
Angle of incidence,o
Fig. 5. Angular dependence of Liulin DRD 01 unit at 252 Cf neutron source; dependences for total D(Si) as well as that above 1 MeV deposited energy are given.
3.3. MDU angular responses at fast neutron sources All four available Liulin units were compared at IRSN laboratory exposed 1 m from Pb-covered AmBe neutron source. The results obtained are presented in Fig. 4. One can see first that the angular dependencies are actually very similar for all four units studied, both for high Edep region as well as for total dose. The response in high Edep region decreases roughly twice when comparing parallel (90◦ ) to perpendicular (0◦ ) incidence, by about 25% for the total. Studies at 252 Cf neutron source were performed with the unit DRD 01, for angles of incidence up to the irradiation from the back of the unit. The results obtained are presented in Fig. 5. One can see that the response in high Edep region is constant to about 30◦ , afterwards decreases to about 0.4 for parallel incidence, a little more than for AmBe neutrons. As far as the total response at 252 Cf source is concerned, it is
All results presented until now were obtained with more or less parallel radiation beams. The irradiation geometry at CERN is different. The radiation field is more isotropic, with the preferential direction from a shield. Besides, angular dependence studies at CERN reference field at top shield positions are complicated due to the presence of a muon background (Mitaroff and Silari, 2002). It seemed preferable to us to study the angular dependence at side shield positions, where muon background is absent. A complete study was performed for the angular and position dependence of Liulin response using DRD 02 module, including self-shielding positions. The results obtained are presented in Table 2. One can see there that: • The variations of the readings are actually registered. • They are the highest for the low Edep region (relative 1 of mean about 10%), less important for the high Edep region, and the least for the total signal (relative 1 of mean about 3%). Similarly, less extensive tests were also performed for FAR 02 unit, with similar results; relative standard deviation was 11% for Elow , 9% for Ehigh . 3.5. Liulin angular response onboard aircraft The values of H ∗ (10) calculated from the readings of DRD 01 and DRD 02 units during two long-haul returned
F. Spurný et al. / Radiation Measurements 39 (2005) 323 – 327
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Table 3 Influence of the orientation of MDU units onboard aircraft, head of DRD 01 unit oriented from Prague to sky, to Prague to bottom, for DRD 02 inversely Flight
D(Si)a in Gy estimated with DRD 01 Low Edep
Prague–Montreal 10.0 Montreal–Prague 8.5 Prague–Dubai 5.7 Dubai–Prague 6.9 a Relative uncertainty (2) 10% for low E dep ,
DRD 02 High Edep
Total
2.7 12.7 2.1 10.6 1.0 6.7 1.3 8.2 20% for high Edep and total.
flights mentioned are presented in total and for both Edep regions in Table 3. One can see that the differences observed for both Edep regions and total value of H ∗ (10) are less important than the estimated uncertainty.
Low Edep
High Edep
Total
9.9 8.2 5.4 6.4
2.2 1.9 1.1 1.2
12.1 10.1 6.5 8.6
to estimate still more quantitatively the importance of this effect.
References 4. Conclusions The main conclusions based on the results presented can be summarized as: 1. The influence of the detector’s orientation is observed for all radiations tested. 2. For isotopic sources, this influence is relatively less important in the case of photons. It can be calculated from the data presented that the average response at 2isotropic impact is about 0.93 of that for perpendicular incidence for 60 Co photons, and still higher for lower photon energies. 3. Angular dependence is a little more important for fast neutrons. It can be calculated from the data presented, that the average response at 2-isotropic impact would be about 0.78 of that for perpendicular incidence for 252 Cf neutrons, and higher for AmBe neutrons. 4. The influence of detector orientation is also observed at CERF high-energy reference fields. The influence is, however, limited in extent, as can be seen in Table 2. 5. Onboard radiation field is still more isotropic than CERF field; the influence of angular dependence registered during two long-haul flights mentioned is actually practically absent. Nevertheless, more complete studies are needed
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