Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 42, No. 9, p. 768–778 (September 2005)
ORIGINAL PAPER
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges Tatsuhiko SATO, Daiki SATOH, Akira ENDO and Yasuhiro YAMAGUCHI Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195 (Received April 18, 2005 and accepted in revised form July 21, 2005) A new inventive radiation dose monitor, designated as DARWIN (Dose monitoring system Applicable to various Radiations with WIde energy raNges), has been developed for monitoring doses in workspaces and surrounding environments of high energy accelerator facilities. DARWIN is composed of a phoswitch-type scintillation detector, which consists of liquid organic scintillator BC501A coupled with ZnS(Ag) scintillation sheets doped with 6 Li, and a data acquisition system based on a Digital-Storage-Oscilloscope. Scintillations from the detector induced by thermal and fast neutrons, photons and muons were discriminated by analyzing their waveforms, and their light outputs were directly converted into the corresponding doses by applying the G-function method. Characteristics of DARWIN were studied by both calculation and experiment. The calculated results indicate that DARWIN gives reasonable estimations of doses in most radiation fields. It was found from the experiment that DARWIN has an excellent property of measuring doses from all particles that significantly contribute to the doses in surrounding environments of accelerator facilities—neutron, photon and muon with wide energy ranges. The experimental results also suggested that DARWIN enables us to monitor small fluctuation of neutron dose rates near the background-level owing to its high sensitivity. KEYWORDS: dose monitor, radiation protection, high energy neutron, background dose, liquid organic scintillator, digital oscilloscope, pulse shape discrimination, G-function, DARWIN, SCINFUL-QMD
I. Introduction High energy proton accelerator facilities have been constructed in some countries1–3) for pursuing frontier research in nuclear physics, material science, transmutation, radiotherapy and so on. At these facilities, accelerated protons produce neutrons with a wide energy distribution by causing complex nuclear reactions in the spallation targets and accelerator structures. Development of dose monitors applicable to higher energy neutrons—above 20 MeV—is one of the key issues in radiation protections at the facilities, since conventional moderator-based survey instruments, so-called rem-counters, are not sensitive enough to measure doses from such high energy neutrons. New devices of rem-counter named LINUS4) and WENDI-II5) were designed by inserting heavy metal shells into their moderators in order to extend their applicable ranges to higher energies. The instruments can measure ambient dose equivalent H (10) from neutrons with energies from thermal to 5 GeV. However, their sensitivities are insufficient to monitor small fluctuation of dose rates near the background-level within a short time, where dose rates in usual workspaces and surrounding environments of accelerator facilities are at this level. Furthermore, the structures of those instruments must be redesigned when the radiation weighting factor is modified by the International Commission on Radiation Protection (ICRP).6) With this specific problem in mind, we have developed a dose monitor applicable to neutrons with energies from
Corresponding author, Tel. +81-29-282-5803, Fax. +81-29-2826063, E-mail:
[email protected]
thermal to 80 MeV, and potentially up to several GeV. In addition, the monitor satisfies the following features: (1) high sensitivity and precision, (2) capable of monitoring dose from not only neutrons but also photons and muons with wide energy ranges, (3) adjustable to the revision of the dose conversion coefficients without redesigning its structure and (4) light weight for mobility. The first item is indispensable for monitoring small fluctuation of dose rates due to the operation of an accelerator. The second item enable us to obtain the total dose in surrounding environments of accelerator facilities. The dose is dominantly contributed by (1) neutrons, photons and muons produced by the accelerator, (2) neutrons and muons originated from cosmic-rays, and (3) photons generated from decay of radionuclides, but none of the existing devices are able to monitor doses from all of these particles except for tissue equivalent proportional counters (TEPC), of which sensitivities are generally insufficient for routine-monitoring of dose in accelerator facilities. The dose monitor is composed of our originally-developed phoswitch-type scintillation detector, which consists of liquid organic scintillator BC501A surrounded by ZnS(Ag) scintillation sheets doped with 6 Li, and a data acquisition (DAQ) system based on a Digital-Storage-Oscilloscope (DSO). Dose from neutrons below 1 keV is evaluated from the number of scintillations from the ZnS(Ag) sheets, while those from neutrons above 1 MeV as well as photons and muons are estimated from the light output of scintillations from BC501A by applying the G-function method7) which directly relates the light output to the corresponding dose. Our previous experiments8–10) verified the accuracy of the dose evaluation method for the neutrons by employing
768
769
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges Li doped ZnS(Ag) sheet
6
High voltage power supply (-1600V)
glass cell BC501A 12.4φ x 12.7 cm
Anode signal Al case
~ 0.1mV
Digital Storage Oscilloscope Waverunner6100A, LeCroy 0.1mV 2V~ ADC ADC ~2V
Channel 1 attenuator
Channel 2 attenuator
Channel 3 attenuator
Photo-multiplier tube R4144, HAMAMATSU
Channel 4 attenuator Not used
Fig. 1 Schematic view of DARWIN
the phoswitch detector or a simple cylindrical BC501A combined with a DAQ system that consisted of several NIM and CAMAC modules, where the DAQ system was very complicated and inadequate for applying to a dose monitor for routine-survey. This paper describes the details of the invented dose monitoring system, designated as DARWIN (Dose monitoring system Applicable to various Radiations with WIde energy raNges), together with its dose estimating method for photons and muons as well as neutrons. The results of experimental verifications of the system will be also presented below.
II. System The schematic view of DARWIN is illustrated in Fig. 1. The detector consists of a cylindrical glass cell with 12.4 cm in diameter and 12.7 cm in length containing BC501A scintillator surrounded by 6 Li-doped ZnS(Ag) sheets and an aluminum cover. BC501A has been widely applied to the neutron and photon spectroscopy. This is attributed to its property that both neutrons and photons can stimulate the scintillator by producing secondary charged particles whose energy is related to the light output of the scintillation. BC501A is also sensitive to muons, but it is hardly feasible to be employed for the muon spectroscopy, since the light outputs of the scintillation are almost independent of their incident energies except for those induced by low energy muons that stop in the detector. On the other hand, 6 Li-doped ZnS(Ag) has been used as a thermal neutron detector, since the cross section of the 6 Li (n; ) reaction increases with a decrease of the neutron energy, and the secondary alpha particles cause the scintillation of ZnS(Ag). Note that scintillations of BC501A caused by incidences of neutrons, photons and muons, and that of the ZnS(Ag) sheets stimulated by alpha particles are hereafter abbreviated to neutron, photon, muon and alpha scintillations, respectively. The signals from the two scintillators are amplified by a photo-multiplier tube (R4144, Hamamatsu Photonics). The supply voltage was set to 1;600 V in order to prevent the saturation of the photo-multiplier due to scintillation with a large light output such as several tens of MeVee. The amplified signal diverges into 3 branches, and their waveforms are recorded by a DSO (Waverunner6100A, LeCroy) with a bandwidth 1 GHz. This DSO includes 2 sets of analog-todigital converters (ADCs) that sample waveforms at the maximum rate of 10 Giga-Sampling per second (GS/s) to VOL. 42, NO. 9, SEPTEMBER 2005
8 bit code. Each ADC is shared by 2 channels, and the maximum sampling rate thus decreases to 5 GS/s while more than 3 channels are in operation. The 3 branches are specialized in close-up digitizing of a waveform with the different voltage ranges—below 0.1 V, from 0.1 to 2 V, and above 2 V, respectively—by changing the resistance of attenuators equipped with each channel. The data obtained from the channel 1 and the others are used for analyzing the decay and peak parts of waveforms, respectively. The fluctuation of dark current can be also estimated from the channel 1 data. In order to accomplish the wide range dose monitor, 3 channels are at least required for the precise measurement of waveforms of scintillations with light output from 0.1 to several tens of MeVee by an 8-bit DSO. We designated this tactics as the signal branching method. The digitized data are stored into the internal hard-diskdrive (HDD) by every 1,000 events, and during the storage of the data, the DSO is unable to record a new waveform. The dead time of the system is thus proportional to the storing time, which significantly depends on the sampling rate. We therefore set the sampling rate at 1 GS/s for each channel, since a higher resolution does not improve the precision of the measurement. The digitized waveforms of the 3 branches are recombined in off-line analysis. Each event is identified as the photon or muon, neutron and alpha scintillations by means of the pulse shape discrimination (PSD) technique,11) since their mean decay times are different from one another—approximately 10 ns, 100 ns and 3 ms, respectively. The light output of scintillation is obtained by integrating its waveform with respect to time, and converted into the corresponding dose by using the G-function method.
III. Dose Evaluation Method 1. Principle of G-function Method In general, dose estimation with a scintillator applicable to spectroscopy such as BC501A is performed by the following 2 steps; (1) determining the energy spectrum from its light output distribution by using the unfolding method, and (2) multiplying the obtained energy spectrum with the corresponding fluence-to-dose conversion coefficients. However, a real-time dose monitor cannot be accomplished by means of this method, since the light output distribution with a small statistical uncertainty is indispensable for the first step, where the measurement for a long duration is generally re-
770
T. SATO et al.
quired to obtain such precise data. The G-function method, on the other hand, enables us to measure the dose in real-time. This is due to the assumption adopted in the method that dose can be uniquely determined from the light output of each scintillation L by correlating the two quantities directly with a function named G-function. Under the assumption, total dose D due to the scintillations with a light output distribution NðLÞ can be obtained by Z Lmax D¼ NðLÞGðLÞdL; ð1Þ Lmin
where GðLÞ denotes the G-function, and Lmin and Lmax are respectively the minimum and maximum light outputs of the scintillations. The applicability of the method to high energy neutron dosimetry was validated in our previous papers.8–10) The G-function can be calculated from the response function of the detector RðE; LÞ, where E represents the energy of incident particles, combined with fluence-to-dose conversion coefficients d ðEÞ by solving the equation Z Lmax d ðEÞ ¼ RðE; LÞGðLÞdL ð2Þ Lmin
by employing the unfolding code SAND-II.12) A more detailed calculation method was also described in our previous papers.8–10) 2. Evaluation of Neutron Dose (1) Conventional G-function Dose from higher energy neutrons is evaluated from the light output distribution of the neutron scintillations by applying the G-function method. The response function of BC501A with respect to the neutron scintillations was computed by the Monte-Carlo based scintillator response calculation code SCINFUL-QMD13) coupled with the Particle and Heavy Ion Transport code System PHITS.14) The two irradiation geometries—isotropic and parallel to the front surface—were assumed in the calculation. The PHITS code provided the source information to SCINFUL-QMD by performing the particle transport simulation outside BC501A. Although SCINFUL-QMD is capable of estimating response functions for the incident energies up to 3 GeV, we calculated those up to 80 MeV, since the applicability of the code to the higher energy is currently under investigation.15) For d ðEÞ, we adopted the fluence-to-ambient dose equivalent conversion coefficients given in ICRP publication 74.16) On the other hand, dose from low energy neutrons is simply estimated from the number of the alpha scintillations multiplied by an experimentally determined dose conversion factor; i.e. the G-function for the alpha scintillations is irrespective of their light output. This is due to the fact that the light output from 6 Li-doped ZnS(Ag) is almost independent of the incident neutron energy. (2) Spectrum-guessed G-function This method, however, cannot measure dose from neutrons with energies between 1 keV and 1 MeV (abbreviated to keV-order-neutrons, hereafter), since both BC501A and 6 Li-doped ZnS(Ag) are not sensitive enough to them.10) We have therefore developed a modified G-function method for the purpose of solving this problem. In this method, the
fluence-to-dose conversion coefficients d ðEÞ adopted in the G-function calculation are enlarged at the energy region around 2 MeV, which is slightly over the lower threshold energy of neutrons measured by BC501A, in order to estimate dose from neutrons below the threshold. This method requires an appropriately guessed-spectrum of neutrons with energies around the threshold level. In radiation fields at workspaces and surrounding environments of high energy accelerator facilities, neutrons below 5 MeV are generally produced by the evaporation process of nuclear reactions induced by high energy neutrons. The spectrum of the evaporated neutrons is not influenced by the incident energy very much, and hence, the low energy neutron spectra in those fields are expected to be similar. We investigated the spectra at workspaces in several high energy accelerator facilities,17) and found that a guessed spectrum with constant neutron fluence per lethargy—1=E spectrum—gives us a reasonably conservative estimate of dose from keV-order-neutrons in most situations (as described later in this section). We therefore determined the enlarged dose conversion coefficient dL ðEÞ to satisfy the relation Z 5 MeV Z 5 MeV d ðEÞdE dL ðEÞdE ¼ ; ð3Þ E E Ethre 1 keV where Ethre is the lower threshold energy of neutron measured by BC501A. The modified G-function (named spectrum-guessed G-function) was calculated by means of the SAND-II code by substituting dL ðEÞ for d ðEÞ in the Eq. (2). (3) Additional-dose G-function Since the practically measurable dose by BC501A is only a small fraction of the total in those fields, the spectrumguessed G-function is inapplicable to radiation fields where fluences of neutrons at keV-orders are much larger than those at a few MeV. Concerned with workspaces at high energy accelerator facilities, the radiation field outside a pure iron shield satisfies this condition, although such case is out of the common since the shielding materials are generally composed of several elements such as hydrogen, carbon and iron. Under this situation, we have to use a conventional remcounter as well as DARWIN, and add doses measured by two monitors to obtain the total neutron dose. The G-function specially applied to this purpose (abbreviated to additional-dose G-function) was calculated by adopting the reduced dose conversion coefficient dR ðEÞ in order to avoid double-counting of dose from neutrons with energies at the overlapped region in terms of their sensitivities—from a few MeV to several tens of MeV. The value of dR ðEÞ was determined by dR ðEÞ ¼ maxfd ðEÞ Rrem ðEÞ; 0g;
ð4Þ
where Rrem ðEÞ denotes the response function of the co-used rem-counter, while that of the Andersson-Braun-type remcounter18) shown in the IAEA technical reports series 40317) was adopted in our calculation. (4) Results of Calculated G-functions The obtained G-functions are plotted in Fig. 2 for the isoJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
771
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges 10
2
10
5
Environment (ER–II Exp.)
19)
10
Neutron Spectrum (/lethargy)
G–function (pSv/count)
Environment (Tohoku Univ.)
1
Conventional Spectrum–guessed Additional–dose 10
0
10
0
10
21)
J–PARC (concrete shield) 17)
10
10
1
20)
FFTP Facility
0
17)
IHEP (concrete shield)
17)
CERN (concrete shield) 17)
CERN (iron shield)
17)
SSRL Linac –5
Light output (MeVee) Fig. 2 Calculated G-functions for the neutron scintillation for the isotropic irradiation
tropic irradiation. The calculations were performed by setting the lower threshold of light output at 0.25 MeVee, which corresponds to the maximum light output of the 1.2 MeV neutron scintillations. The values of the G-functions at the light outputs close to the threshold are different from one another very much. This tendency is attributed to the fact that they are significantly influenced by the dose conversion coefficients at a few MeV, and the values adopted in the calculations of the spectrum-guessed and additional-dose G-functions are much larger and smaller than those done in the conventional calculation as predicted from Eqs. (3) and (4), respectively. (5) Applicability to Various Neutron Fields In order to verify the applicability of the dose evaluation method to various neutron fields, we calculated the responses of DARWIN imaginarily located in workspaces and surrounding environments of several high energy accelerator facilities.17,19–21) In the calculation, we assumed that there
–5
10
10
0
Neutron Energy (MeV) Fig. 3 Neutron spectra adopted in the calculation of the responses of DARWIN and AB rem-counter. The absolute values are arbitrarily adjusted for each spectrum in order to make each line distinctive
is no cosmic-ray neutron in the workspaces of the facilities, and no accelerator-based neutron in surrounding environments, i.e. the cosmic-ray neutron spectra on the ground were employed as representatives of the latter. The neutron spectra adopted in the calculation are shown in Fig. 3. The graphical presentations of the calculated responses are given in Fig. 4 in comparison to the corresponding calculated doses, which were obtained by integrating the spectra multiplied with the fluence-to-dose conversion coefficients up to 80 MeV. It should be noted that the ratios for DARWIN with additional-dose G-function represent those of the sum of the calculated responses of the AB rem-counter and DARWIN by employing the additional-dose G-function
19)
DARWIN with conventional G-function
Environment (ER-II Exp.)
20)
Environment (Tohoku Univ.)
DARWIN with spectrumguessed G-function
21)
J-PARC (concrete shield)
DARWIN with additionaldose G-function
17)
FFTP Facility
AB rem-counter 17)
IHEP (concrete shield)
17)
CERN (concrete shield) 17)
CERN (iron shield)
17)
SSRL Linac
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Calculated Response / Calculated Dose
Fig. 4 Ratios of calculated responses of DARWIN and AB rem-counter imaginarily located in various neutron fields to the corresponding calculated doses
VOL. 42, NO. 9, SEPTEMBER 2005
772
3. Evaluation of Photon and Muon Doses Doses from photons and muons are also evaluated from the light outputs of the corresponding scintillations by applying the G-function method. In the calculation of the G-function for photon, we adopted the fluence to ambient dose equivalent conversion coefficients given in ICRP7416) for photon energies below 1 MeV. Above 1 MeV, those calculated by the Monte-Carlo electromagnetic transport code EGS422,23) were employed, since the Kerma approximation, which was used in the derivation of the ICRP74 values, becomes hardly appropriate with an increase of the energy. The fluence to ambient dose equivalent conversion coefficients calculated by the Monte-Carlo particle transport code FLUKA24,25) were adopted in the calculation of the G-function for muon. The response functions of BC501A with respect to the photon and muon scintillations were computed by EGS4 and PHITS, respectively, under the conditions of the isotropic and parallel irradiation geometries. In the response calculation, we simply estimated the deposit energy in the detector, and regarded the value as the light output of the scintillation in the unit of MeVee. Figure 5 depicts the G-functions for photon and muon for the isotropic irradiation. Photons with energies between 0.15 and 100 MeV were considered, while muons with energies between 1 MeV and 100 GeV were done in the calculation. The photon and muon scintillations, however, cannot be distinguished by PSD, since their decay times are very close
G–function (pSv/count)
to the corresponding doses. The ratios for the stand-alone AB rem-counter are also depicted in the figure. It is found from Fig. 4 that the calculated responses of DARWIN by employing the conventional G-function as well as those of the rem-counter are smaller than the corresponding doses. This is attributed to the ignorance of the keV-order neutron dose in the derivation of the conventional Gfunction, and the insufficient sensitivity of the rem-counter for neutrons over 20 MeV. The ratios for DARWIN with additional-dose G-function are generally close to 1. This tendency indicates that co-use of the AB rem-counter and DARWIN enables us to estimate the doses from neutrons below 80 MeV with a good precision. The calculated responses of DARWIN by employing the spectrum-guessed G-function are slightly larger than (or close to) the corresponding calculated doses except for the iron shielding case at CERN. The slight overestimation is due to the conservative guess—unit fluence per lethargy— for the keV-order neutron spectrum in the G-function calculation. The exception indicates that the spectrum-guessed method is hardly adequate for the radiation fields outside an iron shield, as described before. From these considerations, we concluded that the spectrum-guessed G-function should be adopted in the routinesurvey of doses in workspaces and surrounding environments of high energy accelerator facilities. The use of the conventional G-function is limited for studying the characteristic of DARWIN in calibrated neutron fields. The additional-dose G-function can be employed only in the situation that a rem-counter is in operation beside DARWIN, as described before.
T. SATO et al. 10
1
10
0
10
–1
Photon Muon 10
–2
10
0
10
1
Light output (MeVee) Fig. 5 Calculated G-functions for the photon and muon scintillations for the isotropic irradiation
to each other. We therefore discriminate between the photon and muon scintillations by their light outputs based on an assumption as the following: (1) all scintillations with light outputs below 6 MeVee are regarded as the photon scintillation and (2) the others are estimated to be stimulated by the particle having a larger value of the G-function in order to estimate the total dose conservatively. It should be noted that the values of the G-function for muons generally exceed those for photons because of its greater dose conversion coefficient, and hence, most scintillation with light outputs above the threshold of 6 MeVee is identified as the muon scintillation. This assumption is generally pertinent, since photon scintillations with light output above 6 MeVee are seldom observed in most radiation fields.
IV. Experimental Verification 1. Experimental Condition Characteristics of DARWIN were studied experimentally in some different radiation fields: (a) the thermal and (b) the continuous energy neutron fields based on 241 Am-Be and 252 Cf sources at Facility of Radiation Standard (FRS)26) of Japan Atomic Energy Research Institute (JAERI) Tokai, (c) the quasi-monoenergetic neutron field of 65 MeV at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA)27) of JAERI-Takasaki, (d) the photon fields based on 133 Ba, 137 Cs, 60 Co and 88 Y sources at FRS, and (e) background field at Monitoring Station 1 (MS1) of JAERITokai. The neutron dose rates at the 1 m distance from the 241 Am-Be and 252 Cf sources were 26.9 and 1.04 mSv/h, while the corresponding photon dose rates from 133 Ba, 137 Cs, 60 Co and 88 Y sources were 0.0210, 0.0312, 0.108 and 0.00522 mSv/h, respectively. Figure 6 illustrates the experimental arrangements. The radioisotopes were generally located at the distance of 98 cm from the detector surface except for 88 Y, which was set at a closer distance of 28 cm because of its smaller radioactivity. The experimental setups were similar to those adopted in our previous studies,10) but the DAQ system comJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
773
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges 164 150 Unit : cm
98*) Detector
Graphite Pile 40 164
109
Detector
Neutron or photon sources
Detector Cf Source (at the center of pile) 252
Ground
*) 28 for 88Y
e) Background at MS1
b) & d) neutron and photon fields at FRS
a) Thermal neutron field at FRS
1290
Li target
Unit : cm
Bending Magnet
220
94
Detector
Proton Beam
Iron Collimator Concrete Collimator
Fission Chamber Faraday Cup
Beam Dump (Iron)
c) Quasi-monoenergetic neutron field at TIARA
Fig. 6 Experimental arrangements for studying characteristics of DARWIN
posed of the combination of conventional NIM and CAMAC modules was replaced by the DSO-based system. 2. Data Analysis (1) Waveform Figure 7 depicts typical waveforms due to the neutron, photon (or muon) and alpha scintillations recorded by DSO. It is evident from the figure that the ratios between
Inversed output (V)
10
0
Neutron Photon (or muon) Alpha 10
–1
10
–2
10
–3
0 used for base line restoration
100
Time (ns)
Fig. 7 Typical waveforms due to the neutron, photon (or muon) and alpha scintillations recorded by DSO
VOL. 42, NO. 9, SEPTEMBER 2005
the light outputs of the scintillations in the initial 30 ns and the rest parts (abbreviated to fast and slow components, respectively) are different from each other, but the differences between the neutron and photon scintillations are not manifestly apparent. The signal branching method enables us to clarify the small difference in compatible with avoiding the saturation of DSO. Each waveform was integrated with respect to time, and the obtained value in the unit of Vns (voltnano-second) was converted into its light output in MeVee, where the relation between the two quantities was determined by comparing the experimental and EGS4-calculated response functions of the detector irradiated by several photon sources. (2) Pulse Shape Discrimination Method We adopted a simple pulse shape discrimination (PSD) technique called the gate integration method,11) which discriminates scintillations by means of the 2-dimensional plot of the fast and slow components as shown in Figs. 8– 10, although a more sophisticated technique based on digital waveform analyzers had been developed.28) This is because the latter technique employs time-consuming methods such as the least-square fitting in the analysis of each waveform, which brings an increase of the dead time of the system in the case of on-line analysis. Our final goal is to establish a real time dose monitoring system, and hence, those techniques are not suitable for our purpose. In the gate integration method, the base line restoration is
774
T. SATO et al. 4
Thermal neutron field
Slow component (V.ns)
Alpha scintillation
Neutron scintillation
2
n
latio
intil
sc ified
t
iden
Un–
Photon scintillation 0 0
5
10
15
Fast component (V.ns) Fig. 8 Two-dimensional plot of the fast and slow components for the (a) thermal neutron field
2 252
Slow component (V.ns)
Alpha scintillation
Cf source
Neutron scintillation Threshold for neutron 0.86 V.ns = 0.25 MeVee 1
ifie
ent
d n–i
on lati
ntil
ci ds
U
Photon scintillation 0 0
5
10
Fast component (V.ns) Fig. 9 Two-dimensional plot of the fast and slow components for the (b) continuous energy neutron field based on 252 Cf
30
Slow component (V.ns)
65 MeV quasi–monoenergetic neutron field Alpha scintillation 20
Neutron scintillation 10
Un–identified scintillation 0 0
Photon & muon scintillation 100
200
Fast component (V.ns) Fig. 10 Two-dimensional plot of the fast and slow components for the (c) quasi-monoenergetic neutron field of 65 MeV
essential for keeping the discrimination border stable, since the slow component of scintillation with a small light output is significantly influenced by the fluctuation of the dark current. It is very difficult to restore the base line with the NIM and CAMAC module-based DAQ system, but it is easy by the DSO-based system by analyzing the waveform at the pre-trigger region (see Fig. 7). (3) Results of Pulse Shape Discrimination Figures 8–10 show the 2-dimensional plots of the fast and slow components for (a) the thermal and (b) the continuous energy neutron fields based on the 252 Cf source, and (c) the quasi-monoenergetic neutron field of 65 MeV, respectively. The particle discrimination borders are also depicted in the figures. We found that the DSO based DAQ system has a better property of PSD in comparison to our former system,10) which consisted of several NIM and CAMAC modules. It is obvious from Fig. 8 that the alpha scintillation can be clearly distinguished from the other scintillations. Figure 9 indicates that the discrimination between the neutron and photon scintillations becomes difficult with a decrease of their light output. We assigned a relatively high value of 0.86 Vns, which corresponds to 0.25 MeVee, to the lower threshold of the total light output of signals that can be identified as the neutron scintillation. This value was decided to prevent the contamination of the neutron scintillations due to the photon scintillations, since even if only a small fraction of the photon scintillations was miss-identified as the neutron scintillation, the contamination would have induced a significant overestimation of the neutron dose at the background level due to much greater abundance of photons than that of neutrons. On the other hand, a lower value of 0.21 Vns, which corresponds to 0.062 MeVee, was assigned to the threshold for the photon scintillation. These thresholds correspond to the maximum light outputs of 1.2 MeV neutron and 0.15 MeV photon scintillations, respectively. The G-functions shown in Figs. 2 and 5 were calculated by using these parameters. It should be noted that the thresholds could be extended down to lower values by increasing the supply voltage to the photo-multiplier tube. However, a higher voltage is not suitable for our purpose, since scintillation with a large light output causes the saturation of the photo-multiplier as well as DSO. It is found from Fig. 10 that many events were observed in the gap region between the neutron and photon scintillations, which is denoted as un-identified scintillation in Figs. 8–10, for the incidence of higher energy neutrons. These scintillations were probably triggered by neutrons producing a high energy proton that escaped from the detector. We regarded these events as un-identified scintillations, and discarded in the dose evaluation. For avoiding the underestimation of neutron dose due to the discard, the neutron scintillation with producing the escaping proton was also excluded in the response calculation of the detector by the SCINFULQMD code. (4) Response Function Figure 11 gives a graphical presentation of the measured response functions in (b) the continuous energy neutron field by the 252 Cf source. For the neutron scintillation, the calcuJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Response (/MeVee/s)
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges
10
Exp. Cal.
2
Photon scintillation Neutron scintillation 10
0
0
Alpha scintillation
2
4
Light Output (MeVee) Fig. 11 Measured and calculated response functions in the (b) continuous energy neutron field by 252 Cf
lated result by the combination of SCINFUL-QMD and PHITS is also plotted in the figure for the neutron scintillation. An excellent agreement is observed between the measured and calculated data. Doses due to each scintillation type were obtained from the response functions by applying the corresponding G-functions. 3. Results Table 1 summaries the measured doses by DARWIN together with the reference doses in the radiation fields. In the dose evaluation at the (c) quasi-monoenergetic field at TIARA, we extracted the scintillation due to neutrons with the peak energy by performing the time-of-flight (TOF) measurement simultaneously in order to obtain the dose from 65 MeV mono-energetic neutrons. The (d) mixed source denotes the radiation field composed of the all photon sources—133 Ba, 137 Cs, 60 Co and 88 Y. The reference doses except for the background measurements represent the calculated doses based on the fluxes of the particles inducing the primary scintillation—thermal neutrons, neutrons with energies from 1.2 to 80, 65 MeV neutrons and photons for the (a) to (d) field cases, respectively, at the effective detection points. The fluxes were determined from the source activities except for the (c) field case, where the value of 65 MeV neutron was evaluated from the numbers of nuclear reactions occurred in the fission chambers located near the target. We assumed that the points for the neutron and photon scintillations were at 4 and 5 cm depths from the front surface of the BC501A, which corresponded to the calculated effective detection points of 2 MeV neutron and 1 MeV photon, respectively. Doses measured by other detectors, on the other hand, were employed as the reference doses for the background measurements. The photon background doses were measured by the spherical NaI(Tl) scintillator by applying the G-function method.29) A conventional rem-counter (TPS451, Aloka) was used to obtain the neutron backgrounds. The statistical errors in the photon and neutron doses are approximately 1% and 10%, respectively. The difference between the neutron backgrounds at FRS and MS1 is attributed VOL. 42, NO. 9, SEPTEMBER 2005
775
to the existence of many neutron sources in FRS, since a lot of low energy neutrons were leaked from the container of the sources. The first 4 columns of the measured dose by DARWIN show the data obtained from the light outputs of the neutron, alpha, photon and muon scintillations by applying the corresponding G-functions for the isotropic irradiation. The conventional G-function was adopted in the evaluation of neutron dose, i.e. the values represent the doses from neutrons with energies between 1.2 and 80 MeV. The data shown in the 5th column denote the doses due to the primary scintillation by coupling with the G-function for the experimental irradiation geometries—the half isotropic and parallel to the front surface for the thermal neutron field and the others, respectively. The 6th column data were obtained from the doses shown in the 5th column by subtracting background doses, and hence, they are the experimental data that should be compared with the reference doses. The effects of scattered neutrons in the (b) continuous neutron fields were also excluded in the 6th column data, where these effects were estimated by performing experiments with shadowing the detector by an iron cone. Note that these six doses are abbreviated to neutron, alpha, photon, muon, primary and comparison doses hereafter. 4. Discussion (1) Comparison with Reference Dose The comparison doses (6th column of Table 1) generally agree with the reference doses (7th column of Table 1) fairly well, though slight differences can be observed. This tendency indicates the applicability of DARWIN to these radiation fields. The small discrepancies are probably due to the simplified calculation methods for the reference doses such as the assumption of the unique effective detection point for each scintillation type. For the background measurements, the photon dose rates of 7:91102 and 7:67102 mSv/h agreed quite well with those obtained by the NaI(Tl) scintillator of 7:88102 and 7:40102 mSv/h, respectively, at FRS and MS1. On the contrary, the neutron dose rates of 1:34102 and 6:35103 mSv/h were smaller and larger than those measured by the conventional rem-counter of 2:6102 and 3:6 103 mSv/h at FRS and MS1, respectively. This tendency is attributed to the underestimations of the keV-order and over 20 MeV neutron doses by DARWIN and the rem-counter, respectively. While not shown in the Table 1, the neutron dose rates from the spectrum-guessed G-function were 2:80 102 and 1:06102 mSv/h at FRS and MS1, respectively. The dose rate of 1:06102 mSv/h at MS1 was much greater than the corresponding reference dose rate of 3:6103 mSv/h. This is partially because the guessed-spectrum—unit fluence per lethargy—generally gives a conservative estimation of the keV-order neutron dose, although the primary reason of the disagreement is laid on the less sensitivity of the rem-counter to high energy neutrons. The spectrumguessed neutron dose rates of 2:80102 mSv/h was closer to the reference dose rates of 2:6102 mSv/h at FRS, since the average neutron energy was lower at the facility because of the existence of leaked neutrons from the source contain-
776
T. SATO et al. Table 1 Measured doses by DARWIN together with the reference doses Measured dose and its erroraÞ by DARWIN (mSv/h)
Radiation field (Primary scintillation)
Facility
Real time (s) (Dead time %)
Isotropic irradiation geometry Neutron
(a) (b) (b) (c) (d) (d) (d) (d) (d) (d) (e)
252
Cf:thermal FRS (Alpha) 252 Cf:continuous FRS (Neutron) 241 AmBe FRS (Neutron) 65 MeV neutron TIARA (Neutron) 133 Ba FRS (Photon) 137 Cs FRS (Photon) 60 Co FRS (Photon) 88 Y FRS (Photon) Mixed source FRS (Photon) Background FRS (Photon & neutron) Background MS1 (Photon & neutron)
2,140 (20) 1,445 (32) 966 (75) 1,256 (53) 2,029 (29) 1,003 (29) 1,305 (36) 813 (28) 1,016 (46) 5,413 (22) 20,020 (21)
1.08E-02 (4.92E-04) 6.93E-01 (5.05E-03) 2.58E+01 (5.80E-02) 1.03E+01 (3.26E-02) 1.22E-02 (5.70E-04) 1.34E-02 (8.46E-04) 1.41E-02 (8.10E-04) 1.32E-02 (9.35E-04) 1.44E-02 (9.95E-04) 1.34E-02 (3.51E-04) 6.35E-03 (1.43E-04)
Alpha
Photon
Exp. geometry Muon
Primary
3.59E-02 7.12E-02 2.76E-02 2.54E-02 (4.00E-04) (1.38E-04) (4.31E-04) (2.83E-04) 4.86E-03 1.24E-01 2.89E-02 7.42E-01 (1.95E-04) (2.34E-04) (5.88E-04) (5.49E-03) 6.91E-02 7.48E-01 3.49E-02 2.72E+01 (1.49E-03) (1.49E-03) (1.26E-03) (6.20E-02) No data No data No data 1.30E+01 (4.12E-02) 1.82E-03 9.59E-02 2.79E-02 9.32E-02 (9.78E-05) (1.56E-04) (4.76E-04) (1.54E-04) 1.88E-03 1.03E-01 2.86E-02 1.00E-01 (1.42E-04) (2.29E-04) (6.85E-04) (2.27E-04) 2.09E-03 1.81E-01 2.85E-02 1.77E-01 (1.38E-04) (3.03E-04) (6.29E-04) (3.00E-04) 1.73E-03 1.18E-01 2.72E-02 1.16E-01 (1.50E-04) (2.98E-04) (7.39E-04) (2.95E-04) 2.37E-03 2.36E-01 3.04E-02 2.31E-01 (1.82E-04) (4.03E-04) (8.09E-04) (3.97E-04) 2.07E-03 7.91E-02 2.86E-02 (6.12E-05) (8.91E-05) (2.82E-04) 5.15E-04 7.67E-02 3.60E-02 (1.58E-05) (4.61E-05) (1.63E-04)
Comparison 2.33E-02 (2.91E-04) 6.87E-01 (5.69E-03) 2.58E+01 (6.28E-02) 1.30E+01 (4.12E-02) 1.99E-02 (2.66E-04) 2.71E-02 (3.14E-04) 1.04E-01 (3.70E-04) 4.28E-02 (3.66E-04) 1.58E-01 (4.53E-04)
Reference dose (mSv/h) 2.30E-02 6.94E-01 2.26E+01 1.31E+01 1.98E-02 2.94E-02 1.02E-01 4.51E-02 1.56E-01 2.6E-02bÞ 7.88E-02cÞ 3.6E-03bÞ 7.40E-02cÞ
aÞ
Statistical error only. The error values are in parentheses. Neutron dose measured by the rem-counter of TPS-451. cÞ Photon dose measured by the NaI(Tl) scintillator with applying the G-function method.29) bÞ
er. These tendencies indicate that the difference between the doses measured by DARWIN and the rem-counter depends on the hardness of neutron spectrum at the detection point. (2) Contamination due to Miss-identified Scintillation The neutron dose rates in the (d) photon fields of the 133 Ba, 137 Cs, 60 Co, 88 Y and mixed photon sources in Table 1 were kept at the background level of approximately 1:3 102 mSv/h. The results indicate that the contamination in the neutron dose due to the miss-identified photon scintillations was negligibly small, i.e. the neutron and photon scintillations were perfectly discriminated by the PSD technique. The muon dose rates at FRS were also kept at the background level of approximately 2:8102 mSv/h except for the (b) 241 Am-Be continuous neutron field of 3:49102 mSv/h. The exception is attributed to an unsuitable assumption of the upper threshold of the photon scintillation— 6 MeVee, since 241 Am-Be produces high energy photons that can cause the scintillation with light output above the threshold. The muon dose rate of 3:60102 mSv/h at MS1 also exceeded the background level of approximately 2:8102 mSv/h at FRS. This may not be due to the contamination of the photon scintillations but the thinner shielding of the MS1 building, since a larger number of muons came into the building by penetrating the shield. (3) Directional Dependence The dependence of the dose measured by DARWIN on the direction of incident particles can be estimated by comparing the primary doses (5th column in Table 1) with the
corresponding doses obtained from the G-function for the isotropic irradiation (1st column for the (b) and (c) fields, 2nd for (a), and 3rd for (d) in Table 1). A smaller value of the primary dose indicates a forward directivity, since the values of the G-functions for the parallel-beam irradiation are smaller than those for the isotropic case. It should be noted that a smaller value of the G-function is relevant to a higher efficiency of the detector. For thermal neutrons, DARWIN has a forward directivity since the primary dose rate of 2:54102 mSv/h was smaller than the corresponding value for the isotropic irradiation of 3:59102 mSv/h. This is because the ZnS(Ag) sheets are attached only on the front and side surfaces of BC501A. For fast neutrons, conversely, it has an anti-forward directivity especially for the (c) 65 MeV monoenergetic neutron field, where the primary and the corresponding dose rates were 1:30101 and 1:03101 mSv/h, respectively. The dominant reason for causing the directivity is that the effect of surroundings of BC501A such as the glass and aluminum cases becomes significant with an increase of incident energy for the isotropic irradiation, since a larger number of neutrons cause nuclear reactions in the surroundings and generate high energy secondary particles that can induce the scintillation of BC501A. For photons, on the other hand, DARWIN has little directivity. As an example, the primary dose rate of 1:77101 mSv/h was very close to the corresponding value of 1:81101 mSv/h for the 60 Co photon field. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Development of Dose Monitoring System Applicable to Various Radiations with Wide Energy Ranges
(4) Sensitivity The neutron sensitivity of DARWIN is much higher than those of the rem-counters. As an example, DARWIN needs only 15 min to obtain background neutron dose data with a statistical error within 10%, while the rem-counter of TPS451 does approximately 5 h. With this property, DARWIN enables us to monitor small fluctuation of neutron dose rates near the background-level. The photon sensitivities of DARWIN as well as other systems such as the NaI(Tl)-scintillator-based dose monitor29) are sufficient enough to measure the photon dose rates near the background-level within a short time. On the other hand, the dead time of DARWIN is very large—approximately 20% even for the background measurement, since the system employs the time-consuming data analysis processes such as storing the waveform of each scintillation into HDD. Hence, it is hardly adequate to use DARWIN in radiation fields with high dose rates such as in the target room of an accelerator. A neutron dose rate of 100 mSv/h is the approximate maximum value that can be measured by the current version of DARWIN. It should be noted that the maximum dose rate is limited by the performance of the DAQ system, and therefore, the applicable dose rate range can be extended by the future improvement of DSO in terms of the data analysis speed.
V. Conclusions A dose monitoring system applicable to various radiations with wide energy ranges, DARWIN, has been developed for monitoring doses in workspaces and surrounding environments of high energy accelerator facilities. The spectrumguessed G-function method enables us to estimate the keV-order neutron doses; while the DSO-based DAQ system makes it possible to accomplish the stable PSD with an excellent property. The applicability of DARWIN to various neutron fields was examined by calculation. The calculated results indicated that DARWIN gives reasonable estimations of doses in the most radiation fields. Characteristics of DARWIN were also studied experimentally in several radiation fields. It was found from the experiments that DARWIN is capable of measuring neutron, photon and muon doses with enough accuracy. The experimental results also suggested that DARWIN can monitor small fluctuation of neutron dose rates near the background-level owing to its high sensitivity. The extension of measurable neutron energy up to 1 GeV and the development of on-line analysis software are under progress to achieve real-time monitoring for wider energy.
Acknowledgments This work has been carried out as a part of the JAERI-Cooperation Research Program at TIARA. This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 16560738, 2004. The authors wish to thank Dr. E. Kim for her great contribution to this work, Mr. Su. Tanaka for his support in the experiment in TIARA, Mr. M. VOL. 42, NO. 9, SEPTEMBER 2005
777
Tsutsumi, Mr. Y. Kajimoto, Mr. K. Fujii, Mr. Y. Uchida, Dr. Y. Tanimura and Mr. M. Yoshizawa for their help in the experiment in FRS, and Dr. T. Nakamura for his advices on this work.
References 1) Joint project team of JAERI and KEK, The Joint Project for High-intensity Proton Accelerators, JAERI-Tech 2000-003, Japan Atomic Energy Research Institute, (2000), [in Japanese]. 2) The NSNS collaboration, National Spallation Neutron Source Conceptual Design Report, Volume I, NSNS/CDR-2/VI, Oak Ridge National Laboratory, (1997). 3) ESS study group, The ESS Project, Volumes I, II, III and IV, European Spallation Source Project Report, Forschungszentrum, (2002). 4) C. Birattari, A. Esposito, A. Ferrari, et al., ‘‘The extended range neutron rem counter ‘LINUS’; overview and latest developments,’’ Radiat. Prot. Dosim., 76[3], 135 (1998). 5) C. H. Olsher, H. H. Hsu, A. Beverding, et al., ‘‘WENDI: an improved neutron rem meter,’’ Health Phys., 79[2], 170 (2000). 6) International Commission on Radiological Protection, To be published in, Recommendations of the International Commission on Radiological Protection. 7) Y. Oyama, K. Sekiyama, and H. Maekawa, ‘‘Spectrum weight function method for in-situ fast neutron and gamma-ray responses measurements in fusion integral experiments with an NE213 scintillation detector,’’ Fusion Technol., 26[3], pt2, 1098 (1994). 8) E. Kim, A. Endo, Y. Yamaguchi, et al., ‘‘Measurement of neutron dose with an organic liquid scintillator coupled with a spectrum weight function,’’ Radiat. Prot. Dosim., 102[1], 31 (2002). 9) A. Endo, E. Kim, Y. Yamaguchi, et al., ‘‘Development of neutron-monitor detectors applicable to energies from thermal to 100 MeV,’’ J. Nucl. Sci. Technol., Suppl. 4, 10 (2004). 10) T. Sato, A. Endo, Y. Yamaguchi, et al., ‘‘Development of neutron-monitor detector using liquid organic scintillator coupled with 6 Li+ZnS(Ag) Sheet,’’ Radiat. Prot. Dosim., 110, 255 (2004). 11) A. Bertin, M. Bruschi, V. M. Bystritsky, et al., ‘‘Performance of a coincidence neutron spectrometer with double pulse-shape discrimination,’’ Nucl. Instrum. Methods, A337, 445 (1994). 12) W. N. McElroy, S. Berg, A Computer-automated Iterative Method for Neutron Flux Spectra Determination by Foil Activation, AFWL-TR-67-41, U.S. Air Force Weapons Laboratory, (1967). 13) D. Satoh, N. Shigyo, K. Ishibashi, et al., ‘‘Neutron-production double-differential cross sections of iron and lead by 0.8 and 1.5 GeV protons in the most-forward direction,’’ J. Nucl. Sci. Technol., 40[5], 283 (2003). 14) H. Iwase, K. Niita, T. Nakamura, ‘‘Development of a generalpurpose particle and heavy ion transport Monte Carlo code,’’ J. Nucl. Sci. Technol., 39[11], 1142 (2002). 15) D. Satoh, T. Sato, A. Endo, et al., ‘‘Study on response function of organic liquid scintillator for high-energy neutrons,’’ Proc. Int. Conf. on Nuclear Data for Science and Technology, Santa Fe, USA, Sep. 26–Oct. 1, 2004, in press. 16) International Commission on Radiological Protection, Conversion Coefficients for used in Radiological Protection against External Radiation, ICRP Publication 74, Ann. ICRP 26[3/ 4], Oxford; Pergamon Press, (1996).
778 17) International Atomic Energy Agency, Compendium of Neutron Spectra and Detector Responses for Radiation Protection Purposes, Technical reports series 403, IAEA, Vienna, (2001). 18) I. O. Andersson, J. A. Braun, ‘‘Neutron rem-counter with uniform sensitivity from 0.025 eV to 10 MeV,’’ Proc. IAEA Symp. on Neutron Dosimetry, Vienna, 2:87–95, (1963). 19) P. Goldhagen, J. M. Clem, J. W. Wilson, ‘‘The energy spectrum of cosmic-ray induced neutrons measured on an airplane over a wide range of altitude and latitude,’’ Radiat. Prot. Dosim., 110, 387 (2004). 20) T. Nunomiya, S. Abe, N. Hirabayashi, et al., ‘‘Sequential measurements of cosmic-ray neutron energy spectrum and ambient dose equivalent on the ground,’’ J. Nucl. Sci. Technol., Suppl. 4, 466 (2004). 21) N. Matsuda, Japan Atomic Energy Research Institute, Private communication. 22) W. R. Nelson, H. Hirayama, W. O. Rogers, The EGS4 Code System, SLAC-265, Stanford University, (1985). 23) O. Sato, S. Iwai, S. Tanaka, et al., ‘‘Calculations of equivalent dose and effective dose for photons form 1 MeV to 10 GeV,’’ Radiat. Prot. Dosim., 62[3], 119 (1995).
T. SATO et al. 24) A. Fasso, A. Ferrari, J. Ranft, et al., ‘‘FLUKA: Status and prospective for hadronic applications,’’ Proc. Monte Carlo 2000, Lisbon, Oct. 23–26, 2001, 955 (2001). 25) A. Ferrari, M. Pelliccioni, M. Pillon, ‘‘Fluence-to-effective dose conversion coefficients for muons,’’ Radiat. Prot. Dosim., 74[4], 227 (1997). 26) J. P. Dumais, M. Yoshizawa, Y. Yamaguchi, Evaluation of Scattered Neutrons in Fast Neutron Calibration Field using 252 Cf and 241 Am-Be Neutron Source, JAERI-Tech 98-005, Japan Atomic Energy Research Institute, (1998), [in Japanese]. 27) M. Baba, Y. Nauchi, T. Iwasaki, et al., ‘‘Characterization of a 40–90 MeV 7 Li(p,n) neutron source at TIARA using a proton recoil telescope and a TOF method,’’ Nucl. Instrum. Methods, A428 454 (1999). 28) N. V. Kornilov, V. A. Khriatchkov, M. Dunaev, et al., ‘‘Neutron spectroscopy with fast waveform digitizer,’’ Nucl. Instrum. Methods, A497, 467 (2003). 29) M. Tsutsumi, K. Saito, S. Moriuchi, Spectrum-dose Conversion Operators, G(E) Functions of NaI(Tl) Scintillators Adapted for Effective Dose Equivalent Quantities, JAERI-M 91-204, Japan Atomic Energy Research Institute, (1991), [in Japanese].
JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
