Neutron transport calculation for Activation Evaluation for Decommissioning of PET cyclotron Facility

In order to evaluate the state of activation in a cyclotron facility used for the radioisotope production of PET diagnostics, we measured the neutron flux by using gold foils and TLDs. Then, the spatial distribution of neutrons and induced activity inside the cyclotron vault were simulated with the Monte Calro calculation code for neutron transport and DCHAIN-SP for activation calculation. The calculated results are in good agreement with measured values within factor 3. Therefore, the adaption of the advanced evaluation procedure for activation level is proved to be important for the planning of decommissioning of these facilities.


Introduction
About 150 cyclotrons have been installed in the hospitals and clinics in Japan to supply radioisotopes for PET diagnosis of cancer and aging [1].Several cyclotron facilities are in decommissioning stage recently.In these facilities, it is important to estimate the volume of radioactive wastes in advance.
There are two causes for activations.In case of the activation of charged particles, activated area is limited in a small area.On the other hand, neutrons are also emitted from the target by nuclear reactions and are widly distributed in a cyclotron vault.The evaluation of neutron activation of concrete around the cyclotron affects the cost of decommissioning.The purpose of this study is to estimate degrees and ranges of activation in a cyclotron vault.
In the previous report [2], we used the Monte Calro [MC] calculation code such as PHITS [3] or MCNP [4] for calculation of neutron spectrum emitted by the 18 O(p,n) 18 F reaction [5] and the induced radioactivity calculation code DCHAIN-SP [6].The results were compared with the measurement results in case of the PET Center of Dokkyo Medical University.In this work, we attempt to use PHITS with the revised nuclear reaction mode, in order to re-evaluate the activation results.

Facility
The cyclotron of Dokkyo Medical University is a CYPRIS-HM18 made by Sumitomo Heavy Industries, Ltd (Figure 1).This cyclotron has a target box to shield the radiation from several taget chambers.Target box is composed of polyethylene, lead and iron.In this facility,

18
F is mainly produced by the typical operation condition of the 18-MeV protons of 50PA for one hour in every morning on weekday.Two target (A and B) ports are used alternatively.Cyclotron has been operated about 200 days per year.

Measurements
The gold (Au) foils (6mm in diameter and 20Pm in thickness) with and without the cadmium (Cd) cover (1mm in thickness) were used for measuring the thermal and epi-thermal neutron flux(Figure 2) and the aluminium (Al) plates were used to detect a fast neutron flux by the 27 Al(n,D) 24 Na reaction(Figure 3) .TLD UD-813PQ4 (Panasonic Co. Ltd.) (Figure 4) were used for measuring the neutron dose.The TLD has four phosphor elements.Two of the four elements (elements 1 and 2) are 6 Li2 10 B4O7(Cu), which is sensitive for thermal neutrons and gamma rays, and the others (elements 3 and 4) are 7 Li2 11 B4O7 (Cu), which is sensitive for J-rays only.A neutron dose for a TLD can be obtained by subtracting the average reading of elements 3 and 4 from that of elements 1 and 2.
Three TLDs were set inside the neutron monitor case UDS893-P1 for measuring the Ambient Dose Equivalent H*(10), respectively.The neutron monitor case is a polyethylene cube (outer size:225×225×220mm).This case is designed to moderate a fast neutron to thermal neutron and to increase a TLD reaction rate for measuring neutron ambient equivalent dose.The outside view of the neutron monitor case is shown in Figure 5.
These detectors were placed on the floor, wall, target box shielding and cyclotron surface.Irradiation of 18 MeV proton was performed for 55min in the target port A.
The location of cyclotron components and detectors for neutron measurements in the cyclotron vault are shown in Figure 6.
In addition, for the verification of the activity calculation, several metal samples, bolts and screws, were obtained in the room (Figure 7).These samples are located at the same place all the time during operation period.The location of activity samples were shown in Figure 6.where, N (atoms/g) ; aluminium atomic density, (E) (n/cm 2 /sec) ; neutron flux at energy E, (E) ; the 27 Al (n, ) 24 Na reaction cross section (Fig. 8) of JENDL/D-99 library [12], ; decay constant and t; experiment irradiation time.

Activation calculation
Radioactivitiy induced in samples is calculated by using the DCHAIN-SP code.DCHAIN-SP code needs the neutron spectra which were calculated by the PHITS [T-Dchain] method for the activation calculation.A flow chart of linkage calculation between PHITS and DCHAIN-SP is shown in Figure 9.The [T-Dchain] tally method in PHIS code generates an input file for DCHAIN-SP.This is a merit to use the PHITS code and the DCHAIN code.
For the calculation of activity, the elemental composition (especially parent elements of radioactive nuclide) of samples were measured by an X-ray fluorescence analysis(Table 1).These samples have been set at the sampling positions since the start of this facility.In this facility, two target ports have been used alternatively.But in order to calculate induced activity simply, we supposed that the target port A has been only used, because the neutron fluxes during irradiation between target port A and B were alomost same level at the wall of cyclotron vault.

Spatial distribution of neutrons
The spatial distribution of neutron dose is shown in Figure 10.At the target port A, neutrons are produced by protons and neutrons are partially shielded by the target box shielding and the magnet yoke.For the estimating a radioactivitive volume, understanding the location of the maximum neutron flux at the wall is important.
In this facility, the location of maximum neutron flux is not occurs near the target because of the target box shielding.Results of neutron dose calculation and TLD measurement are shown in Table 2.The results of calculated neutron dose are in good agreement with the measured results to within the factor (C/M; calculation / measurements) 0.79 to 0.99.The thermal neutron flux distribution in the cyclotron vault is shown in Figure 11.The results of calculation and measurement obtained by activation of gold foils are shown in Table 3.The calculated results of thermal neutron flux are in good agreement with the in-room measured results to within the factor (C/M) 1.1 to 3.0.
Thermal neutron flux on the concrete surface may vary according to the concrete composition like as moisture content.The calculated and measured values of 24 Na with activity of R(Bq/g) were shown in Figure 12.The

Activity calculation of samples
For the activity metal samples in the cyclotron vault, the activity calculation results are shown in Table 4 and Table 5.These measured and calculated data are in good agreement with each other.Detected nuclides in a screw were Cr-51, Fe-59 and Co-60, which are produced by thermal neutron capture reactions.The C/M ratios of the activities are comparable to the C/M ratios of thermal neutron fluxes.In this activity calculation, samples are very small, so activity self-shielding factor is not so large.

Fig. 2 .
Fig. 2. A set of gold foils with and without Cd cover.

Fig. 3 .
Fig. 3.An aluminium foil set on the cyclotron surface.

A
calculation results are smaller than measurement values.The cause of this discrepancy might be came from the simple modelling of the 18 O target.We only considered the proton beam attack 18 O target and a Havar foil only.Any other proton beam loss point such as a vacuum foil may slightly contribute the emission of high energy part of neutrons.

Table 1 .
material composition for activilty samples calculation * *based on JSS Standard stainless value

Table 2 .
Neutron dose obtained by calculation and measurement.

Table 3 .
Thermal neutron flux obtained by calculation and measurement.
Fig. 12.The comparison of the results of calculation and measuremet value of 24 Na activity in aluminium plates which were set in the cyclotron vault X(cm)

Table 4 .
Radioactivity induced in a screw tapped on a cabinet obtained by calculation and measurement.

Table 5 .
Radioactivity induced in a key set at the safety box obtained by calculation and measurement.It was found that a newer version of PHITS/DCHAIN-SP with a proton induced cross section library can be applicable to estimate radioactivity for PET cyclotron facility.The combination of MC calculation code and DCHAIN-SP code is suitable to estimate the induced activity and to depict the 3 dimensional map inside the cyclotron vault and concrete wall.