RPC aging by irradiation

One of the major concerns related to the neutron flux and dose rate in the experimental areas at LHC is the material damage.
According to the energy of the neutrons different processes can take place in organic materials such as those used in RPCs.
Within the reaction with the nuclei of the atoms of an irradiated medium fast neutrons transfer a considerable amount of their energy. Each struck nucleus is ejected as a recoil ion that interact with orbital electrons of the medium thus producing molecular excitation and ionization. The initial neutrons are degraded to a thermal-energy state as a result of the several scatterings.

A big fraction (95 to 98 percent) of the energy of fast neutrons is thus transferred to the medium through ionization and excitation and since organic compounds and covalent materials are highly susceptible to these processes a considerable damage result.

Thermal neutrons undergo nucleus capture and the resulting emitted radiation (gamma in the MeV range for the most probable reaction with hidrogenated compounds) is responsible for the subsequent excitation and ionization via secondary processes (mainly Compton scattering and Photoelectric effect)

Other kind of capture reactions might be possible with emission of electrons (n-beta reaction with chlorine), protons ( N14(n,p)C14) and alfa particles (n-alfa reaction with Boron and Lithium).
Despite the different mechanisms and the different secondary radiation production for fast and slow neutrons the equivalence in damage for equivalent energy absorption (and hence dose) is a rather good approximation for covalent materials such organics.

The expected dose rate in the CMS barrel region will not exceed 1 Gy/year [1]. A factor 100 more is foreseen in the forward region. The previous dose rate (1 Gy/year) is consistent with what one expects from a particle dosage of fast neutrons (> 1 MeV) equivalent to some 1010 n/cm2 (according to the exposed material) [2].
In the case of bakelite, for example, the fluence of fast neutrons corresponding to a deposit of 100 Rads (1 Gy) is 4.6 1010 n/cm2. Similar fluences, for the same dosage, are nedeed for Mylar (6), Polythene (2.2) and PVC (5.8).

A rough estimate of the fraction F of molecules affected by the damage for a particular absorbed dose D (in Rads) in a medium of molecular weight M is :

F = 10-12G M D
where G represent the number of molecules affected per 100 eV absorbed, being this value about 4 for covalent materials.

The above foreseen particle dosages are well below the values nedeed to measure a relevant degradation of the mechanical and electrical properties of the materials such as bakelite or PVC. These limits, for a variation of 25% to 75% in the measured parameters are show in table 1 [2]. for some of the components of bakelite RPCs.

Table 1 Dosage to give 25 and 75 % decrease, 107 Rads Particles dosages each equivalent to 107 Rads
Material Tensile strenght Elongation Elastic modulus Shear strenght Impact strenght Thermal neutrons,
1017 n/cm2
Fast neutrons (> 1MeV),
1015 n/cm2
-25% -75% -25% -75% -25% -75% -25% -75% -25% -75%
Bakelite 2.2 40 2.2 40 25 > 300 2.2 40 2.2 40 5.0 4.6
Polyethylene 800 > 1000 9.0 37 130a 220a 11a 94a 9.0 37 2.7 2.2
Polystyrene > 600 > 600 > 600 > 600 > 600 > 600 > 600 > 600 > 600 > 600 4.5 3.5
PVC 180 370 12 83 0.068 5.8
a The dosage gives a positive change in the measured parameters.

We have used the 250 KW Triga Mark II research reactor located in Pavia to irradiate small samples of bakelite RPCs. After irradiation the samples have been analized by means of visual inspection with a 600X magnification.
An initial heavy irradiation (about 4.5 1016 thermal n/cm2) has been performed in order to analyze the radioisotope content of the samples. The results, obtained with a gamma-spectroscopy (in the range 13 KeV - 2 MeV) and presented in table 2, show a certain activation.
Though the beta and gamma emission of these radioisotopes will probably not account for aging effects, safety procedures might be nedeed when handling the detectors after several years of LHC running. In table 2 the results of the activation of the linseed-oil is also shown.

Table 2 Gamma activity of irradiated RPC samples
Sample Weight (mg) Activity (Bq)
PVC + Polyethylene 60 24Na 5000 65Zn 133 82Br 375 198Au 73
Al + polyestere 25 24Na 1702 122Sb 31802 82Br 22 198Au 78 124Sb 572
Foam (PVC) 60 24Na 246340 122Sb 76 82Br 377 198Au 93
Al+PVC+Polistyrene 160 82Br 224410 60Co 383
Bakelite + PVC 390 24Na 665100 122Sb 1169 82Br 633 198Au 104 124Sb 19 46Sc 213 51Cr 2737 54Mn 16 59Fe 557 58Co 15 60Co 85 65Zn 70 140La 549 141Cs 3.8
Al+ Foam(PVC)+Fiberglas Polyestere 330 24Na 54600 122Sb 183 82Br 742 198Au 1010 124Sb 8 46Sc 23 51Cr 240 54Mn 17 59Fe 143 60Co 334 65Zn 70 140La 1311 141Cs 4.4
Bakelite + Graphite 350 24Na 104000 122Sb 1114 82Br 244 198Au 71 124Sb 24 51Cr 106 59Fe 452 140La 210 65Zn 43
Al 10 24Na 17036 122Sb 10 82Br 19 198Au 46 46Sc 1 51Cr 13 54Mn 2 59Fe 24 60Co 1 65Zn 7
Polystyrene 20 82Br 229060 60Co 384 198Au 203
Linseed Oil 1000 82Br 40 60Co 400

In a second heavy irradiation, corresponding to a particle dosage of about 100 MRads, we have noticed the blackening of PVC as shown in fig. 1

Fig. 1: comparison of weight equivalent PVC samples before and after a dose absorption of about 100 MRads.

Fig. 2 shows the damage of thin foils of aluminium (strips) and of polystirene.

Fig. 2: Aluminum strip and polystirene before and after a 100 MRads dose absorption. The blackening of a PVC layer is also shown.

More realistic exposures ( 10 LHC year equivalent) of the bakelite samples have been performed and no damages have been noticed on the surfaces when inspected with a 600X magnification.
Also resistivity measurements before and after the irradiation have given equivalent results within a factor 2. A measure of the bakelite surface wrinkledness after such irradiation showed no degradation at the level of few microns.
Dynamics tests are foreseen in 1998 with RPCs exposed to fast neutron beam.

[1] CMS Technical Proposal, pg 123
[2] Nuclear Engineering Handbook, H. Etherington Editor - McGraw-Hill (1958)