Simulation of a suite of generic long-pulse neutron instruments to optimize the time structure of the European Spallation Source

Abstract

We here describe the result of simulations of 15 generic neutron instruments for the long-pulsed European Spallation Source (ESS). All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to a) the peak flux and b) the duty cycle to a power of approx. 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3-5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single Figure-ofMerit for a full facility and compare with evaluations of the individual instrument classes.

Optimization of Performance, Price, and Background of Long Neutron Guides for European Spallation Source

“…The monolith shielding ends at 6 m from the moderator [4]. This distance is therefore a natural place to position the first instrument choppers, which are the pulse shaping choppers [6,31]. Since the open and close times of choppers depend on the guide width, it has been decided to narrow the beam at this point.…”

“…Although not directly comparable to our work due to the difference in wavelength bands and phase space volume, the previous results indicate the magnitude of BT values to expect for at least the WB3a and WB3b bands. (The modified moderator size is not likely to have any effect on the results, since it was found that all neutrons reaching the sample within the correct divergence range would anyway have originated from within the middle of the moderator, approximately 4 × 4 cm 2 [6]).…”

“…The European Spallation Source (ESS) will be the first long-pulsed neutron source and novel instrument concepts have been developed to utilize the unique time structure [2][3][4][5]. Many instruments will be optimally placed at long distances, ∼170 m, from the moderator [3,6]. In fact, 8 of the first 15 instruments selected for ESS are placed at this distance [4], which is about twice as long as the longest instruments currently in use [7].…”

“…In fact, 8 of the first 15 instruments selected for ESS are placed at this distance [4], which is about twice as long as the longest instruments currently in use [7]. For this reason, a dedicated guide simulation effort has been coordinated by ESS and performed by several groups during the last decade [5,6,[8][9][10][11][12][13]. The results from these works are that long neutron guides are indeed feasible, when ballistic guide concepts [14,15] are used.…”

CAMEA ESS – The continuous angle multi-energy analysis indirect geometry spectrometer for the European Spallation Source

“…From the neutron moderator there is a guide feeder to a virtual source at 6.5 m [14], where a pulse shaping chopper is placed at the closest possible distance to the moderator. To be able to use the full length of the 2.86 ms neutron pulse of the 14 Hz ESS, filling the counting window, a pulse shaping chopper at 6.3 m gives a natural length of 165 m [15]. The neutron guide is then two ellipses separated by a small angular kink in the guide to remove line-of-sight from the neutron source.…”